b'at theedgesprior to coveringthe wholeroof.Wherethelines along which themovementwill beconcentrated arenotknown,asforinstancein thecrackingof ascreed, partial bonding of the feltcoveringgreatly reducestherisk of failure. Butylrubberandpolyisobutylenesheetsstretch much morethanbitumenfelt-perhaps50percentof their length.Thusthereis lessrisk of tearing thesheets. Howeverthe lappedjoints arestill vulnerable. Sprayed,trowelled or brushedcoatsare moresensitivethan thesheet materials becausethey arefully bondedandgenerally thinner.Joints wheremovementisto be expectedcanbetapedasforbitumenfeltcoverings,but thereisnotmuchthat can bedoneaboutcracksthat formaftertheroofcovering hasbeen applied.Undoubtedly thesesite appliedcoatingsareat their bestonsurfacesthat are alwaysin compression,forinstance,onshell roofs. Mentionshould bemadeof the weaknessoffully bonded coveringsin that they areliableto blisteronamicroporoussurfacesuch asconcreteor asbestoscement. A smallpressure,aslowaslouncepersquareinch issufficient to causeablister. Thispressurecandevelop fromentrappedmoisturevapour whentheroof warmsupin sunshine.Withopen-textured materials,thepressureisreleased;hencetheabsence of blistering whereasphaltis laidoveraflaxfelt underlay or wherebuiltupbitumen feltis laid overfibre-board. Condensation Theair in occupied buildings alwayscontainsmoremoisturethanthatoutside. Unlessprecautionsare taken,this humid air will findits wayinto colderpartsof the roof and will beentrapped thereby theimperviousroofcovering.In cold weather deposition of moistureontheundersideof thecovering and withintheroofstructure is theninevitable. Onewayof avoidingthiscondensationisto provideavapourbarrier onthewarm sideoftheroof.Thiscanbedonesuccessfully onsometypesofroofdecking,for instance,metal deck,by laying bitumenfeltbeforefixingtheinsulating material. Attemptsto provideavapourbarrier onsuspendedceilingsarenotsosuccessful.The barrieris almostsureto bepunctured byservice pipes andfittings,particularly thelightfittings.Thereis also difficulty in makingavapourtightjointat the junction betweenceiling andwalls.It wouldbe wise,therefore,to regard thistreat- , mentasproviding avapourcheckratherthan avapourbarrier.Inconsequencesome meansof disposingof moisturevapourthat leaksthroughtheceiling i ntot heroof spaceis desirable.Ventilationoftheroofspaceto theopenair istheeasiest wayofdoingthis.It isestimatedthat,forsmall buildings, squareinch opening per footrunontwooppositesidesof thebuildingshouldgivesufficient ventilation. It is acommonpracticeto putsmallventsthroughthecoveringon flatroofs . Whilst these donotallow sufficientair movementtodryoutawetscreed or to preventthebuild upofcondensed moisturein adverseconditions,they doprovidea pressurerelease.In this waythey avoid waterbeing forceddownthrough cracksand conduitswhensunshinewarmstheroofsurfaceandincreasesthevapourpressure underneath. Entrapped Water In all partsof Britain therainfall exceedstheevaporation from afreewater surface.Consequently,aroofdeck exposedtothe weatherbecomes,on average,wetter -4-'