Stone Slate RoofingTechnical advice note

2005

08268_Cover_10Black 4/5/05 9:30 am Page 1

Contents

1 Introduction . . . . . . . . . . . . . . . . 2

2 Stone as a roofing material . . . . . . . . . . . . . . . . . . 22.1 Terminology . . . . . . . . . . . . . . 22.2 Stone types. . . . . . . . . . . . . . . 42.3 The production of stone

slates . . . . . . . . . . . . . . . . . . . . 4

3 The principles of stoneslate roofing. . . . . . . . . . . . . . . . . 4

4 Planning and historicbuildings legislation . . . . . . . . . . 5

5 Inspection andmaintenance . . . . . . . . . . . . . . 6

6 Investigation andrecording . . . . . . . . . . . . . . . . . 6

7 The repair and reroofingof stone slate roofs . . . . . . . . . . 77.1 Materials for repair and

reroofing . . . . . . . . . . . . . . . . . 77.1.4 Stone slates . . . . . . . . . . . . . . 77.1.5 Ridges . . . . . . . . . . . . . . . . . . . 87.1.6 Fixings . . . . . . . . . . . . . . . . . . . 87.1.7 Torching, roofing felts and

vapour barriers. . . . . . . . . . . . 107.1.8 Bedding and pointing . . . . . . . 107.1.9 Roof timbers . . . . . . . . . . . . . 11

7.2 Construction and workmanship . . . . . . . . . . . . . 11

7.2.1 Specification . . . . . . . . . . . . . . 117.2.2 Stripping, sizing and stacking . . 117.2.3 Reroofing . . . . . . . . . . . . . . . . 117.2.4 Detailing . . . . . . . . . . . . . . . . . 147.2.5 Insulation and ventilation . . . . 15

7.3 Minor and localised repairs . . 167.4 Choosing a slater . . . . . . . . . . 16

8 Costs . . . . . . . . . . . . . . . . . . . . . . . 178.1 Life-cycle costs . . . . . . . . . . . . 178.2 Grants. . . . . . . . . . . . . . . . . . . 17

Appendix 1 Stone slate roofs:checklist for recording constructiondetails . . . . . . . . . . . . . . . . . . . . . . . . . 18

Appendix 2 Stone slate roofs:checklist for recording materials . . . . 19

Glossary . . . . . . . . . . . . . . . . . . . . . . 20

References . . . . . . . . . . . . . . . . . . . . 21

Further information . . . . . . . . . . . 22

Sourcing stone slates . . . . back cover

1 Introduction

If any regard is to be had to the general beautyof the landscape, the natural material of thespecial countryside should be used instead ofimported material. (William Morris, On theexternal covering of roofs, 1890)

1.1 Sandstone and limestone slating is ahighly regionalised roofing form,fundamental to the distinctive localcharacter of buildings in many parts of thecountry. However, local character anddistinctiveness are under threat and stoneslate roofing is becoming increasingly rare.As locally produced stone slates come tothe end of their natural lives, decliningsupplies of new ones have led to the use ofimported or artificial substitutes whichhave little to do with local buildingtraditions. If the special qualities of ourbuilt heritage are to be conserved, anyrepair or reroofing should be based on theprinciple of replacing ‘like with like’. Stoneslate roofs are no exception. Keeping thelocal stone slate roof of an historic buildinghelps to conserve its significant character.

1.2 The gradual decline of the stone slateroofing industry has led to a loss ofexpertise of both roofers and specifiers,and there is little written informationavailable on best practice methods forreroofing. The revival of traditionalmethods and the adaptation of historicdetails so that roofs meet modernperformance standards has createdconfusion and resulted in the unnecessaryfailure of a number of roofs. This leafletattempts to clarify these issues andprovides information on best practice aswe understand it today.

1.3 This technical advice note has beendeveloped from several documents writtenwhen stone roofing was a live art; frompractical experience, and from advicegiven by stone slaters, manufacturers,conservation officers, architects andsurveyors. Some of the issues raised arematters of conservation principle ratherthan technical fact, and others may need

2

to be reviewed as more information comesto hand and experience grows. Researchinto the construction of stone slate roofs isa neglected field. Future research willreveal more information on traditionalpractice and help us to understand betterthe regional variations of stone slateroofing.

2 Stone as a roofing material

2.1 Terminology

2.1.1 Stone slates are known in differentparts of the country as grey slates, flags,flagstones, thackstones, slats, flatstones,stone tiles, and tilestones. Geologistsprefer the term tilestone as theselimestone and sandstone products are not,geologically, slates. That is to say that theyare made from sedimentary, rather thanfrom metamorphic rocks, andconsequently split along bedding ratherthan cleavage planes. Stone slate ishowever the most commonly used termand has been adopted in this leaflet.

2.1.2 The word ‘delph’ (plural ‘delves’)has been used in preference to ‘quarry’and ‘quarrying’ because of the unpopularimage the latter have in the public’sperception. Other names are used locally,including ‘quar’ in Dorset and ‘pit’ over awide area of the country.

2.2 Stone types

2.2.1 Stone slates have been used forroofing in Britain since Roman times.Wherever a rock could be split to form areasonably thin slab, it has been exploitedfor roofing and examples exist for almostevery geological period and rock type.Until the second half of this century, stoneslates were quarried or mined extensivelythroughout Britain and although mostquarries were small and served a smalllocality, collectively they formed asignificant industry. The table identifiesthe regions in which stone slates havebeen manufactured and used, their

Technical advice note

This technical advice note gives guidance for architects, surveyors, and building owners, on the repair andreroofing of traditional stone slate roofs in England. It deals specifically with stone slates fromsedimentary rocks (mainly limestone and sandstone) rather than with real slates, which are frommetamorphic rock.As the material and methods of stone slate roofing vary from region to region, itconcentrates on general principles. For guidance on the specific local roofing types and methods, adviceshould be sought from the local planning authority. Some local authorities provide leaflets on regionalroofing practice. Further advice will be available in “Stone Roof Conservation: a best practice guide” (seeReferences).

08268 26/4/05 10:33 pm Page 2

3

Figure 1 Geological formations important for stone slates in England, and some historical locations for production and use

08268 26/4/05 10:33 pm Page 3

geological period and formation names,and is a reminder of how extensively theywere once used (Fig 3).

2.3 The production of stone slates

2.3.1 Roofing stones are produced fromeither weathered rock (subjected to glacialand post-glacial weathering) or fromdeeper, unweathered rock. Depending onthe lie of the ground and the amount of

overburden, the block for stone slateproduction has traditionally been obtainedby open quarrying or delving (exploitingthe near-surface weathered rock), oralternatively by mining (mainly to extractunweathered rock). Sometimes delvingbegan with the extraction of the weatheredrock and progressed to mining. In bothcases it was (and remains) a manualprocess although, today, small excavatorsare sometimes used to loosen the rock

before selecting, splitting, and dressingare done by hand.

2.3.2 Stone slates were mined atPurbeck in Dorset, at Collyweston andDuston in Northamptonshire, atStonesfield and elsewhere in theCotswolds, in Yorkshire and occasionallyin Derbyshire. Access to the fissile rockwas by adits or shafts with the rockworked along galleries. At Collywestonand Stonesfield, the splitting was carriedout by frost action. The raw block waseither stored underground or taken to thesurface where it was wetted and coveredin earth until the frosts came. The frostthen swelled the natural moisture withinthe stone and split it into slates. Frost-split slates may be thinner and thereforelighter than those split by hand (Fig 2).

2.3.3 However, most stone slates weresurface delved rather than mined. Wherethe rock has been weathered, it isweakened along the bedding planes,allowing it to be split into thin layers of asuitable thickness for roofing. In someareas, the weathering actually completesthe splitting process and stone slates arefound at the surface or turned up byploughing. Traditionally the delves tendedto be small in scale, supplying one villageor even a single farmstead (Fig 3),although some developed into largeoperations supplying industrial towns.Even so, their markets were always local.Today the remaining delves often supplyslates far beyond their traditional markets.

2.3.4 Once the rock had been split tothe required thickness, it was dressed tosize and shape. In some cases the faces ofthe slates were also tooled to provide aflatter surface. The slates were holedusing a pointed tool, from one or bothsides. This provided a tapered orhourglass hole which gripped the timberpeg commonly used to hold the slate inplace on the roof (Figs 4 and 5).

