When building an accessory structure in the backyard or trying to decide whether you need to shovel off your roof after a storm, you need to know that your roof materials are up to the weight -- expressed as pounds per square foot -- of the snowfall in your area. This pressure is increased or decreased based on the steepness of the roof and weight of the snow, which may range from 7 to 20 pounds per square foot, depending on how fluffy or dense it is. You can compute real-time snow load for quick decision-making or use official figures to design a new building.
On a Flat Surface
Push the ruler or yardstick into the snow vertically in a spot that is representative of the overall snow depth and record the depth in inches.
Convert your depth measurement to a figure expressed in feet. For instance, using a snow depth of 15 inches: 15 divided by 12 -- the number of inches in a foot -- yields 1.25 feet.
Collect a cubic foot of the snow on the ground, choosing a sample that is representative of the overall snow pack, and weigh it. Don't pack the snow into whatever container you use. This will give you an inaccurate weight.
Multiply the depth of snow in feet by the weight of a cubic foot of snow. Let's say your snow weighed 9 pounds per cubic foot: 1.25 times 9 comes to 11.25, so the amount of pressure the snow is exerting on the ground is 11.25 pounds per square foot.
Estimate the total weight of snow in your yard or flat roof by multiplying the area in square feet by the snow load, expressed as pounds per square foot. For example, using a 25 by 20 backyard: 500 times 11.25 yields 5,625 pounds of snow.
Expected Snow Load on Pitched Roofs
Obtain the recorded snow load -- the maximum load expected to accumulate on the ground -- for your area. This information would be available from your local permitting authority, or you can obtain it from a website such as "Ground Snow by Zip." A search will turn up many such websites.
Measure the horizontal distance from eave to ridge -- the point where the roof is tallest -- and record the result labeled as "run."
Find the rise. This is the difference between the height of the highest point in the roof, called the "ridge," and the height at bottom of the eave -- the lowest point on the roof. Label this number the "rise."
Convert the "rise over run" information to a ratio of 12. For example, if rise is 10 and run is 40, the calculation is a fraction: 10 over 40, which reduces to the fraction 1 over 4. This is equivalent to 3 over 12. Expressing this in ratio form, our sample roof has a 3:12 pitch. It rises 1 foot for every 4 feet of horizontal distance from the eave.
Go to a snow load calculator, such as the Cornell University calculator in "Resources" and enter the information requested. For the Cornell calculator, this includes the ground snow load for your area, terrain, exposure, roof type, pitch, and run, which is expressed as "W" in the calculator. Based on an expected ground snow load of 30 pounds per square foot, the snow load on the 3:12 roof is 18.9 pounds per square foot.