b'and wetter.Ideallyit shouldbecoveredirrL~ediately withi ts waterproofpr otection, butthisisnotalwayspossible.Wherethereareconcretescreeds,forinstance,they mustbeallowedtimet oharden beforeworltcanbecommencedontherooffinish.This waiting periodissometimesreferred to mistakenly as"allowingthescreedto dryout!\' Onewayoutofthedifficultyisto usedryconstructionandapply thecovering immediately.Wherethisisnotpossible,othertechniquesmustbeadopted,Attempts havebeenmadeto provi\'deatemporary water-sheddingsurfaceby,forinstance,coveringscreedswith bitumenemulsion,butnoreal successhasbeenachieved.Themore sensible approachseemsto beto providearoofthatletsm oft herain passthrough ost it until such timeasthecoveringisputon .Thisisthebasisof theno-fineslight weightscreededrooftha0hasbeenrecorrrraendedforsometimeby theBuilding Research Station.Thescreedismadewith such anopentexture that waterrunsthroughit with little hindrance_:thestructural roofbelowi tispuncturedwithdrainageholesat all lowpointssoasto removethewaterassoonasit reachesthatlevel.Thistechnique hasgiventroubleonly wherethedrainageholeshavebeenprematurely filled.Aperiod aslong astwomonthsma.ybeneededbefore waterstopsdripping. TheNorwegianBuildingResearchInstitutehaverecommendedventilation ductsin all flatroofs.In thiscountrytheextraexpenseofinstalling ductsmaybewarrantedonlyin high humidity buildings.Somefurtherdi scussionisgivenin Building Research DigestNo.8andNo.51in theSecond Ser ies. Drainag2 Drainageofflatroofsisoftenoverlooked.With modularplanning andsystem building hascomeanincreaseintheuseof roofswithoutfallsfordrainage.Because of thedeflectionsoftheseroofsunder theirownweightmost ofthem tur::1into a seriesofsaucerswith waterlyingonthemthroughoutthewinter andthrough muchof thesurnrner . Thereareseveralobject ions\':,otheprovisionofr oofsthatdonothavefalls fordrainage.Principleamongtheseobjectionsare: (1)Thecoveringisliabletodeteriorat emorerapidly becauseof (a)Sharpthermalgradientsat theedgesofponds. (b)Crackingof driedmudaroundtheedgesofpondsleadingt o correspondingcrackingin abituminoussurfacebelow . c) (Slowpenetrationoftheles;,resistantcoverings)e.g.mineral surfacedbitumenfelt. (2)If aleakoccurs,alot of damageis donebeforetherepaircanbemade. Conclusion Thereisnodoubtthatflat roofscanberr:adeastrouble-freeaspitchedroofs. Thereis,however,notradition toensurethatthey aresoundlyconstructed;design thereforeshouldstart fromfirst principlesandincorporatethefeaturesthathave been mentionedabove. * * ** * **** *** **** * *** RCG\'IERREVISION Toprovideusanaccuratearidup-to-dateroster,pleasecompleteandreturnt he enclosedca1dirmnediately.'