7.0 Roof Framing
Transcript of 7.0 Roof Framing
ThE homE REfEREnCE Book ©
STRUCTURE
128
7.0 Roof Framing
DESCRIpTION Theroofframingisanassemblyofwoodorsteelcomponents.Theroofframingtiesthebuild-ingtogether,addingrigidityandprovidingasurfacefortheroofcovering.Theroofframingalsosupportstheceilingfinishesbelow.Wewill lookattheindividualcomponentsofroofframingsystems.
7.1 Rafters – Wood
DESCRIpTION Rafterscarry the loads fromthe roofsheathing, roofcovering,wind,water, snowand ice.Theseloadsaretransmittedthroughtherafterstobearingwallsorbeamsbelow.Thetermrafterisassociatedwithslopedroofs.Whenthesemembersarefoundonaflatroof,theyarehorizontalandcalledroofjoists,althoughtheydothesamejob.
Rafters can usually be seen overhead when standing in the attic. Some rafters supportfinishedceilings,creatingacathedralceiling.Inthiscase,insulationisoftenfitbetweentherafters.
Woodraftersaretypically2x4s,2x6sor2x8s,spaced16to24inchesoncenter.Conventionalraftershavebeenreplacedbytrusses inmostmodernhomeconstruction.Theengineeredtrussescanspangreaterdistanceslessexpensivelythanconventionalrafters.
When calculating the span of a rafter system, the horizontal span rather than the actuallengthoftherafterisused.Kneewallsorpurlinsmayprovideintermediatesupport,reducingthespan.Collartieshelpkeepraftersinplace.Ceilingjoistsarehorizontalmembersthatoftentiethebottomsofopposingrafterstogether,makingastrongtriangle.
129
STRUCTURE
ThE homE REfEREnCE Book ©
STRUCTU
RE
CommonProblemswithWoodRafters
OVERSpaNNED/ If rafters are overspanned or spaced too far apart, the roof will sag. If rafters are notRaFTER SpREaD adequatelysecuredto thewallsat thebottomedge, the raftersmayspreadapart.This is
commononolderhouses,particularlywithgableroofs.Itisoftennotedattheeaves,becausethesoffitspullawayfromthehousewallastheraftersspread.Inothercases,thespreadingrafterspushthetopofthewalloutward.Thiscanbeveryserious.
Raftersmayalsoseparateattheroofridgeifconnectionsandsupportarepoor.
ROT/INSECT Wood rafters are susceptible to rot, insect, fire and mechanical damage. Rafters mayDamagE/SplIT splitunderload.
CONDENSaTION Attics in cold climates with good insulation but poor ventilation may be susceptible tolEaDS TO ROT condensation problems. Condensation will attack the roof sheathing and the rafters. Left
unchecked,thiscanleadtoroofstructurefailure.Correctiveactionincludesimprovedventila-tionandreplacingdamagedwood.Moldoftendevelopsinatticsasaresultofcondensation.
Sag– Concentrated loads may be a problem around roof dormers. The load from a dormer isCONCENTRaTED transferredtotheraftersoneithersideofthedormer.Unlesstheseraftersarestrengthened,
lOaDS theroofmaysagarounddormers.
In cold climates, roofs mayfail because of a concentratedsnow load. Split-level housesaresusceptibletothisproblem,forexample.Itisnotunusualforsnowdriftstoformonthelowerroof,nearthewallofthehighersection. This leads to big loadsontheraftersortrussesintheseareas.Theraftersmaycrack,sagorspreadatthebottom.Trussesmaycollapse.
7.2 Roof Trusses – Wood
DESCRIpTION Rooftrussesareengineeredassembliesthatperformthesamefunctionasrafters,collarties,kneewalls,purlinsandceilingjoists.Therooftrusscarriestheroofsheathingandshingles,andtheliveloads,transferringtheroofloadstotheoutsideorbearingwalls.Thebottomofthetrussalsosupportstheceilingfinish.
ThereareotherengineeredwoodframingsystemsincludingwoodI-joists,andotherconfigu-rations.Adesignreviewoftheseisbeyondthescopeofahomeinspection.Theperformanceevaluationissimilartotrussesandrafters.