3 The principles of stoneslate roofing

3.1 Local stone slating traditionsevolved from the slater’s response to theparticular geology and climatic conditionsof an area – it is a highly regionalisedactivity. Local practice varies from regionto region, with the production of thestone slates, design of the roof, roofingmethods and detailing all contributing to

4

Table Stone slates: local usage and sources

Limestones, including sandy limestonesDorset JURASSIC:

Purbeck, Forest Marble

Somerset JURASSIC:(South and East) Lias, Forest Marble

Cotswolds JURASSIC:East Gloucestershire, Purbeck, Corallian, Forest Marble,North Oxfordshire, Hampen Marly Beds,Trougham and Taynton and North Wiltshire Stone (Stonesfield Slate), Fullers Earth,

Chipping Norton Limestone

East Midlands JURASSIC:Northamptonshire, Blisworth Limestone, Upper Estuarine Lincolnshire, Limestone, Chipping Norton Limestone Cambridgeshire, Lincolnshire Limestone, (Collyweston Slate),and Leicestershire Northampton Sand, Lias

Yorkshire JURASSIC:(North-east) Scarborough Limestone

Nottingham to South Shields, East PERMIAN:Derbyshire (Whitwell) Magnesian limestone

Sandstones, including calcareous sandstonesSurrey, Sussex,West Kent CRETACEOUS:

Horsham stone

West Gloucestershire, Bristol CARBONIFEROUS:Upper Westphalian/Coal Measures– Pennant Measures

Welsh Marches DEVONIAN – OLD RED SANDSTONE Gloucestershire, and SILURIAN – PRIDOLI:Herefordshire, Dittonian, Downtonian,TilestonesWorcestershire,and Shropshire ORDOVICIAN:

Hoar Edge Grit, Cheney Longville Flags,Hope Shale (Corndon Hill)

South Pennines CARBONIFEROUS:North Staffordshire, Lower Westphalian/Coal MeasuresNorth Derbyshire, Namurian/Millstone Gritand East Cheshire

Lancashire,Yorkshire, CARBONIFEROUS:Durham, Northumberland Lower Westphalian/Coal Measures,

Namurian/Millstone Grit

Cumbria PERMIAN:New Red Sandstone– Penrith Sandstone

CARBONIFEROUS:Namurian/Millstone Grit

Note: for more detail see Hughes in Wood (2003)

08268 26/4/05 10:33 pm Page 4

the special qualities of an area’s builtheritage.

3.2 Stone slates are randomly sized (seeGlossary) and laid in diminishing courses.The principle of random roofing is thatthe range of stone slate sizes reflectedwhat was available at that time from thequarry and maximised the usable rock.Unfortunately, some manufacturers todayhave limited the range of sizes theyproduce and this alters the appearance ofa roof. The selection of replacementmaterials should reflect the original mix ofsizes. Stone slates should diminish in size

regularly up a roof slope and represent, asfar as possible, the range of sizes typicallyused in the region (see front cover image).

3.3 In many parts of the country stoneslates were used at the eaves courses incombination with thatch or tiles for avariety of reasons. Eaves slating has soundpractical advantages: it evolved as a meansof protecting the eaves of thatched buildingsfrom harm by animals, but also provides ahard edge for resting ladders and a longoverhang for water runoff. In most cases,however, where stone slates are present onlyat the eaves, they are the remnants of an

earlier stone roof which has been graduallylost because of lack of make-up slatesduring successive re-slating.The appearanceis now considered to be traditional andshould be conserved (Fig 13).

3.4 There is no British Standard forspecifying either materials or constructionmethods for stone slate roofing, althoughmany of the details and recommendationsmade in BS 5534: 2003 Code of practice forslating and tiling are applicable to stoneslates. However, it should be recognisedthat this British Standard does not deal indetail with random roofing. Many of thetraditional details typical of stone slateroofing, which have performedsatisfactorily for centuries, conflict with itsrecommendations. However, departurefrom British Standard practice should bebased on sound, proven knowledge oflocal traditional practice.

4 Planning and historicbuildings legislation

4.1 The controls for listed buildings, andbuildings in conservation areas, are

5

Figure 2 Collyweston, Northamptonshire: stone laid out for splitting by winter frost (Photograph: Daniel Martin)

Figure 4 The rock is split along the beddingplanes into a suitable thickness for roofing.

Figure 5 Once split to a suitable thickness, thestone is dressed roughly square and a fixing holemade.

Figure 3 The duck pond of this West Sussex farmhouse may originally have been the quarry for the Horshamslates on the roof.

08268 26/4/05 10:33 pm Page 5

covered by the Planning (Listed Buildingand Conservation Areas) Act 1990, andPlanning policy guidance: planning and thehistoric environment (known as PPG 15 –see References). Consent is needed foralterations to a listed building if thesewould affect its character as a building ofarchitectural or historic interest.Obviously, stone slate roofs are veryimportant features whose loss will almostalways be resisted by the local planningauthority. For all but the most minorrepairs it is prudent to speak to the localplanning authority to ascertain whether ornot Listed Building Consent will berequired for the works.

4.2 Unlisted buildings in conservationareas do not have the same protection.Planning permission is required for anychange which would represent a materialalteration to the external appearance of abuilding if it is not in residential use.Permission is not usually required if abuilding is in residential use. However, itis always advisable to check in case thelocal planning authority has any additionalpowers under an Article 4 Direction (ofthe 1990 Act), which could requirepermission to be obtained to remove oralter a stone slate roof.

4.3 An application for a new building (oralterations to an existing building) in aconservation area will be subject to localplanning policies, so an early consultationwith the local planning authority’s Designor Conservation Officer is recommended.Development proposals are required topreserve or enhance the character orappearance of the area so, where stoneslate roofs are common, new buildingsmay be required to incorporate the sametype of roof covering. This is especiallylikely if the relevant appraisal confirmstheir positive contribution to the overallcharacter.

4.4 Detailed recommendations for theapplication of planning legislation to stoneroofs is available in Stone RoofConservation: a Best Practice Guide atwww.stoneroof.org.uk/best.html

5 Inspection andmaintenance

5.1 Planned and regular inspection andmaintenance are the best ways of ensuringthe continued satisfactory performance ofa roof. Inspections, which can be carried

out annually by the building owner and(for example) quinquennially by anarchitect or building surveyor, will enableminor problems to be identified and dealtwith before they become serious.Inspections should include an internal andexternal examination, checking for slippedslates, loose ridges, and defectiverainwater goods including flashings andgutters. Signs of decay may be indicatedinternally by moisture staining of the rooftimbers, ceilings and walls, presence ofmould or fungal growth, and by timberdecay and loss of torching. Leaks can be asymptom of more serious design orconstruction problems, so it may benecessary to inspect the overall structure.If, for example, the lap of the slates isinadequate, or the pitch of a roof is tooshallow, wind-driven rain will penetrate.

5.2 Regular maintenance is part of theday-to-day responsibility of all owners andoccupiers, and a well maintained stoneslate roof can last for many generations.Excessive plant growth may affect theperformance of a roof but some raremosses live on stone roofs and may beprotected. Before removing a mossbuilding owners should check its statuswith the local nature conservationorganisation. With care it is possible toremove unprotected plants withoutdamaging a protected species. Ensuringthat the roof is free of excess moss (whichcan retain moisture and acceleratedelamination) and creepers such as ivy(which can dislodge slates) will save onfuture repair bills. Leaves, accumulatedsilt, and debris should be regularlyremoved from the valleys, gutters, gullies,rainwater pipes, drains, and channels.

6 Investigation and recording

6.1 If the roof covering requires repair,specialist advice from someone familiarwith stone slate roofing in the area (eitheran experienced architect/surveyor, or aspecialist stone slating consultant orcontractor) should be sought to ensurethat the causes of failures are correctlyidentified. It is particularly important todistinguish between leaks due toinadequate construction (gaps etc) andlong-term deterioration of, for example,slates, nails or pegs.

6.2 The roof coverings should bereinstated in their existing form exceptwhere inappropriate and unsympatheticearlier changes have occurred, or where

there are known technical defects in theexisting details. Thus it is essential torecord the details of the roof coveringsand the supporting structure, both beforeit is disturbed and during stripping. Thematerials or products, their dimensions,shape, composition, colour, and textureare all important, as is the way they havebeen assembled. The checklists inAppendices 1 and 2 should be completedas far as is possible and appropriate, andshould be supported with photographsand drawings. When photographing slatingalways try to include views straight on tothe surface, including a scale to provide arecord of the size mix (markings on tapemeasures rarely show up on photographs:a metre length subdivided into 25mmbars, painted alternately black and white,is suitable for most purposes). If acomprehensive investigation cannot beundertaken before the work begins, thechecklists should be used as strippingtakes place and the reslating methodsreviewed as relevant.

6.3 The type and size of the variousmaterials should be recorded as well astheir condition and, if they havedeteriorated, whether they need to bereplaced or can be repaired. The conditionof the roof timbers and the slates will bedifficult to assess until the roof has beenstripped and the slates can be examinedon the ground or on a wide, load-bearingscaffold.

6.4 It is important to record as muchinformation as possible about thematerials and the detailing of the roof. Forthe stone slates, information about theircolour, texture, edge treatments (square orbevelled, dressed or sawn), and sizes mayall be helpful in identifying their origin.Often building owners or local slaters willbe able to identify the type of slates andeven the source quarry. It is also useful torecord any vegetation growing on slates,its type (lichen, moss etc), and its colour –which may be indicative of the slates’mineral composition and origin (Fig 16).

