How to Calculate the Snow Load for a Roof Pitch

Check a U.S. Forest Service website for information on snow loads by state; it will provide figures or refer to sources that calculate snow loads by region, often using Natural Resources Conservation Services SNOTEL -- SNOwTELemetry -- statistics gathered by automated weather stations. These stations record such information as depth of snow, water content and temperature (which affects melting) to calculate a ground snow load for a specific area.

Apply ground snow load statistics to roofs using American Society of Civil Engineering standard 7-05. This takes into account such variables as pitch of the roof, roofing materials, thermal characteristics of the structure and exposure to wind, all of which affect the accumulation of snow on a roof. Steep slope roofs shed snow better than low slope, for instance, and metal roofing sheds snow better than rough surfaced shingles.

Consider other factors, such as one roof sloping onto another, so snow piles up on the lower roof, or drifting, where wind forces snow to pile up in one section of a roof. Adjust snow loads to account for frequency of snows and accumulations; frequent snows in areas where temperatures remain near freezing will accumulate more than those in regions where snow melts quickly.

Adjust roof pitch to snow load conditions. Flat or low slope roofs work well in mild climates where snow rarely falls. Medium pitch roofs, from 5 to 8 inches of rise per foot, are most common in temperate zones which rarely get deep, heavy snows that accumulate. Steep pitches of 9 to 12 inches per foot are used in mountain regions or areas where heavy snows are common and snowpacks build up.