ThE homE REfEREnCE Book ©
STRUCTURE
130
Most trusses in homes are made of wood. The top and bottom members of the truss arecalledchords.Theinteriormembersofatrussarecalledwebs.Trussmembersarefastenedtogetherwithgussetplates.Thesemaybemadeofplywoodorsteel.Differentconfigurationsoftrusseshavedifferentstrengths,andengineerscanusetheshapeandcomponentsizethatbestsuitsthem.Trussesareengineeredsystems.Trussesarenormallyspaced24inchesapart,butthiscanvary,againdependingonthespansanddepthoftrussdesired.
Therearetwocommontrusstypesusedresidentially.TheFinkor“W”haswebmembersthatforma“W”.TheHowetrusscanbeidentifiedbyverticalwebmembers,includingaverticalwebrunninguptothepeak.Therearemanyvariationsofthesefoundinresidentialconstruc-tion.
Ineithertrusstype,thewebsshouldbeatleast2x4s,unlessspecialengineeringconsiderationhasbeengiven.Wherethecompressionwebsarelongerthansixfeet,theyaresusceptibletobucklingunderheavyloads.Braces,suchas1x4s,shouldbefastenedtothemidpointsofthesewebs.
Where the bottom chord hasa long span between supportpoints, it may not be strongenoughtocarrytheceilingload.If the span is more than tenfeet between support points,thebottomchordshouldbeatleast a 2x5. If the span is morethan 12 feet, the bottom chordshould be a 2x6. Again, specialengineering consideration canresult in deviations from theseguidelines.
CommonProblemswithWoodRoofTrusses
Likeanywoodmember,trussesaresubjecttorot, insectdamage,mechanicaldamageandfire.
CUT Individualchordsorwebsthatarecutordamagedcanbeaseriousproblem.Cuttingatrussinonespotmayseriouslycompromisetheentiretruss.Wheretrussesarecuttoaccommodatechimneysorotherinterruptionsintheroofline,engineeringconsiderationshouldbegiven.
FaSTENINg Trussesmustbewellsecuredtoperformwell.
OVERSpaNNINg Overspanningoftrussescanleadtodeflectionand,inworstcases,roofcollapse.Heavierroofmaterialsuchasconcretetile,orgreatersnowloadsthanexpected,duetounusualconditionsordriftingmaycausetheproblems.Overspanningcannotbeidentifiedduringahomeinspec-tion,butevidenceofdeflectionorfailureisnoted.
131
STRUCTURE
ThE homE REfEREnCE Book ©
STRUCTU
RE
REpaIRS – Reinforcing overspanned or damaged trusses is more difficult than strengthening a rafterDESIgN NEEDED roofsystem.Anengineershoulddesigntherepair.
lONg WEBS – Trusses with web members longer than six feet may be subject to buckling. BracesBRaCINg NEEDED shouldbeaddedtothemidpointsofthewebs.Onebraceattachedtoeachwebwithtwonails
shouldconnectseveralwebsinadjacenttrusses.Thebracesshouldbeatleast1x4s.
INaDEqUaTE UndersizedbottomchordsshouldbestiffenedtopreventceilingsagandcrackingofceilingCEIlINg finishes.Addingasecondmembertothebottomchordwouldnormallybesatisfactory.
SUppORT
TRUSS UplIFT Aphenomenonknownastrussupliftisrelativelycommonincoldclimates.Thetemperatureandhumiditychangesintheatticduringthewintermonthsaffectthesectionsofthetrussabovetheinsulationleveldifferentlythanthebottomchord,whichisburiedintheinsulation.Thisresultsinanupwardbowingofthebottomchord.
Theresultoftrussupliftisthatthecentersectionofthebottomchordmovesup,andgapsaslargeas1-1/2inchesappearatthetopoftheinteriorwalls,wheretheyjointheceiling.Theceilingispickedupbythetruss.Itisalsopossiblethattheinteriorwallbelowwillbeliftedup,andseparationwilloccurbetweenthewallandthefloor.
