Historic ScotlandHistoric Scotland

Technical Summer School 2010Technical Summer School 2010

Masonry RepairsChuck Jones

A. R. Powys1881-1936

““The life of a building is longer than that of a The life of a building is longer than that of a man, but in the end both become dust. It is man, but in the end both become dust. It is for us to secure the longest possible life for for us to secure the longest possible life for that which is fine and noble, neither does that which is fine and noble, neither does this desire for security of the old conflict this desire for security of the old conflict with the urge to have new and gracious with the urge to have new and gracious

buildings about us”buildings about us”..

Masonry Decay and Repair

A building in need of repair

• The primary purpose of repair is to restrain the process of decay without damaging the character of the building, altering the features which give them their architectural identity, historic importance and without damaging and disturbing sound original and historic fabric.

• The repair should aim to achieve a sufficiently sound structural condition to ensure its longest term of survival.

• The repair work should be kept to the minimum required to stabilise the building or monument.

St Albans Cathedral 1805 showing the cracked and leaning Weathampsted Window and decay to areas of masonry.

Lord Grimthorp’s repair strategy was the complete renewal of the West front.

The principle stages in masonry repairs.

• Analyse the Historical development of the building.

• Analyse the cause of defects.

• Adopt a repair strategy

The Historical development of the site

• An understanding of the historical development of the site is essential preliminary to its repair.

• Archaeological investigations

• Architectural investigations

• Documentary research• Recording and

interpretation of the particular structure

• Such processes may need to continue during the works

• Preserve all findings and records of works for the future

Analyse the cause of defects

• Survey the building and include long-term observations from the owners.

• The nature and condition of the material. (don’t just look from the street, get up close and feel the building)

• The cause, process and rates of decay.

• To repair or replace decayed fabric without carrying out these investigations could be to invite the repetition of problems.

Problems, Decay and Causes• Environmental

– Frost– Salts– Thermal movement– Moisture movement

• Chemical– Pollutants– Oxidisation of metals

• Mechanical– Impact– Vandalism

• Biological– Lichen – acid release– Vegetation growth

• Problems can include: Spalling, Scaling, Splitting, Powdering, Staining and Pitting of masonry elements.

• The reasons behind The problems, decay and causes must be fully understood to be able to choose the correct remedial work to masonry.

Problem causes, Many are water Bourne.

• Acid attack on calcareous limestone

•Blistering and decay from wetting and drying cycles

• Acid attack on Ferruginous Sandstone

• Argillaceous, Clay-rich Sandstones

• Expansion and contraction of the clay rich minerals on wetting and drying.

• Salt crystallisation.

• Honeycomb effect on weaker areas of the stone caused by sea salts and weathering out of soft beds.

• Frost attack Freeze thaw cycles

Salt crystallisation

• Rusting of iron cramps• Chemical decay through the oxidisation of

the iron fixings

Desegregation of the stone matrixThe matrix of the stone is being compromised by chemical reaction

and/or internal stresses from Salt expansion.

• Fire Damage

Alveolar erosion

• Associated with maritime locations where wind and rain driven salts cause physical wind erosion and accelerated drying and salt crystallisation forming depressions that get larger.

Pollution crusts

• Build up of sulphur and carbon forming crusts on the masonry.

• On limestone, the sulphur and calcium carbonate converts to gypsum!!

Organic

• De-Lamination

• From incorrect bedding• Decay assisted by

freeze thaw cycles, wetting and drying cycles and salt contamination

• De-lamination is different from Contour Scaling

• Contour Scaling.• Appears the same as de-lamination but follows the face

contour of the stone rather than the bedding plane. Resulting from stress fatigue of the outer-layers caused by the formation of gypsum from the mortar or atmosphere.

• Structural

• Vandalism• Impact

• Poor choice of materials and workmanship

Adopt a repair strategy.

• Stone like us all, has a limited life.

• It will decay, be subjected to damage and may lead to an impaired aesthetic appearance or worse affect its structural stability, and proper remedial action will be needed.

Remedial Action• Traditional method of repair to masonry

has been to cut out and renew all weathered or otherwise defective stone.

• Disadvantages of new stone include: 1. Less of the original work is preserved.2. Removal of the old to make way for the new

can loosen and damage adjoining masonry.3. An unpleasant visual and geological match.4. Quality of new material and workmanship may

not match original.

Options for Repair

• Modification of the External Environment

• Provide Weather Protection

• Re-route water channels

• Introduce DP membrane's

• Remove harmful causes

Weather Protection

Weather protection to Weathering Detail

Weather protection to the Whole

building

Re-route water channels

Introduce a DPM, Clay tiles

• Remove the Cause of decay

Options for Repair

• Modification of the Internal Environment

• Humidity

• Temperature Controls

• Restrictions on Visitor numbers

Options for Repair

• Provide sacrificial layer

• Lime Render

• Lime Wash

• Lime Harl

Lime Harl

Lime wash

Lime Render

Mortar repair

Lithomex” Stone Repair- Beware of its

ingredients we want to go forward not

backwards.

Options for Repair

• Insert Tile Repair • SPAB Repair

• Honest Repair

• Can replace whole missing stones or stabilise large areas

Options for Repair

• Piece in new Stone to decayed areas.

• Easier to find small pieces of existing original stone for partial replacements.

• Only the decayed areas are replaced

• Areas are cut back to the “sound” to accept new piece.

• Indents secured with S-Steel Dowels.

• Avoid impermeable epoxy and polyester resins to secure new to old. Variable wetting and drying rates.

• Dowels secured with neat lime, casein, resin etc.

Indent repair, also known as piecing in.

Options for Repair

• Micro Stitch or Jointing of Decayed / damaged stones

• Threaded stainless steel or fibreglass rods/dowels inserted through drill holes

• Fractures can be injected with lime grout

• Fractures not to be glued but can be bonded with slurried lime

Option for RepairThe Complete Stone Replacement.

• Stone should match original geologically taking into account Porosity and Permeability. Info from BGS, BRE, SSLG.

• Carefully matched to original size and profile if possible.

• Obtain and accept only the highest possible quality of workmanship that must match original.

Replace only what's necessary its repair not restoration.

• Stone brought out to original profile although beware of water traps which can be filleted of with mortar

• Avoid six side sawn stone from the quarry

• The original finish should be matched accurately

• Stones can be dated and should certainly be recorded as replacements

• The structural function and stability of the individual stone is often the reasons behind complete replacement.

• Access and cost can determine replacement or repair

• Weathering details and structural elements are often replaced when decayed.

Consider these in your choice

• The age and character of the building;

• The structural function of the individual stone in question, and the nature and cause of its defect;

• A careful assessment of its rate of decay, taken in the context of the building as a whole.

Whoever expects to find a stone that will stand from century to century, deriding alike the

frigid rains and scorching solar rays, without the need of reparation, will indeed search for

“the philosophers stone”.

C.H Smith in his lecture to RIBA in 1840

ENDS