b'Technical ArticleThe Art and Science of Snow Mitigation on Metal RooftopsBy Rob HaddockThefollowingarticleoriginallyWhat Causes a Rooftopfreeingthetemperature-sensitivewhich reduce ice-damming ten- Site-Specific VariablesappearedinIIBECInterfaceandAvalanche? bond.Themeltwaterlubricatesdencies but do little to mitigateAsnowguardisadeviceor is reprinted with permission from thetheinterfaceofthesnowwithavalanchepotential,whichissystemthatmechanicallypro-InternationalInstituteofBuildingDuringsnowfall,sunlightistheroofsurface,thusreducinggovernedbyrooftemperature. occluded and snowpack verticallythesurfacefrictionbetweentheRegardlessofthesecontribut- videsaresistanceinterfacebe-Enclosure Consultants. loads (with the snow weight) thetween the roof and the snow, so smooth metal roof and the snow. ing factors, the surface warming E ach year, rooftop avalanchesroof surface. This load translatesBecausethesnowbankinsulatesfrominvisiblerayscantipthethat snow clears from the roof in causecostlypropertydam- to a vector force parallel to thethe roof, this scenario can happena predictable and controlled fash-roofsurface(thatis,theforcescales to precipitate thaw of theionthroughevaporation,subli-age, personal injury and eveneven when ambient temperaturesbond. The author has repeatedlymation and thaw, rather than by death.Snowpackcansuddenlyofsnowtryingtoslideofftheare well below freezing. Studies ofobservedopen-gabledcanopy releaseanddumptonsofsnowroof), which is often referred toglacial melting1indicate that thea sudden and dangerous rooftop as drag load or gravity load.visible-lightalbedo(reflectivity)roofs (with subfreezing ambientavalanche. The resistance of the belowtheeavesinamatterofThevectorforceisresistedbytemperaturesbothaboveandsnow guard system must exceed seconds, endangering building el- two bonds. The first is a strongof snow/ice is 90 per cent or low- below the roof) where the snow- the vector force in service, or the ements, adjacent roofs, landscape,buttemperature-sensitivebonder, but the remaining 10 per centbankreleasesfromthesouth- system will fail. Preventing such a vehicles,propertyandpedestri- ofvisiblerayspenetratedeeplyfacing slope, but not the north-ans.Nonexistentorinadequate(surfacefrictionandcrystallineand are quite effective at meltingfacing one. failure requires science and math snowretentionsystemscreatefusion)betweentheblanketofice.ThealbedoduetoinfraredWheninter-surfacethawoc- thatfewvendorsunderstand snowandtheroofsurface.Thelight is much lower, at five to 30or apply.safety issues and potential liabil- second is a cohesive bond at theper cent. When these rays hit thecurs, the vector force of the snowCalculatingthevectorforce ity for building owners, designersridge where the snowpack inter- roof, they are converted to ther- blanket is only resisted by the co- applied to a snow guard system is and contractors.laces with snowpack on opposingmal energy and cause breakdownhesive bond at the ridge, whichrelatively simple, but the calcula-The best way to mitigate ava- roof planes (Figure 1). When vec- of the fusion. is too weak, causing the blankettion must include the following lanche danger on a metal roof andtor forces exceed the strength ofRoom-sideheattransmissionof snow to shear and a suddensite-specific variables, all of which reduce liability caused by slidingthe two bonds on a slippery roof,through the attic and roof assem- releaseofsnowfromtheroof.are known to the design team: snowandiceistoinstallasci- a rooftop avalanche occurs.blies can also accelerate this thaw.This can happen on one side ofDesignroofsnowload entifically tested and engineeredWhenthesnowstormpasses,This facet can be minimized withthe ridge or on both sides simul- (Figures 3 and 4)snow retention system specific toinvisible rays from the sun passatticfloorre-insulationandat- taneously, depending on the ori- Roof slope the snow loads expected on thethroughthetranslucentsnow- ticventilationtoexhaustwarmentation of each side to the sunRoof(rafter)lengthfrom particular metal roof profile.pack, warming the roof below andair from the attic spaceall of(Figure 2). eave to ridge18ONTARIO ROOFING NEWSISSUE 3 2021 THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ORN'