6.5 A careful distinction needs to bemade between apparent defects andoriginal subtleties of the construction thatcontribute to the character and technicalperformance of the roof. For example, it iseasy to assume that the roof has sagged ormoved. Apart from the visual implicationsof correcting this (the charm of manystone roofs is their unevenness), they mayoriginally have been constructed as they

6

08268 26/4/05 10:33 pm Page 6

appear today. If, for example, the raftersare installed with a slight concavity it aidsthe laying of the slates. Elsewhere, roofswith gable copings may have beenconstructed with the rafters at a slightlysteeper pitch than the copings to allow forthe reducing thickness of the slates up theslope, thus making the roof surface andthe copings parallel. In some regions itwas common practice to upturn eachcourse of slates slightly at the abutment inorder to direct water away from thisvulnerable junction. This was often doneby raising the end rafter and usingnarrower slates to accommodate the tilt.

6.6 Where there is a need to record theconstruction of the existing roof theimportant dimensions are the battengauge (see Fig 9) and the slate length(below the fixing hole) at each batten.The headlaps at each slate length and theminimum sidelap should also be recorded,although because of movement in the roofthese may not be as originally installed. Ifthese measurements are to be useful, arecord should also be made and related tothe above measurements, of any waterpenetration or timber rotting.

6.7 The interpretation of theconstructional details of stone roofs is acomplex exercise which is best carried outby specialists with an understanding ofhow roof slating works and experience oflocal variations.

6.8 While the appearance of a roof maybe of interest, photographs of the exteriorwill be most unlikely to supply usefulinformation about the way the roof wasslated.

6.9 Bats Before the roof covering isdisturbed, the roof should be inspectedinternally for evidence of bats. If there isany possibility that bats may be present, orany evidence of occupation is found,English Nature must be consulted beforework begins. Because bats return to thesame place year after year a bat roost isprotected even if bats are not alwayspresent. All British bats and their roostsare protected and it is illegal for anyonewithout a licence intentionally to disturb,injure or kill a wild bat or obstruct accessto any place used as a roost. Some timber-preservative chemicals are toxic to batsand must not be used.

6.10 Plants Some protected plants, forexample a moss Grimmia ovalis, grow on

stone roofs. The local nature conservationorganisation should be consulted before aroof is cleaned.

7 The repair and reroofing ofstone slate roofs

7.1 Materials for repair andreroofing

7.1.1 Stone slates are reusable and canlast for hundreds of years; a stone slateroof, providing it is well maintained, canlast for at least a century and possiblymuch longer. The need to repair a roofwill most frequently arise from thedeterioration of the nails or pegs used tofix the slates to the laths or battens, thedecay of the laths or battens or of therafters, or the delamination or cracking ofthe slates, especially at their top edges.Sometimes the problems will have beencompounded by the misguided applicationof bitumen or mortar patches or coveringto the entire roof.

7.1.2 In order to retain historic integritywithout compromising performance, asmuch as possible of the original roofcovering and supporting structure shouldbe retained during repair and reroofing. Ifthe defects are extensive enough, reroofingmay be necessary and Listed BuildingConsent may be required (see 4 Planningand historic buildings legislation). It isalmost impossible to determine theprecise cost of reroofing until the roof hasbeen stripped, the materials examinedmore closely, and the extent of damage tothe roof timbers and the condition of thestone slates determined. However, anexperienced stone slater should be able togive a reasonable cost estimate beforestripping the roof although it should berecognised that it may need to be revisedwhen the roof has been opened up.

7.1.3 The introduction of man-maderoofing products, new performancecriteria, and the consequent changes inroof detailing have resulted in astandardisation of roof construction. Ifunsatisfactory results or even roof failuresare to be avoided, the adaptation oftraditional details to accommodatemodern materials must be carefullythought through – will the felt be piercedby the pegs, will the introduction ofinsulation and ventilation affect the lie ofthe slates and result in leaks? Roofinnovations such as the introduction of

insulation may well require the roofsurface to be elevated, leading to problemsat eaves, verges and abutments. A fullythought-out solution, which will probablyneed specialist advice, may lead to theconclusion that traditional details are bestfor traditional materials.

7.1.4 Stone slates7.1.4.1 Stone slates are remarkablydurable. It is usually possible to salvage alarge proportion for reuse, making themamong the most sustainable of all roofingmaterials. As long as the slates are sound,with no cracks or delamination (seeGlossary) through their whole length, theywill be suitable for reuse. Where they havesoftened or delaminated at their heads, orwhere they have been damaged along theiredges, they should be redressed, usingappropriate hand tools, rather than sawn.This means that salvaged slates are oftensmaller than the originals. Slates shouldbe discarded if delamination is extensiveand severe. Where the fixing hole hasbroken or enlarged, a new hole can bemade at the same end of the slate.

7.1.4.2 Wherever possible, new stoneslates rather than second-hand should beused. Reclaimed slates should be usedonly on the building or group of buildingsfrom which they were removed. The tradein second-hand slates inevitably leads tothe unnecessary destruction of roofs andundermines the viability of quarriesproducing new slates; it also often resultsin inappropriate regional exchanges andthe use of non-local slate. Substitutematerials, such as artificial slates made offibre resin, concrete tiles, ‘reconstitutedstone’ and so on, are inappropriatealternatives to real stone slates and are notsuitable for use on historic buildings.

7.1.4.3 New stone slates should matchthe existing ones as closely as possible interms of geological type, colour, texture,size, thickness and edge dressing. It isimportant to recognise the slatesparticular to an area and know that evenwithin the same region that a variety ofslates was sometimes used. If difficultiesare encountered in achieving a goodmatch locally, it may be acceptable tosource slates from outside the regionprovided that they are geologically andvisually similar. However, it is importantto ensure that they have suitableweathering and durability characteristics,and are finished in the local tradition.Sandstone and limestone slates and

7

08268 26/4/05 10:33 pm Page 7

ridges, including sandy limestones, shouldnever be substituted for each other, norshould they be used together.

7.1.4.4 Some manufacturers are offeringstone slates sawn to thickness. These canbe technically and aestheticallyunacceptable and, if they fail to meetthese criteria, their use on historicbuildings should be resisted. If traditionalsplit slates are available they should alwaysbe used in preference. If they are notcurrently available but might be speciallymanufactured in a traditional way, thisoption should be investigated. If fissilestone is no longer available a local andbuilding specific decision will have to bemade whether to use sawn slates, orsimilar imported slates, or some othersubstitute. If sawn slates are used theymust always be sawn along the naturallyoccurring bedding planes or they willweather differentially and may eventuallybreak.

7.1.4.5 Slates should preferably bedressed rather than sawn to size. Sawingreduces the likelihood of producing thenatural mix of random sizes, and isaesthetically unpleasing, leaving a straight,blunt edge. Sawn edges can be dressedsubsequently – albeit at an additional cost.

7.1.4.6 Slates should be reused withtheir original orientation (top face up, topedge up). This is because the undersideand the head of the slate deteriorate mostrapidly and may not be as weatherresistant as the originally exposed surface.

7.1.4.7 Laths and battens The issue oflike-for-like repair or replacement of lathsor battens will depend on the particularsituation. Wherever possible and practical,the retention of historic detail should beachieved in all elements of re-roofing.Vernacular details such as cleft laths formintrinsic parts of historic roofs and year byyear they become rarer.

7.1.4.8 Cleft laths are split fromhardwood and whilst they can be nailed torafters, they are unsuitable to receiveslating nails. Battens, on the other hand,are sawn from softwood, being nailed torafters and able to receive nails whichretain slates or tiles in place. Decisions onthe retention or replacement of existinglaths may be based on many issues:

• the age of the roof

• the importance and condition of theroof

• the listing grade of the building

• the rarity of riven lath in a particulararea

• whether or not the appearance of theunderside of the roof is important

• whether or not the laths are part of theceiling construction

• if the use of slate nails is desirable oracceptable

• the extent to which other componentsof the roof structure are authentic

• if insulation and/or an underlay are tobe incorporated

• if the roof is to have batten spaceventilation installed.

The basic principle which should beapplied is to change as little as possibleunless there are sound legal or technicalreasons for more extensive changes.

7.1.5 Ridges7.1.5.1 Ridges were traditionally madefrom stone, sawn or chiselled from ablock, and should be reused if they aresound. It is possible to obtain frommasonry quarries new ridges selected tomatch the original type, colour, andtexture of the stone. The exterior faces ofa new ridge should be dressed or tooled inthe manner traditionally employed in thelocality. In some regions clay ridge tilesare traditional.

7.1.5.2 Ridges are back-bedded inmortar so that, as far as the undulations inthe slating permit, the mortar does notshow at the long edges. They are alsohaunched at the butt joints.

7.1.6 Fixings7.1.6.1 Traditionally, stone slates werefixed with pegs hung over laths, frequentlyhead bedded in lime mortar, and theunderside was often torched. Laths weresplit from oak, fir, or more rarely sweetchestnut, and are again readily available.From the nineteenth century, sawnsoftwood battens and nails began to be

8

Bedding mortarhidden behindridge

Verticaledge

Figure 6 The edges of the ridge should be formedas shown to avoid large and unsightly mortar fillets.Similarly ridges should not be edge pointed, ratherthe bedding should be neatly struck off as workprogresses.

introduced. Modern preservative-treatedsoftwood sawn battens are morecommonly used today, and the slates arefrequently nailed rather than pegged.Whether modern methods are acceptableor not will depend on the specificconservation objectives and theimportance of the building; the localConservation Officer should be consulted(Fig 7).

