How Much Snow Will Collapse a House Roof?

Local Building Codes

• Roofs must be strong enough to support not just their own weight, but also the pressure of wind and weight of precipitation peculiar to their latitude and elevation. Building codes in a state such as Arkansas, where snow is infrequent and does not stay on roofs long enough to form thick ice dams, may require roofs to support weights of up to 25 pounds per square foot. On the other hand, where snow piles up for long periods, as in North Dakota, codes require roofs to support 30 to 40 pounds of weight per square foot. Roofs that do not conform to local codes may bend or collapse, but, unless the winter’s snow is extreme, roofs built to code will carry their load.

Snow Versus Water

• A cubic foot of water weighs 62.43 pounds, a weight that would seriously stress most roofs. Snow contains from 5 to 33 percent water depending on whether it’s the dry, fluffy or wet, heavy variety, according to Dr. Karl VanDevender and Doug Petty of the University of Arkansas Agriculture extension. A roof with a 25 pound-per-square-foot snow load could carry 96 inches of dry snow or 14 inches of heavy, wet snow. Unfortunately, light snow grows heavier on the roof over time as convective heat from the house and radiant heat from the sun melt it. Melting snow runs over the roof surface underneath the snow to the gutters where it freezes in the night air. As ice dams form, their weight increases, straining the edges of the roof.

Determining the Load

• Your roof is designed to carry a snow load determined for your area, say 20 pounds per square foot. A 12-inch snowfall of light, fluffy snow will yield less than 5 pounds per square foot. Dump a foot of heavy snow -- which weighs a bit more than 20 pounds -- on top of that, and you’ve exceeded your roof’s load limit. You can accurately measure the load by collecting a cubic foot of snow, melting it, measuring the new volume and multiplying 62.43 by whatever fraction of a cubic foot of water that you have. Your local building inspector can tell you your local snow load requirements.

Warnings

• Occasionally, local officials caution homeowners to remove snow from their roofs. Frequently, these warnings are due not just to the weight of snow but to the probability that ice dams will form, damaging the roof and allowing water to seep into the house itself. Raking snow off the lower part of the roof keeps it from piling up and gaining weight in an ice dam.