Snow loading is the downward force exerted on structures by the weight of accumulated snow. The roof of any building is the area subject to the greatest weight, temperature, and moisture extremes. A roof in an older building may be "underbuilt" and therefore more vulnerable to structural problems. A roof may also have been subjected to common abuses that include:
- insulation without adequate roof deck ventilation,
- reroofing over three or more layers of shingles, and
- employing ineffective practices to correct symptoms of structural problems.
Snow loading is determined by factors influencing snow and ice accumulation on the structure, and vary considerably by geographic location. The fact that a roof has endured many winters of heavy snow does not mean it will last indefinitely. Roofs of most older buildings were built with little or no insulation, so snow melted fairly quickly and roofs carried snow loads for very brief periods. If insulation has been added to a roof, less heat escapes, so snow and ice don't melt as rapidly and snow loads accumulate to new and sometimes excessive weight levels. The roof deck may not have been properly ventilated when insulation was added, therefore ice dams and subsequent backup of snowmelt under shingles and excessive condensation due to moisture transfer from the interior of the building may occur. This creates favorable conditions for deterioration of roof structural members and a decrease in fastener holding ability.
Another variable affecting a roof's snow-bearing capacity is roof dead weight, or the weight of all the roofing materials (sheathing, roofing paper, felt, shingles, or built-up membrane).
Several factors affect the amount of snow that can build up on a roof. They include:
- roof pitch (snow is less likely to slide off flatter roofs--3/12 pitch or less),
- drifting (wind blowing snow around other buildings and trees can create huge snowdrifts and uneven snow loads),
- lean-tos or other low roofs that receive snow or ice sliding off roofs above them,
- shingles or other roof decks which do not shed snow as easily as metal roofs,
- roof valleys or other roof areas which collect a lot of snow.
How much weight can a roof take? That varies. Snow loads for agricultural buildings in southern and western Minnesota are generally around 20 LB per square foot, not including the weight of the wood that makes up the truss or rafters, a ceiling if one is added to the lower cord of a truss, any equipment hung from the trusses or rafters, or wind loads. Many roofs for livestock barns and machine sheds are designed for a total load of 25 to 30 LB per square foot. Most house roofs in the eastern and northern parts of North Dakota should be built to hold 30 to 40 pounds of snow per square foot. In the southwestern part of the state, where snowfall is typically lighter, roofs are built to hold less; about 30 pounds per square foot.
The weight of the snow on a roof also varies, in turn varying the snow loading on the roof. A cubic foot of snow can weigh from seven pounds for snow that is new and dry to 30 pounds for old, compacted snow. Rain falling on accumulated snow will add more weight. Drifting snow can put excessive loads on the areas where it piles up, such as against equipment or penthouses, or at walls between roof levels.
Additional information on snow loading can be found at the following places:
Ice and Snow Accumulations on Roofs
Describes ways to determine a roof's snow-bearing capacity. From the the University of Arkansas, United States Department of Agriculture, and County Governments Cooperating.
Calculating Loads on Headers and Beams by Paul Fisette
Understanding how loads are transferred through a structure and act on structural members is the first step to sizing headers and beams. From Building Materials and Wood Technology at the University of Massachusetts.
Heavy Snow Loads
By Curt A. Gooch, Senior Extension Associate, Cornell University.