7.1.6.2 Pegs were most commonly ofoak, although other woods and, in someparts of the country, animal bones wereused. New oak pegs and treated softwoodversions are available in a variety ofdimensions. Green pegs should not beused as they are likely to shrink, and slipfrom the peg holes. Pegs should beroughly square sectioned and driedbefore use so that after fixing absorbedmoisture makes them swell, locking theminto the holes. The fixing holes forwooden pegs were traditionally taperedor hourglass shaped: this also helps togrip the pegs and is still good practice.

7.1.6.3 Large headed non-corrodingmetal, preferably copper, pegs 38–63mmlong are also used to hang stone slates.Aluminium pegs should be avoidedwhere they may be in contact withalkaline materials such as limestone slatesor mortar. If the existing peg holes arelarge, pegs with large heads will lock inposition and prevent the slates slipping.Where the stone slates are to be drilledor re-drilled, for nailing or metal peghanging, the holes should be parallelsided at a slightly larger diameter thanthe peg shank and countersunk to receivethe head. Electric drills without hammeraction are suitable and each slate,properly supported, should be drilledindividually. Attempting to drill througha pile of slates will inevitably damagethem.

7.1.6.4 Nails for fixing slates should belarge-headed, large-gauge copper. Nailsfor fixing battens, laths, or boards may bestainless steel for maximum durability,provided that they are strong enough todrive into hard timber, otherwisegalvanised or sheradised nails areadequate. Length, gauge, and type shouldbe appropriate to the batten size andwind uplift requirements in the particularlocation (BS 5534, 2003).

7.1.6.5 All metal fixings should bechecked for compatibility with the

08268 26/4/05 10:34 pm Page 8

9

7i Traditional fixing method:pegs and single torching

7iv Insulation at ceiling level and verges

7v a Insulation below the rafters

7v b Insulation between the rafters

7v c Insulation above the rafter(which changes the roof line)

7ii Traditional fixing methods:pegs and full torching

7iii Nail fixed stone slates

felt

felt supported to allowventilation and to avoidponding

eaves

to rid

ge ven

tilatio

n

possible use ofproprietary eavesvents

ventilation

vapour barrier

vapour barrier

eaves

to rid

ge ven

tilatio

n

eaves

to rid

ge ven

tilatio

n path

min

50mm co

unter

batte

n

eaves

to rid

ge ven

tilatio

n path

min

50mm co

unter

batte

n

alternative positionsfor vapour barrier

stop batten to prevent insulationsliding down the roof slope underload

vapour barrier

Figure 7 Fixing methods and the use of different types of torching and roofing felts for stone slating

08268 26/4/05 10:34 pm Page 9

materials they come in contact with.Verylarge, wide slates may require double fixing.

7.1.6.6 Where methods of fixing arealtered, the appearance of the roof may beinadvertently changed. For example, thehorizontal alignment of the tails of slateswill be much more even and regular whenthe stone slates are drilled and nailed thanwhere they are pegged and hung on laths.This can be avoided by drilling the fixingholes freehand rather than in a jig oragainst a stop.

7.1.7 Torching, roofing felts, vapourpermeable membranes and vapourbarriers7.1.7.1 Pegged roofs were traditionallytorched to render them windproof.‘Mossing’ (driving moss into the joints ofthe slates with a mossing iron) wasanother means of achieving the sameresult. These methods have now beenlargely superseded by the use of roofingfelts. Torching, or tiering as it was alsoknown, was traditionally a mix of lime andaggregate (sand and/or crushed stone),usually with animal hair added, and wasapplied to the underside of the roof eitheras half or full torching. Half, or singletorching, was applied at the upper edge ofthe laths, and full torching filled the spacebetween the laths. The torching had to beapplied carefully avoiding forcing mortardown between the joints, which woulddraw in moisture. Torching also served tohold the pegs in place and stop themtilting and falling out (Fig 7i and ii).Traditional roofs and roof spaces werevery well ventilated. Any change in theconstruction, especially introduction offelts or vapour permeable membranes(VPM) with or without insulation, maywell reduce this and lead to deteriorationof the fabric. Careful consideration shouldbe given to all the aspects of suchinnovations. If the roof can be wind-proofed without the use of felt or VPMthis may well be the best option.

7.1.7.2 It is preferable to retain allhistoric details, including torching. This isstill a successful and appropriatetechnique if carried out correctly and,properly applied, allows the battens to dryout even after occasional wetting bydriving rain. A mix of 1:3 mature limeputty to aggregate, with approximately onehandful of clean hair, well teased out,added to one bucket of the mortar, andevenly mixed, will produce a satisfactorytorching mortar.

7.1.7.3 If the slates are pegged over felt,thicker battens, or counter battens of anappropriate thickness will be required toprevent the felt being pierced (Fig 7iii). Ifthe slates are pegged but not torched, large-headed metal pegs may be used to stop thepegs tilting. In recent years, doublebattening has been adopted to prevent pegstilting but for conservation purposes themortar methods are more appropriate.

7.1.7.4 Underslating felt should conformto BS 747 type 1F or 5U, and be fixed tothe rafters with large-headed clout nails,with a minimum vertical lap of 150mmbelow 35 degrees pitch and 100mm at 35degrees and above, and a horizontal lap ofat least one rafter space. It should benailed to each rafter. The use of roofingfelt is no substitute for goodworkmanship, which by itself will result ina weather-tight roof (Fig 7, iii-v).(NB BS 747 is to be withdrawn.Thespecification for 1F and 5U felts isgiven in BS 5534:2003 Annex A.)

7.1.7.5 If insulation is introduced intothe roof, increased ventilation may berequired and it may be necessary to use aVPM and a water-vapour barrier.There isa British Standard specification for VPMs– BS 4016 – and their selection and useshould be based upon calculations usingrealistic (extreme not average) parametersfor the internal and external environments.Vapour barriers may be formed by closed-cell insulation with taped joints, or by useof 500 gauge polythene or similar material.BS 5250: 2002 and BRE 2002 (seeReferences) give further guidance onperformance requirements for suchmaterials (Fig 7iv and v).

7.1.7.6 Ventilation can be introducedinto gables, at the eaves, at the ridge, orwithin the roof slope. Proprietary eavesvents are usually unobtrusive but mayrequire adjustment of the eaves tilt toensure that the slates lie properly.Proprietary ridge vents do not matchstone slates, thus the use of gableventilation is the preferred solution wherethis is an option. Where clay ridges aretraditional vent ridges may be acceptable.Individually designed ridge ventilationsolutions such as open joints, raising ofridges and so on, should be designed withcare, as they may not provide adequateventilation and may leak. There arecurrently no slate-vents suitable for stoneroofs, so in-slope ventilation is not apractical option.

7.1.8 Bedding and pointing7.1.8.1 Ideally, mortar (other thantorching) should not be used in stoneslating, as there is always a danger ofdrawing moisture between the stones andinto the roof. However, the use of mortarbedding and pointing seems to beregionally specific and local practiceshould prevail. It is preferable and cheaperto increase the lap rather than to rely onthese methods to make a roof water-tight,and the use of mortar for bedding andpointing should be carefully considered.Spot bedding in lime mortar is used insome parts of the country, but only as ameans of helping the stone slates to lie flatwithout rocking. This is also a methodsometimes employed in minor repairs toisolated patches on roofs. It should beused very sparingly and the spots ofmortar kept away from the tails of theslates and the perpendicular joints toavoid drawing moisture into the roof. Insome very exposed areas, the heads ofslates are bedded in a thin band of limemortar.

7.1.8.2 Pointing applied at the tail of theslate and at the perpendicular joint is amethod employed in some regions andcan be carefully reinstated where itrepresents traditional practice. As withspot bedding, it has the potential to drawmoisture into the roof structure. It alsodries out very slowly after rain, with theconsequence that the roof structure canbe wet for long periods. Furthermore,pointing can exacerbate frost damage tothe backs of the slates. In some regions,pointing is held back from the edges ofthe slates, or raked out, so it is not visible;elsewhere, it is exposed – this is visuallyundesirable. If it is applied, pointing mustpenetrate upwards or sideways betweenthe slates as little as possible to minimisethe extent of resultant wetting. If the roofis laid to the correct pitch and sufficienthead and side laps are applied, pointingwill not be necessary to keep the roofweather tight.

7.1.8.3 Pointing and bedding shouldnever be combined with torching. It isalmost impossible to prevent the twocoming into contact and forming a wickwhich continuously draws in water. Thiscan result in total failure of a roof within25 years of reslating.

7.1.8.4 Where bedding and pointing areused careful consideration should be givento the use of underslating felt. Where they

10

08268 26/4/05 10:34 pm Page 10

are used together the humidity in thebatten space may be high for extendedperiods resulting in the deterioration ofthe slates and battens.

