b'thatsolarheatoftencausesear ierfai lureSuchfailuremaytake l o theformofblistering,especiallyi fmoistureispresentinsome .componentoftheroofsystemoSolarheathasalsobeenobservedto causewrinklingandbucklingwit hresultantcrackingofthemembrane, aswellasslippageofthemembr a neinroofshavingaslopeofone inchperfootormoreoSolarheathasbeenfoundtoaccelerate chemicaldegradationofbitumeni nt hem whilethermalshock embrane (rapidchangeintemperature)acceleratesthephysicaldeterioration ofthebitumeno Manyprematurefailurescanbetrac eddirectlyorindirectlyto thermalmovementwithi naspecificcomponentoftheroofsystem,or todifferentialmovementamongt hesystemcomponent s(decktinsulation, membrane,orflashings )oWhent hecoefficientsoflinearexpansion ofthecomponentsdiffer 9t heamountofmovementdiffers 1thereby causingseparationofthecomponent s cThisholdstrueforanymetallic appurt enanceconnectedtothemembrane~suchasflashings o Inconjunctionwiththermalshock ~t herma lmovementmaycausesplittingand membraneruptureso Toobtainbasicdatathatm helpinpreventingprematurefailight uresofroofsystems,t heBureauconductedtwoseriesoftestsoThe firstserieswasdesignedtodeterm et hemagnitudeoftemperature inchangewithinthemembrane .Thesecondser ieswassetuptodetermine thecoefficientoflinearexpansi onofthemembraneonly,asthecoefficientsoflinearexpansionformostmaterialsusedintheother componentswerealreadyknown o Tostudythedegreeofsol arheati ngandradiativecoolingin built-uproofsystems 9twentys pecimenswereconstructedandexposedontheroofofabuildingatt heBureaufromJune1962toJune 1963 0Tenhalf- inchplywooddecks,18by18ino,andtendenseconcretedecks,2by18by18in o~werecoveredwithconventionalfourplyasphalt- saturatedorganicfe l tbuilt- upmembranes~halfofeach grouphavingexpandedpolyurethaneasinsulationbetweenthedeck andthemembraneoOneoffivepr otect ivesurfaceswasthenapplied toeachspec\'imenThesurfusedwereAsphalt( smoothblack), aacesi crushedstone(darkgrey),scoria(mar oon)jmarblechips(white), andapolyvinylfluoridefilm( smoot hwhi te )oThesubstratesand surfacesusedwereselec tedasbeingrepresentativeoftheextremes inmaterialspresentlyusedinser-vice oCopper- constantanthermocoupleswereinstalledineachspecimenbetweenthemembraneand insulationorbetweenthemembraneanddeck oTemperatureswererecordedcontinuouslytwenty- fourhoursaday " Testsshowedthattemperaturec hangesofroofsurfacesdepend uponthemass,density,andspecif icheatoft hesubstrate;color andtextureofthesurface;anduponatmosphericconditions(wind velocity,rain,cloudcover~ha ze ~etco ) cTheroofsurfacemaybe sub- cooled,i oe o 1rooftemperaturemaybelessthan thatofthe ambientairbecauseoftheradiativepropert iesoftheroofsystemo Themagnitudeofthesub-coolingisdependentuponthemass,density, andspecificheatofthesubstrat e;whetherthesurfaceexposedis metallicororganic;andupona t mospher i cconditions oForallpractical purposestheradiativecoolingofor ganicbuildingmaterialis'