The author has repeatedly observed open-gabled canopy roofs (with subfreezing ambient temperatures both above and below the roof) where the snowbank releases from the south-facing slope, but not the north-facing one. Regardless of these contributing factors, the surface warming from invisible rays can tip the scales to precipitate thaw of the bond. This facet can be minimized with attic floor reinsulation and attic ventilation to exhaust warm air from the attic space-all of which reduce ice-damming tendencies but do little to mitigate avalanche potential, which is governed by roof temperature. Room-side heat transmission through the attic and roof assemblies can also accelerate this thaw. When these rays hit the roof, they are converted to thermal energy and cause breakdown of the fusion. The albedo due to infrared light is much lower at 5% to 30%. Studies of glacial melting 1 indicate that the visible-light albedo (reflectivity) of snow/ice is 90% or lower, but the remaining 10% of visible rays penetrate deeply and are quite effective at melting ice. Because the snowbank insulates the roof, this scenario can happen even when ambient temperatures are well below freezing. The meltwater lubricates the interface of the snow with the roof surface, thus reducing the surface friction between the smooth metal roof and snow. When the snowstorm passes, invisible rays from the sun pass through the translucent snowpack, warming the roof below and freeing the temperature-sensitive bond. When vector forces exceed the strength of the two bonds on a slippery roof, a rooftop avalanche occurs. The second is a cohesive bond at the ridge where the snowpack interlaces with snowpack on opposing roof planes ( Fig. The first is a strong, but temperature-sensitive, bond (surface friction and crystalline fusion) between the blanket of snow and the roof surface. This load translates to a vector force parallel to the roof surface (that is, the force of snow trying to slide off the roof), which is often referred to as “drag load” or “gravity load.” The vector force is resisted by two bonds. The best way to mitigate avalanche danger on a metal roof and reduce liability caused by sliding snow and ice is to install a scientifically tested and engineered snow retention system specific to the snow loads expected on the particular metal roof profile.ĭuring snowfall, sunlight is occluded, and snowpack vertically loads (with the snow weight) the roof surface. Nonexistent or inadequate snow retention systems create safety issues and potential liability for building owners, designers, and contractors. Snowpack can suddenly release and dump tons of snow below the eaves in a matter of seconds, endangering building elements, adjacent roofs, landscape, vehicles, property, and pedestrians. RCI-IIBEC Foundation and RCI Foundation CanadaĮach year, rooftop avalanches cause costly property damage, personal injury, and even death.The Guide An Overview of IIBEC Services.Protecting Procurement of Design Services.The Hub: Member Resources, Publications, Technical Articles.Certified Building Enclosure Commissioning Provider – CBECxP®.Registered Building Enclosure Consultant – RBEC®.Registered Exterior Wall Observer – REWO®.Registered Exterior Wall Consultant – REWC®.
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