7.1.8.5 If the water vapour passingthrough VPMs is not vented from theirupper surface they will not be effective.For this reason they should never beused with bedding and pointing unlesseffective through-ventilation of thebatten space is provided by some othermeans.

7.1.9 Roof timbers7.1.9.1 To avoid unnecessary loss ofexisting material, timbers should becleaned down without defrassing (seeGlossary) and inspected for signs ofactive decay or infestation. Replacementof roofing timbers should be restricted to those which threaten the structuralsoundness of the roof. Repairs to rooftimbers should be carried out as far aspossible in the same timber, usingappropriate repair methods whichminimise loss of existing timbers, and do not regularise the appearance of theroof.

7.1.9.2 Where alterations to the roofstructure of a listed building arenecessary, such as the replacement ofrafters, purlins, or valley boards, ListedBuilding Consent may be required.Advice from the local planning authorityshould be sought before any timbers areremoved.

7.1.9.3 New timber should be dried to amoisture content of between 15 and 20per cent and should match the existing insize and species. Softwoods should bepressure-impregnated with preservativebefore being brought to the site and anycut ends or bored holes should be treatedwith insecticide/fungicide before beingused.

7.1.9.4 When timber treatment isconsidered necessary and appropriate, workshould be confined to areas obviouslyaffected by active insect infestation or fungaldecay. Remedial work should be carried outwith reference to current legislationincluding the Control of SubstancesHazardous to Health Regulations 1998,TheHealth and Safety at Work etc Act 1974, andthe Control of Pesticides Regulations 1986. Ifbats are present, advice should be sought onthe suitability of treatments so as not tocause them harm (see 6.9 Bats).

7.2 Construction andworkmanship7.2.1 Specification7.2.1.1 The specification is relied on bythe roofer to provide a full description ofthe works and the quality of the materialsand workmanship required for a job. It isalso likely to become part of any contractdocumentation. Estimates will beprepared on the basis of the specificationand thus it needs to be clear andcomprehensive, ensuring that anythingwhich is likely to have a cost implicationon the work is included. A thoroughspecification will enable a fairer and moreaccurate cost estimate to be prepared bythe roofers pricing the works.

7.2.1.2 It is outside the scope of thisleaflet to provide a model specification,which needs to reflect local roofingtraditions and practice. Some localauthorities provide guidance on suitablespecification clauses. As a minimum thespecification should cover the following

• scaffolding

• security (stone slates are valuable items)

• stripping, stacking, sorting andredressing (see 7.1.4 and 7.2.2)

• timber and carpentry (see 7.1.9)

• roofing felt/torching (see 7.1.7)

• insulation and ventilation (if applicable– see 7.1.7.5–6 and 7.2.5)

• slates, battens and fixings (see 7.1.4 and7.1.6)

• slating (see 7.2.3)

• valleys (if applicable – see 7.2.4.6)

• ridges, abutments, hips and verges (see7.2.4.6–9)

Where new stone slates are needed, thespecification should include source(geological/geographical/delph), sizes,thickness, edge dressing style and, ifrequired, surface dressing.

7.2.2 Stripping, sizing and stacking7.2.2.1 Once the roof covering isrecorded as far as is possible, the strippingof the roof can begin, starting at the ridge.The slates should be carefully removed,cleaned, inspected for damage anddelamination, and then carefully stacked.Damaged slates should be put on one sidefor redressing. Stone slates should bestacked on end, in piles sorted by length(measured from the fixing hole to thetail). They should NOT be stacked flat.Once sorting to length is completed,sorting to thickness can follow if this is thelocal practice (Fig 8). It is not possible tostack, inspect, and sort stone slate at rooflevel unless a large working platform isprovided. Such practice is unsafe on allbut the largest roof platforms, and doesnot encourage the rigorous standards ofinspection and sorting required. Stoneslates are valuable and will need to bestored in a secure place.

7.2.3 Reroofing7.2.3.1 Cover The sorting of the slateswill identify the amount of existing slatewhich is reusable. Sometimes, there willbe sufficient to re-cover the roof withsome patching-in of replacements. Lowerrates of recovery will necessitate theconsolidation of the surviving slates on asingle roof slope, with new slate requiredto re-cover the remaining slopes. Either

11

Figure 8 Stone slates removed from the roof and sorted according to size.The roofer will now be ableto calculate the amount of new material required for reroofing.

08268 26/4/05 10:34 pm Page 11

way, it will now be possible to calculatethe total width of cover available for eachlength of slate, and the quantity of newslate that will be required. The number ofcourses for each slate length and thebatten or lath gauging can also becalculated. This will be dependent on thesize of the available slates and should beadjusted as their length diminishes up theroof to ensure that the minimum head lapis achieved for each course (Figs 9 and10). The risk of unbalancing the loadingon the roof should be considered wherestone slate is used on one pitch and lighterslates or tiles on the opposite pitch.

7.2.3.2 Slating gauge Reslating shouldalways be carried out by measuring howmany courses of each slate length can beachieved with the recovered and newslates together. It is inadvisable to try to

reproduce slavishly an existing pattern ofmargins. This is not the correct traditionalmethod, is unnecessarily expensive andrisks the use of inadequate headlaps. Thepreferable method is to specify the largestand smallest sizes of slate and areasonably even gradation of sizes betweenthem. The specific gradation of themargins will then be the natural outcomeof the mix of sizes and the correctheadlap. In practice new slates in thelargest sizes specified may simply not beavailable due to the limitations of therock.

7.2.3.3 Laying Stone slates should belaid one course or a few courses at a time,depending on local practice, working fromthicker to thinner slates across the roof ifthis is the local tradition. The slates shouldbe laid to avoid rocking on uneven

surfaces, and to avoid variations inthickness from one slate to the next. Ifbedding and/or pointing is to be carriedout, this should be applied as workprogresses, using the minimum mortarrequired.

7.2.3.4 Backers Wider slates shouldgenerally be reserved for use with backers,which are narrow slates placed roughlycentrally over wide slates to makeprovision for the larger number of slates inthe upper courses (Fig 9).

7.2.3.5 Wide slates should also bereserved to close the half bond at verges,hips and abutments.

7.2.3.6 Underslating Historically, stoneroofs were laid without underfelt ormembranes, although they would

12

Figure 9 Measuring slates.The length is measured from the fixing hole to the tail.The head lap is the amount by which a stone slate overlaps the fixing hole ofthe slate in the next course but one below.

head

fixing hole

laths or battens

reduced batten gauge at firstshorter course

batten gauge (variable)

head lap

margin

side lap

backer

effectiveslate length

tail

08268 26/4/05 10:34 pm Page 12

generally have been torched to make themdraughtproof. Where torching is an optionor draughtproofing is not required, theremay be no need to include a felt ormembrane. If a modern specification is tobe adopted, the underslating system mightinclude a vapour barrier, insulation, androofing felt (see 7.2.5 Insulation andventilation). Ideally, the laths or battensshould be fixed in place as the felt is laid(Fig 7).

7.2.3.7 Weather tightness Waterpenetration is resisted by ensuring anadequate pitch, head and side lap and,most significantly, making certain that thelargest slates (which provide the largestside lap) are placed at the bottom of theroof. It is important for the weathertightness of the roof to ensure that thestone slates are properly sorted, selected,and laid, and sit together as well aspossible.

7.2.3.8 Pitch It is for good technicalreasons that stone slates are seldom laid atso low a pitch that they cannot be seen

from ground level. The temptation to alterthe pitch of a roof should always beresisted: traditional pitches may haveevolved in response to regional and verylocalised conditions. They may also differfor slates of differing roughness or sizes,even from a single quarry (Fig 19).

7.2.3.9 Head lap There are no setguidelines for determining the minimumhead lap of stone slates; it seems to varyfrom area to area and relates to the pitchof the roof. Head lap (the amount thethird course overlaps the first, the fourthoverlaps the second, etc) can be specifiedas a value or as a fraction of the slatelength. In either case it should not deviatefrom established local practice (unless thisis known to be inadequate). Lapmeasurements are taken from the pegholerather than the top of the slate. As thestone slates are laid in diminishingcourses, it is important to ensure that theminimum lap is maintained as the slatelength changes up to the roof (Fig 9).Failure to do so will lead to insufficientheadlap indicated by pigs, courses where

the margin is bigger than courses lowerdown (Fig 15). The head lap should notexceed one third of the slate length.

7.2.3.10 Side lap The minimum side lapis commonly not specified. Frequently,local rules of thumb are applied, such as‘not less than 75mm’ or ‘not less than thehead lap’. Genuine local methods knownto be successful should be respected.

7.2.3.11 Gallets or shales In someregions where stone slates are thin, theyare supported with small pieces of stoneslate or metamorphic slate, known asgallets or shales, which are bedded in asmall amount of lime mortar at the headof the course below. The techniqueprevents the slates breaking if, forexample, they are loaded by a roof ladder:it is mainly required at the eaves wherethe tilt produces a gap between successivecourses. Gallets must be used carefully toavoid forming a path which would drawmoisture into the roof. In some limestoneslating traditions similar small pieces ofstone or metamorphic slate known as

13

Figure 10 Section through a typical sandstone slate roof showing structure, diminishing courses, ridge and eaves details.The inset shows a typical Cotswoldcussome detail.