Acommonsolutionistosecureamoldingtotheceiling(butnottothewall).Astheceilingmovesupanddown,themoldingwillslideupanddownthewall,concealingthegap.
Anothersolutionistodisconnecttheceilingfinishesfromthetrussduringconstructionandtocliptheceilingfinishestothewall.Thisallowstheceilingtobendveryslightly,butnottocrack.
Trussupliftisnotaseriousstructuralproblem.
ThE homE REfEREnCE Book ©
STRUCTURE
132
7.3 Steel Framing
DESCRIpTION Steelroofframingperformsverymuchlikewoodframing.Likewood,steelroofframingmaybemanufacturedtrussesorsite-assembledraftersandjoists.Theprinciplesofwoodframingalsoapplytosteelroofframing,althoughcontinuousbridgingontheraftersisoftenneeded,either on the underside, or through the knockouts. The undersides of joists will twist andmoveunlesstheyarerestrained.
Woodsheathingmaterials,suchasplywood,arescrewedtothesteel roof structure. Roofingmaterialsareinstalledoverthissheathingasnormal.
CommonProblemswithSteelRoofFraming
RUST Steelis,ofcourse,vulnerabletocorrosionwithlong-termexposuretomoisture.
hOlES Anyholesthatarecreatedshouldbeatleast10inchesawayfromtheendoftherafter,posi-tionedinthemiddleoftherafter,andshouldnotbewiderthan1-1/2inchesandnotlongerthanfourinches,unlesstheyarereinforced.
NO DESIgN Steelroofsareengineeredsystems,andaswithwoodtrusses,homeinspectorsdonotassessREVIEW theirdesign.
133
STRUCTURE
ThE homE REfEREnCE Book ©
STRUCTU
RE
7.4 Collar Ties
DESCRIpTION Collartiesmaybeinstalledtopreventraftermovement.Thesearetypicallywoodmembers(atleast1x4s)installedhorizontallyacrosstheatticspace.Theyareconnectedateitherendtoopposingrafters.Insomecircumstances,metalstrapsmaybeusedinsteadofcollartiestopreventuplift.
CommonProblemswithCollarTies
mISSINg, WRONg Missingcollartiescanbeaddedreadily.Collartiesmaybeinstalledincorrectlyorfastened lOCaTION OR poorly. pOOR CONNECTION
BUCklED aND When collar ties are installed to prevent rafter sagging, the compression from opposing BROkEN raftersmaybuckleandcrackthecollarties.Addingbraceswillpreventthis.Brokencollarties
canbereplacedasneeded.
7.5 purlins
DESCRIpTION Purlinspreventraftersag.Thesearewoodcomponentsthesamesizeastherafterstheysup-port.Theyarenailedtotheundersideoftheraftersandaresupported,inturn,by2x4braces,whichextenddown,usuallyonanangleratherthanvertically,toabearingwallbelow.Seeillustrationonnextpage.
CommonProblemswithPurlins
Likeanywoodcomponent,purlinsaresubjecttorot,insectandmechanicaldamage.
STRUTS – mISSINg Bracesorstrutsnomorethanfourfeetapartshouldsupportthepurlins.
STRUTS – SlOpE Strutsshouldbeinstalledwithaslopeofnotlessthan45degreesfromhorizontal.Anylessthanthisandthestrutswillnotproperlytransfertheirload.
STRUTS – Thestrutsthemselvesshouldbebracediftheyarelongerthaneightfeet.Astripoflumber UNBRaCED attachedperpendiculartothestrutswillpreventbuckling.
ThE homE REfEREnCE Book ©
STRUCTURE
134
7.6 knee Walls
DESCRIpTION Knee walls are intermediatesupports that prevent raftersag. These small walls in theatticaretypicallybuiltwith2x4woodstuds.Theyrunfromtheatticflooruptotheundersideoftheraftersneartheirmid-point.In 1-1/2 or 2-1/2 story houses,knee walls form the walls of aroomontheupperfloor.Theserooms often have a partlyslopedceilingasaresult.
