7.0 roof framing

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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.

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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.


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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.

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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.


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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.

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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.


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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

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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.


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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.

7.0 roof framing

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