08268 26/4/05 10:34 pm Page 13

shadows or shales are used to improve theweather tightness where the slate'sshoulder is not quite good enough.Usually they are bedded in lime mortarand act by blocking the wind fromblowing through the slating and carryingin water. The way they are laid also turnsany water which reaches them down andout of the slating. This is acceptabletraditional practice although it is better toavoid the need for them by carefulselection of slates of suitable shape. Today,it is also acceptable to use lead soakers forthe same purpose.

7.2.3.12 Eaves slates The under eavesand eaves slates should be laid to projectover the wall and discharge water awayfrom the building, or into the gutter whereone is to be fitted. Local details should beapplied (Fig 10). Gutters were not alwaysused for stone slate roofing.

7.2.4 Detailing7.2.4.1 The detailing of a roof – its pitch,and the treatment of the eaves, valleys,ridges, and dormers – was evolved by localcraftspeople in response to the local stoneand regional climate, and may varyaccording to the building type and period.A roof may exhibit a number of stages of abuilding’s development and incorporate anumber of different details which may beequally important.

7.2.4.2 The existing features of an oldroof, such as the mix of slate sizes withinthe limits described in 7.2.3.2 (which willdetermine the number of courses), verge,

ridge, hip, and valley details should beretained wherever possible. Newtechniques should not be applied withouta careful assessment of their technicalimplications, and the adaptation oftraditional methods to modern practiceshould be treated with caution. Correctdetailing not only ensures that thecharacter of the roof is retained but alsothat it fulfils its function.

7.2.4.3 To ensure the roof waswatertight, the valley, hip, dormer, and

abutment details evolved when lead wasunavailable (or too expensive to be used).These details also influence the pitch ofthe roof. In regions where slates are smalland thin, the roof pitch will be generallysteep (about 50 degrees), enabling curves(such as the swept valleys of theCotswolds) to be formed at theintersection of the roof planes. Wherestone slates are very large and thick, suchas the sandstone slates of the Pennines,the pitches are lower and the roof formsare simpler, with the valleys formed withchevron slates (Figs 11 and 12).

7.2.4.4 The decision to restore originaldetails, such as swept or chevron valleys,which have been replaced by leadwork inthe recent past, should be based uponsound evidence for their former existencerather than upon speculation. Changingthe details of the roof may require ListedBuilding Consent, and advice should besought from the local planning authority.

7.2.4.5 A roof must be capable ofwithstanding wind forces and preventingthe penetration of driving rain, so thefixings must be strong and secure enoughto hold the slates in place withoutslipping. Different parts of the countryhave achieved these objectives by evolvingvarious methods, some of which may bequite localised and unlike those applied tosimilar materials in other regions. Theymay also conflict with therecommendations of the British Standard(BS 5534 2003 does not deal with stoneslate roofing specifically but is theaccepted standard usually referred to).However, many of these traditional detailshave a proven track record and should beconserved where it is practical to do so.Care must be taken when replicatingtraditional details to implement themcorrectly. Modification of traditionaldetails can be problematic: confusion andpoor amalgamations of traditional andmodern methods have resulted in failures(Fig 17).

7.2.4.6 Valleys and hips should be formedas work progresses, using the appropriateweathering technique. Extra fixings maybe required for hips, which should beformed with wide slates. Some valleys aremade without fixings. Solid bedding ofvalleys should be avoided. Sometraditional valley detailing requires thesetting of the valley board at anappropriate level and angle by the slater,in anticipation of the need to align the

14

Figure 11 The limestone slates of the Cotswolds, which are lighter and smaller than most sandstoneslates, require a steep pitch and can accommodate curves in swept valleys.

Figure 12 The heavy, flat sandstone flags of theSouth Pennines are suitable for lower pitches, andthe detailing of the roof is simpler toaccommodate their large dimensions.The chevronvalley is an example of the craftsperson’s skill informing a weather-tight roof.

08268 26/4/05 10:34 pm Page 14

eaves levels of adjacent roof slopes withthe valley slate (Fig 18).

7.2.4.7 Abutments should be formed withlead soakers and cover flashings or filletsto the specified detail. Fillets forabutments to uncoursed stonework andrender should be formed with a suitablemortar which may require a stainless steelreinforcement mesh. Back abutments,such as at chimneys, should alwaysinclude an adequate lead gutter andupstand, and be carried up under theslating. At the intersection of dormervalleys and the upper slope, it istraditional in some regions to use aninverted ridge piece to weather the joint. Ifthis detail is adopted it may need to bebacked up with a discreet lead soaker. Analternative detail carries the dormer ridgeup into the slating of the main slope.

7.2.4.8 Verges should be bedded and thebedding struck off as work progresses,rather than pointed up after completion.Where verges are formed without beddingor an undercloak, they may be pointed inand a fillet formed under the slates afterslating is complete.

7.2.4.9 Ridges should be back bedded inmortar and solid bedded at the joints. Toavoid the need for unsightly pointingalong the ridge it is preferable if ridgestones have a vertical edge at the bottomof the wing. Mortar should not be visibleand the wings should not be pointedexcept where there are large undulationsin the roof surface. Gallets can beintroduced under the ridge joints and overthe vertical joints in the slating where theycoincide (Fig 10).

7.2.4.10 If torching is specified, it shouldbe carried out after slating is completed toavoid damage.

7.2.4.11 The use of large stone slates aseaves courses below thatch, tiles, andmetamorphic slates may be an originaldetail or may be a solution to the shortageof stone slates during the last reroofing. Ineither case, this is now regarded as atraditional detail in many parts of thecountry and should be reinstated as found(Fig 13).

7.2.5 Insulation and ventilation7.2.5.1 Wherever an old roof is altered ina way that reduces airflow through orbeneath it, careful consideration should begiven to the effect on the ventilation of the

roof space. This is especially important ifroofing felt is used. Failure to provide analternative ventilation route can result incondensation and extensive wetting of thefabric, or at least a raised moisturecontent, with an increased risk of mouldformation and deterioration.

7.2.5.2 If insulation is to be introducedinto a roof space, a specialist shouldcarefully consider the implications,

especially the need for ventilation.Insulation can be positioned above orbelow the ceiling, or between, below, or ontop of the rafters (Fig 7iv and v). The firsttwo options are relatively straightforwardand ventilation can be provided fairlysimply. However, when insulation followsthe rafter line, the situation is morecomplex and the success of any methodrelies on good workmanship. Whereinsulation is laid on top of the rafters, and

15

Figure 13 Eaves slating is a traditional roofing practice in some parts of the country. In some areas stoneslate has been gradually replaced by other roofing materials such as metamorphic slate, and the stoneslates at the eaves are a remnant of an earlier roofing material.The example shown here is in the village ofMontacute, where the local Ham Hill stone slates have become increasingly scarce.

08268 26/4/05 10:34 pm Page 15

there is a change in the roof line, ListedBuilding Consent may be required. Inmost cases when insulation is introducedinto the roof space, it will be necessary tointroduce a vapour barrier. It must becomplete, continuous, and sealed at thejoints. This may be difficult or impossibleto achieve in practice and this fact shouldbe taken into account when the ventilationsystem is being designed. BS 5250provides design guidance.

7.2.5.3 Where proprietary ventilationproducts are introduced, it is important tocheck with the manufacturer that they aresuitable for the specific situation.Particular attention should be paid toensure that adequate laps are achieved.Individually designed solutions should becarefully considered to ensure thatadequate ventilation is provided. Iftorching or bedding is employed on aroof, care must be taken not to block thevents or the flow of air under the eaves.

7.3 Minor and localised repairs

7.3.1 It is preferable to repair small areasof deteriorated stone slate roofing ratherthan to strip the whole roof. It may bepossible to reinstate missing or loose slatesand refix them by swinging the adjacentstone slates aside if they are pegged: nailedslates will break. Before a stone slate isreused, it should be examined to ensurethat it is suitable for refixing. If the stoneslates are pegged and torched and the

underside of the roof is accessible, missingtorching should also be replaced to helphold the peg in place. If the underside isnot accessible and the laths or battens arein reasonable condition, it is preferable tonail fix the replacement slates.Replacement slates should be the samelength as the originals and aligned withthe adjacent slates so as to retain the headlap and ensure that the repair is watertight(Fig 14). If the new fixing hole would belower than originally it should be drilledaway from the perpendicular joint of theoverlying slates so as to increase theeffective head lap.

7.3.2 Where supplies of replacement slatesof the same size and thickness areunavailable, it may be impossible to carryout patch repairs. In this case, consolidatingthe salvageable stone slates on one part ofthe building may be a solution.

7.3.3 Where the number of slates to bereplaced is extensive, consideration shouldbe given to renewing whole areas or tocomplete stripping and reslating. Largenumbers, typically five per cent, ofdisplaced or damaged slates probablyindicate that the roof is nearing the end ofits life. If large areas are to be patched, thecoursing must match the remainder of theroof, and the head and side laps must bethe same or larger.