CommonProblemswithKneeWalls
pOORly SECURED/ Ifthekneewallsarenotadequatelysecuredtotheraftersaboveorthejoistsbelow,theywillWEak FlOOR move.Ifthefloorjoistsystembelowisnotstrongenough,orthereisnopartitionbelow,this
canleadtodeflectionanddamageintheceilingbelow.
lOCaTION Ifthekneewallisnotnearthemidspanoftherafters,itmaynotbeeffectiveinpreventingraftersag.
DamagE/ROT/ Likeanywoodcomponent,kneewallsaresubjecttorot,insectandmechanicaldamage. INSECT DamagE
135
STRUCTURE
ThE homE REfEREnCE Book ©
STRUCTU
RE
7.7 Roof Sheathing
DESCRIpTION Sheathingsupportstheroofcovering,transmittingtheloadsfromthecoveringandtheliveloadsduetowater,snowandwindtotherafters,trussesorroofjoists.
Upuntilthe1950s,virtuallyallroofsheathingwaswoodplank.Plywoodroofsheathingin4-footby8-footpanelsbecamepopularinthe1960s,andwaferboardpanelsarrivedinthe1970s.
Plywoodshouldbelaidwiththesurfacegrainperpendiculartotherafters,trussesorjoists.Theeight-footlengthshouldbeacrosstherafterswitheachendsupportedbyarafter.Theotheredgesshouldalsobesupported,typicallybymetal“H”clipslocatedbetweenrafters.Theseclipsnotonlysupporttheedges,butkeepthesheetsslightlyseparatedtoallowforexpansionwithoutbuckling.
Thethicknessofthesheathingisdeterminedbythespacingoftheraftersandtheliveroofloads. Modern construction typically employs trusses or rafters on 24 inch centers andplywoodsheathing3/8inchthick(orwaferboardsheathing7/16inchthick).Thisleadstoafairlyspringyroofsurfacewhenwalkedupon.Unusuallyheavyloadsorslightweakeningoftheplywoodduetohighmoisture levels intheatticcan leadtosaggingof thesheathingbetweenthesupports.Normallythisisnotastructuralflaw,althoughitisunsightly.
ThE homE REfEREnCE Book ©
STRUCTURE
136
CommonProblemswithRoofSheathing
DamagE/ROT/ Likeanywoodcomponent,roofsheathingissubjecttorot, insectdamageandmechanical INSECT DamagE damage.
DamagE – Condensationinanatticcancauseproblems.Plywoodroofsheathingwillbegintodelaminate CONDENSaTION andwaferboardsheathingwillswell.Thiscancauselossofsheathingstrengthandrender
thenailingineffectiveasnailsarepulledoutoftheraftersorthroughthesheathing.Inseverecases,theroofcoveringhastoberemovedandthesheathingreplaced.
Concealedcondensationincathedralroofsisacommonproblemandconsiderabledamagecanoccurbeforethereareanyvisualclues.
mOlD Mold often appears when there has been condensation in the attic. Once the moisture isremoved,themoldstopsgrowing.Removaloftheinactivemoldisnotnecessary,althoughmanyhomeownersremoveitbecauseofthestigmaattachedtomold.
TOO ThIN Sheathingthatistoothinfortheapplicationwillsagunderload.Aesthetically,thisisoftenunacceptable,althoughsaggingtothepointoffailureisunusual.
Sheathingthicknessisdeterminedinpartbythespacingoftheraftersortrussesbelow.Thick-ersheathingisneededwhenthespansaregreater.Sheathingthicknessisalsodeterminedbytheliveloadsfromwindandsnowaswellasdeadloadsfromroofcoverings.
EDgE SUppORT Unsupported edges of roof sheathing may lead to differential movement between twopanels.Thiscanleadtohorizontalridgesappearingintheroofing.Ifthesheathingisunusu-allythick,edgesupportisnotnecessary.
FIRE RETaRDaNT Fire retardant treated (FRT) plywood was recognized as a problem in the late 1980s. TREaTED (FRT) Delaminationandweakeningofthisplywoodcanleadtoalossofroofshinglesandultimately plyWOOD collapse.Whereithasbeguntofail,itshouldbereplaced.