b'FIATROOFS- SOMEEA.SICPROBLEMS ByA.G.Day CrownCopyrightReserved Themostimportantfunctionof aroofisto keepoutrain andsnow.It should serve asan umbrellaandshedthe watertoconvenientoutlets. Howthisis donedependsprimarily uponthepitch oftheroof.With aslope of45,excellent waterproofingcanbeobtained with smalloverlappingunitssuch as slatesortiles.Such aroofcan bedesigned togivesatisfactory servicefor50, 100or even200years.Atasomewhatlowerpitch,interlockingtilescanbeemployed andforstill lowerpitchescorrugatedsheets.Someof thesecan belaid at slopes lowerthan 10andsocouldbeclassed with theso-called"flat roofs." Themoreexpensiveflatroofsareoften covered withnon-ferrousmetalsheets butthe designof thecoveringstill follows,to alargeextent,that of thepitch roofs;themetalcoveringcan belooked uponasaseriesofinterlocking tiles each about8ft x2ftin size.Such aroofcanstill tolerateslight movementbecause ,of thediscontinuitiesin thecovering. It iscommontothinkof flatroofingashaving acontinuouscoveringof,say, asphaltorbuilt-up bitumenfelt.Thiscontinuouscoveringintroducesthreemajor difficulties:(1)it issensitivetolocalised movementsatjointsorcracksand maybesplit unlesstheroof deckis dimensionallystable,(2)thecovering affords avapourbarrier onthecoldsideof theroof which encouragescondensation troubles, (3)moisturein theroof atthetimeof coveringisentrapped. Traditional pitched roofsaremuchlesssensitivein theserespectsandeven with average workmanship areusuallytrouble-free.Why,then,dosomearchitects insistonhavingaflat roofeven onbuildingsofregularshapewhereapitched roof couldbeused withoutdifficulty? Assumingthat aflatroofhasto beemployed,it wouldbewisetolookmore closely into thepointsthat havebeenmentionedabove. ___of theCovering Splitting Mostflat roofdesignsarebasedon theassumption- orhope- thatthecovering will accommodatemovementof theroofdeck.Whetherornotthisistrue will depend, in additionto themagnitudeofmovementsin thedeck,uponthepropertiesofthe coveringandthewayin whichit is attached.Thus,asphaltwill acceptslowmovementsbutbehavesasabrittle material whenthemovementsaresudden andsois l i able tocrack.Usuallyit is laid overaflax felt underlay whichcushionsthemovements ofthedeck.Anylocalised movementin thedeckat acrack orjoint producedripples in theasphalt whichcanserveasawarningbeforefailureoccurs. Built-up bitumenfeltcanacceptonly asmall movementwithoutsplitting.Tests haveshownthatfew,if any,of thefelts usedin roofing will extend morethan5 percentwithoutbeingruptured.Fordesign purposesit wouldbe wiseto assumethe maximumsafeextension to be2percent.Obviouslyif thecoveringisfully bonded acrossajointthat opens or aline whereacrack forms,this2percentislikely 7 to beexceeded androofcovering will then split.Wherelocalised movementcanbe predicted the wisestcourseisto maskthat line with acoverstrip stuckdownonly -3-'