In many cases, a site-specific study of both ground snow and regional climate is required. It is not wise to accept without question snow loads issued by local building officials, as this recent case illustrates: When a snowstorm in northern Idaho (including portions of eastern Washington and western Montana) caused more than 300 roofs (including several new buildings and one new school) to give way, the subsequent investigation showed that the storm's high winds had caused extensive drifting and eccentric loading. Moreover, local building officials had permitted construction based on a uniform snow load that was, in some cases, as little as one-third of the ground-snow load. Had the designer investigated local conditions much property damage could have been prevented (Figure 4).
The architect must also analyze snow loads in the most eccentric configurations conceivable. For instance, at a site in California's Sierra Nevada range, a north-sloping (winter shade) roof in the lee of storm winds normally had 10 to 12 feet of snow on it in winter—at least double the ground-snow load at this site—while the sun-drenched south-sloping roof had only patches of snow on it or none at all (an extremely eccentric snow-load distribution). The north roof had not been designed for such high snow loads, so manual shoveling was required after all major storms to avoid overloading the roof (Figure 5).
Finally, wind tunnel testing or computer modeling should be considered in addition to sun angle shifts and temperature fluctuations.
Ice damming—the blockage of water flow by an ice obstruction—is one of the leading causes of building problems in snow country. For instance, at the Sierra Nevada site described above, the north-sloping roof in the lee of storm winds was plagued by ice dams often more than three feet high, with huge icicles extending as far as two stories down from the eaves (Figure 6).
Besides creating a heavy load on the eaves, ice dams cause snow to remain on the roof in places where it might normally slide off. Vent pipes or flue caps can be blocked or flooded, and glacial action and slippage can shear them off. Water pressure from the meltwater "lake" behind the ice dam can force water through small gaps in the roof, often causing interior leaks. Furthermore, ice dams and icicles will freeze-lock onto rough roofing (such as shakes and tiles) where they can easily pull the roofing and flashings off the building as thawing occurs. So, when thawing causes glacial movement and avalanche, the ice dams and icicles can easily pull the roofing and flashings off the building. This severely damages roofing components and is dangerous to anyone or anything below (Figure 7).