7.3.4 Bitumen-coated fabric applied overroof coverings, or spray-on coatings to

their undersides, should be avoided asthey prevent the salvage of stone slates orreuse and make it difficult to locatedefects. These treatments also serve to sealroof voids, preventing ventilation andleading to fungal attack and infestation ofroof timbers.

7.4 Choosing a slater

7.4.1 Stone slating is a specialist activityand not all roofing contractors are eitherfamiliar with the particular methodsinvolved or skilled in the local roofingtraditions. An assessment and registrationscheme for stone slaters is underdevelopment by the National Federationof Roofing Contractors. It is not alwayssafe practice to import a slate fromoutside the region, and equally it may notbe wise to import a contractor who is notexperienced in the local stone slateroofing techniques. In many areas of thecountry, the scarcity of stone slates andthe decline in their use has resulted in aloss of traditional skills, and care shouldbe exercised when selecting suitablecontractors. They should be familiar withstone slate roofing in the region and beprepared to supply photographs of workthey have carried out. Better still, theyshould provide the opportunity to visitprevious work, in order to demonstratetheir competence. Inspection of a roofer’swork should include a check that thestone slates are laid evenly and sit wellacross the roof, and that the sizes of the

16

Figure 14 Fixings for replacement slates as part of a localised repair must be aligned with the adjacent slates. If the original slates are pegged but the replacementslate is nailed, care must be taken not to compromise the minimum head lap by placing the nail too far below the pegs.

upper courses omitted for clarity

headlap over peg headlap over nail

replacement slate

slates swung aside to fixreplacement

08268 26/4/05 10:34 pm Page 16

subcontract work are at risk of having touse unsuitable or inexperienced roofers.It is equally crucial not to be swayed byprice; to be wary of significantly lowertenders; to check that nothing has beenmissed; to ensure that all the quotationsare based on the same specification; andto feel confident that the job will beundertaken to a high standard.

8 Costs

8.1 Life-cycle costs

8.1.1 Stone slates form a high quality,very durable and beautiful roof covering.They satisfy the aims of sustainabilitybetter than any other roof although, in theshort term, they are also the mostexpensive to purchase and install.However, stone slates can prove aneconomic choice when account is taken ofthe long life cycle of the roof, and theeven longer life cycle of the slates (a highproportion of which will be capable ofreuse when next reslating). Where stoneroofs are properly maintained, totalreroofing will rarely be required.

8.1.2 Stone slates are essentially hand-crafted materials and it is difficult to

reduce production costs significantly.Reusing the salvageable material fromthe same roof will reduce the materialcosts of the roof. However, new slatesshould be used to make up shortfalls, tohelp encourage the market for new stoneslates and secure the future for stoneslate supplies.

8.2 Grants

8.2.1 Grants for repairs to stone slateroofs of Grade I and II* listed buildingsand scheduled monuments are availablefrom English Heritage. Many localauthorities also operate grant schemeswhich include Grade II listed buildingsand occasionally unlisted buildings inconservation areas. The Heritage LotteryFund and English Heritage offer grantsfor ecclesiastical buildings through theJoint Grant Scheme for Churches andother Places of Worship which isadministered by English Heritage. TheHeritage Lottery Fund also providesgrants to registered charities, voluntaryand public organisations, and to someprivate building owners.

8.2.2 In some areas, theEnvironmentally Sensitive AreasScheme offers grants to farmers andland managers for the repair oftraditional buildings, including stoneslate roofing.

8.2.3 For a comprehensive guide toGovernment, private sectororganisations and charitable trustswhich offer grant aid towards the repairof historic buildings, seeFunds for historic buildings in England andWales: this guide is available on theArchitectural Heritage Fund (AHF)website, free and regularly updated, atwww.ffhb.org.uk

17

Figure 15 ‘Pigs’ (where a long margin is visible above a short one) indicate a failure to maintain adequatehead lap and are evidence of poor workmanship, as this roof in West Yorkshire shows.

slates diminish regularly from the eaves tothe ridge: successive margins should beeither the same size or smaller than thosebelow – there should not be equalnumbers of courses of each margin size(Fig 9). Long courses above shortcourses, known as pigs, indicate that thegauge and head lap have not beenproperly adjusted at changes of slatelength (Fig 15). It is sometimes possibleto check the head lap at the verges,provided that the slates are notshouldered. A competent stone slatingcontractor should be able to provide adetailed method statement thatdemonstrates understanding andcompetence in the historical and technicalmethods of the area. This is a usefulmeans of assessing tender submissions.

7.4.2 The local authority’s conservationofficer may be able to provide a list ofspecialist roofing contractors in the area.It is essential to remember that it is theindividuals involved in a job, notnecessarily the firm, who are important, soa check should be made to ensure that itis the team whose work has beenexamined (and has proved itscompetence) which undertakes the newcontract. Roofing contractors who do nothave their own retained slaters but

08268 1/6/05 4:38 am Page 17

18

Ridges Fixing: bedded or nailed, etc

Pitch

Slating Head lap

Number of courses

Margin for each course

Batten/lath gauges

Side lap: minimum and range

Gallets

Single lap

Shadow slates

Dormers Pitched: mono or duo

Hipped

Cheeks: slated or other

Junction weathering

Abutments Weathering: mortar or metal

Gables Plain or coping

Fixing for coping

Flush, overhang or oversail

Hips Mitred

Covered: stone, tile, lead, etc

Detail: roll top, etc

Valleys Open

Mitred

Chevron

Swept

Laced

Other

Eaves Plain or sprocket

Slating detail

Rafter centres Range or average

Fixing system Battens or laths

Slates fixed to rafters

Pegs or nails

Torching Full, partial or none

Slating felt Laps: head (up slope)

Laps: side (across slope)

Appendix 1 Stone slate roofs: checklist for recording construction details

08268 26/4/05 10:34 pm Page 18

19

Ridge Material

Dimensions

Shape

Copings Material

Dimensions

Shape

Slates Limestone

Sandstone/Gritstone

Other

Composition

Dimensions: length range x random

Thickness: average or range

Gallet or shadow and material

Battens/laths Split

Sawn

Species

Dimensions

Fixings

Slate fixings Pegs: sawn or split

wood: species

other: bone, etc

dimensions

Nails: hand made

machine made

wire

dimensions

material and composition

Mortar Bedding

Pointing

Flashings and soakers Material

Dimensions

Torching Composition

Slating felt Composition

Appendix 2 Stone slate roofs: checklist for recording materials

08268 26/4/05 10:34 pm Page 19

Glossary

Backer narrow slates laid centrally over awide slate to accommodate thediminishing size and therefore increasingnumber of slates in subsequent courses aswork progresses up the roof (Fig 9)

Batten sawn wooden support forhanging or nailing stone slates. Synonym:lath. NB: in this technical advice note, theword lath is reserved for cleft supports(Fig 7).

Bedding use of mortar in spots or filletsto prevent stone slates from rocking. Insome areas, it is used to improve weathertightness.

Cussome see Eaves

Defrassing the removal of insectattacked sapwood from timber. Thematerial subjected to insect attack istermed frass.

Delamination splitting of stone slatesalong the bedding planes, most often seenat the heads of slates. Delaminationshould not be confused with surfaceflaking.

Diminishing the system whereby slatesare sorted by length and laid with thelongest at the eaves diminishing to thesmallest at the ridge. It is essential that theminimum head lap is maintained whenthere is a change of slate length betweentwo courses. This also ensures that eachsuccessive margin is the same size as orsmaller than those below (Fig 9).

Double lap stone slates laid so that eachcourse overlaps the course next but onebelow (Fig 9)

Dressing the process of shaping thestone slate and producing the edge detailusing either a chisel edged hammer or abladed tool. Regional differences exist forthe edge detail which may be square orbevelled. Synonyms: trimming, fettling(Yorks, Lancs)

Eaves of stone slates: the short course laidat the eaves under the first full course.The method of placing and supporting theeaves stone slates varies regionally.Synonyms: under eave(s), cussome(Cotswolds)

Fixings nails or pegs

Gallet a small piece of stone slate ormetamorphic slate bedded in lime mortarat the head of a slate to support the slateabove. Synonym: shale

Gauge the spacing of laths or battens upthe roof slope measured from the topedges of successive laths or battens. Instone slating, the gauge is always variable(Fig 9).

Head the top edge of a stone slate as laid(Fig 9)

Head lap the amount by which a stoneslate vertically overlaps the stone slate inthe course next but one below (Fig 9)

Lath cleft wooden support for hangingstone slates (Fig 7). Synonym: batten. Inthis technical advice note, the word batten isreserved for sawn supports.

Margin strictly the area, but morecommonly the length, of the exposed partof the slate

Mossing use of moss or other vegetablematerial to windproof the joints and gapsbetween stone slates

Overburden in quarrying: uselessmaterial which overlies a bed of usefulmaterial

Perp joint Perpendicular joint: thevertical joint between adjacent slates in acourse

Pig a course with a larger margin thanthe course(s) below resulting from poorsetting out and a failure to maintainadequate head lap (Fig 15)

Pitch the angle of the rafters to thehorizontal. The pitch of the stone slateswill be significantly less because they areresting on each other, but this is takeninto account by the traditional rafter pitchand lap relationship for the slate and thelocality.

