How to Calculate Roof Snow Loads

Summary:Whether you are building an accessory structure in the backyard or trying to decide whether you need to shovel off your roof after a storm, you need a way to learn whether your roof materials are up to the weight--expressed as psf, or pounds per square fo...

Whether you are building an accessory structure in the backyard or trying to decide whether you need to shovel off your roof after a storm, you need a way to learn whether your roof materials are up to the weight--expressed as psf, or pounds per square foot--of the snowfall in your area. This pressure is increased or decreased based on the pitch of the roof. You can compute real-time snow load for quick decision-making or use official figures to design a new building.

Instructions

1 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.

2 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.

3.Collect a cubic foot of the snow on the ground, choosing a sample that is representative of the overall snow pack, and weigh it. Snow may range in weight from 7 up to 20 pounds per square foot, depending on how fluffy or dense it is.

4.Multiply the depth of snow in feet by the weight, from Step 3, of a cubic foot of snow. Let's say our 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--multiply the area of the roof in square feet by the snow load.

5.Estimate the total weight of snow in your yard by multiplying the size of your yard in square feet by the snow load, expressed as psf. For example, using a 25 by 20 backyard: 500 times 11.25 yields 5,625 pounds of snow. Use this same calculation to find the total weight of snow on a flat roof.

6.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 (permit-issuing) municipality, or you can obtain it from a website such as "Ground Snow by Zip." A search will turn up many such websites.

7.Measure the horizontal distance from eave to ridge--the point where the roof is tallest--and record the result labeled as "run."

For example, consider a roof with a run of 40 feet.

8.Find the rise. This is the difference between the height of the highest point in the roof (the ridge) and the height at bottom of the eave--the lowest point on the roof. Label this number the "rise."

For example, consider a roof with a rise of 10 feet.

9.Convert the "rise over run" information to a ratio of 12. The rise over run in our example is 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.

10.Go to a snow load calculator, such as the Cornell University calculator listed as the third reference in this article, and enter the information requested. For the Cornell calculator, this includes the ground snow load for your area (from Step 1), terrain, exposure, roof type, pitch, and run (from Step 2), which is expressed as "W" in the calculator. Based on an expected ground snow load of 30 psf, the snow load on our theoretical 3:12 roof is 18.9 psf.

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