Pointing use of mortar to fill the verticaljoints and to seal the tail gap of stoneslating. Pointing may show (undesirable)or be raked or held back. Often associatedwith bedding.

Randomof stone slate: variable length and width of roofing: slates laid with reducing lengthup the roof slope and the widths selected

and placed so that they provide at leastthe minimum side lap over the slates in thecourse below (Fig 9)

Regularly of diminishing slating: thesystem whereby each successive margin isthe same size or smaller than those below.It does not mean that there is an equalnumber of courses of each margin size(Fig 9).

Shadow a thin piece of slate used in theHorsham district to improve the weatherresistance of a roof when, because of ashortage of stone slate, the head lap isreduced to less than the normalminimum. Originally the shadow was athin piece of Horsham stone but it is nownormally a metamorphic slate. It wasalways used in conjunction with mortarbedding and pointing. Technically this isan undesirable method; however, it hasbeen in use for about 100 years andappears to work in many cases.

Shale see Gallet

Shoulder (verb) to remove the topcorners of a stone slate; (noun) the topcorners of stone slates. Excessiveshouldering can result in a leaking roof.

Side lap the amount by which a stoneslate laterally overlaps the stone slate inthe course below (Fig 9)

Tail the bottom edge of a stone slate aslaid (Fig 9)

Tiering see torching

Tilt the lift provided to the eaves courseto ensure that successive courses liecorrectly without gaps at the tail. On themain areas of the roof slope, the tail ofeach stone slate rests on two thicknessesof stone slate. At the eaves, the first fullcourse rests on only one thickness - theeaves slate. Essentially, the tilt replaces themissing thickness.

Torching lime and hair mortar appliedto the undersides of stone slates to renderthem windproof. Synonym: tiering

half torching/single torching: application oflime and hair mortar between the topedge of the lath or batten and theundersides of the slates (Fig 7i)

full torching: application of lime and hairmortar between the top and bottom edges

20

08268 26/4/05 10:34 pm Page 20

of the laths or battens and the undersidesof the slates (Fig 7ii)

Unweathered of stone roofing: rock whichis too deep to have been subjected toweathering and consequently has to besplit by mechanical action or frosting afterextraction

Weathering the process by which rocksare broken down and decomposed by theaction of external agencies such as wind,rain, temperature changes, plants andbacteria. In the development of weatheredstone slates, it is often very thin clay ormica beds which are weathered out.

21

References

British Standards Institution publications:BS 5250: 1989 Code of practice for

control of condensation in buildingsBS 5534: 2003 Code of practice for

slating and tiling

Other publications:Brereton, C, 1995 The repair of historic

buildings: advice on principles andmethods, 2 edn, English Heritage,London

Cotswold District Council, 2000Cotswold stone slate roofs: technical guide,Cirencester

Department of the Environment,Department of National Heritage, 1994Planning Policy Guidance: Planning andthe historic environment (PPG 15),London

Derbyshire County Council, 1996Derbyshire stone slate roofs; guidance forowners of historic buildings, Matlock

——, 1996 Derbyshire stone slate roofs;technical advice and model specification,Matlock

EH, SRWG and IHBC, in prep StoneRoof Conservation: a Best PracticeGuide (Pending print publication, seewww.stoneroof.org.uk/best.html)

Hughes, T G, Macdonald, S, and Strange,P, in Wood, C (ed), 2003 Savingstone slate roofs: the revival andenhancement of the stone slate roofingindustry in the South Pennines, amodel area of Britain, English HeritageResearch Transactions, 9, James andJames, London

Morris, W, 1890 On the external coveringof roofs, SPAB (Society for theProtection of Ancient Buildings),London

North Wiltshire District Council, 2001Stone tiling, Chippenham. Alsoavailable athttp://www.northwilts.gov.uk/ and follow the planning links

Internet only:Architectural Heritage Fund (website

regularly updated) Funds for historicbuildings in England and Wales www.ffhb.org.uk

Collyweston Stone Slater’s Trust, 2002 See www.csst.org.uk

English Heritage Stone RoofWorking Group A format for regionalstone-slate roofing guides,www.stoneroof.org.uk/format.htm

Figure 16 Langley Chapel, Shropshire.This Harnage stone slate roof includes both sandstone and calcareous sandstone, which overlie each other in the samequarry. The calcareous sandstone is identifiable by the presence of white lichen.

08268 26/4/05 10:34 pm Page 21

Further information

Planning and local technical advice:contactLocal planning authority,

Conservation OfficerBats: contacts

Local authority,Nature Conservation Department

English NatureNorthminster HousePeterboroughPE1 1UA01733 455000

Exhibition: English Heritage’s exhibitionThe Roofs of England: A celebration ofEngland’s stone slate roofing traditions isavailable for loan free of charge. Theexhibition provides information onstone slate roofing includingproduction, traditional techniques,conservation, and practical issues. Forenquiries regarding the exhibition or tocomment on this Technical advice note:contact

Building Conservation and Research TeamEnglish Heritage23 Savile RowLondon W1S 2ET020 7973 3314

Grants: for Grade I and II* listed buildingscontactEnglish Heritage23 Savile RowLondon W1S 2ET020 7973 3000

Grants administered by the Heritage LotteryFund: contactThe Heritage Lottery Fund7 Holbein PlaceLondon SW1W 8NR020 7591 6000

For information on funding (various sources):contactThe Architectural Heritage Fund(AHF)Clareville House26–27 Oxenden StreetLondon SW1Y 4EL020 7925 0199www.ffhb.org.uk

Environmentally Sensitive Areas Scheme: forinformation contactThe Department for Environment,Food and Rural AffairsNobel House17 Smith SquareLondon SW1P 3JR020 7238 6907

Regional stone slate types, areas of use, sourcesand manufacturers: contactStone Roofing AssociationCeunantCaernarfonGwynedd LL55 4SA01286 650402www.stoneroof.org.uk

22

Figure 17 The misinterpretation of historic details and their modification with modern materials can cause difficulties.The cement pointing to this Purbeck stoneslate roof is unsightly and technically problematic.

08268 26/4/05 10:34 pm Page 22

23

Figure 19 These small Cotswold stone slates have been laid at a steep pitch to form the huge roof of Great Coxwell Barn, Oxfordshire.

Figure 18 The hip of the roof at Stokesay Castle, Shropshire has been formed with lime mortar, which disguises theintroduction of a lead soaker beneath to ensure that the roof is weather-tight.

08268 26/4/05 11:45 pm Page 23

• New stone slates rather than second-hand should be used.

• Reclaimed slates should be used only on the buildingor group of buildings from which they were removed.

• Substitute materials, such as artificial slates made offibre resin, concrete tiles, ‘reconstituted stone’ and soon, are inappropriate alternatives to real stone slatesand are not suitable for use on historic buildings.

• New stone slates should match the existing ones asclosely as possible in terms of geological type, colour,texture, size, and thickness. It is important torecognise the slates particular to your area. Sandstoneand limestone slates should never be substituted foreach other, nor should they be used together.

• Stone slates which have been sawn to thickness, ratherthan split, can be technically and aestheticallyunacceptable and, if they fail to meet these criteria,should not be used on historic buildings.

English Heritage Customer Services: 0870 333 1181E-mail: conservation@english-heritage.org.ukWebsite: www.english-heritage.org.uk

Authors: Terry Hughes (Slate & Stone Consultants);Chris Wood, Building Conservation and Research Team,English Heritage; Susan Macdonald (1998 text)Editing: Margaret Wood, English Heritage PublishingDesign: Michael McMannProduction: Elaine Pooke, English Heritage Creative ServicesPrinter: Hawthornes

© English Heritage 2005 (text)© 2005 Illustrations and photographs copyright of the authors orsources cited in the captions

English Heritage Product Code 51024

LONDON23 Savile RowLondon W1S 2ETTel: 020 7973 3000

EAST OF ENGLANDBrooklands House24 Brooklands AvenueCambridge CB1 1DJTel: 01223 582700

EAST MIDLANDS44 DerngateNorthampton NN1 1UHTel: 01604 735400

NORTH EASTBessie Surtees House41–44 SandhillNewcastle upon Tyne NE1 3JFTel: 0191 269 1200

NORTH WESTSuites 3.3 & 3.4 Canada House3 Chepstow StreetManchester M1 5FWTel: 0161 242 1400

SOUTH EASTEastgate Court195–205 High StreetGuildford GU1 3EHTel: 01483 252000

WEST MIDLANDS112 Colmore RowBirmingham B3 3AGTel: 0121 625 6820

YORKSHIRE37 Tanner RowYorkYO1 6WPTel: 01904 601901

English Heritage Regional Offices

Front cover Stone slate roofs are laid to diminishing courses, with the longest slates used at the eaves, diminishing to the ridge.This maximises the use of theavailable rock from the quarry or local delph.

Sourcing stone slates

08268_Cover_10Black 4/5/05 9:30 am Page 2