How to Determine the Weight of Snow on a Flat Roof

Measure your roof's surface area. Use a tape measure to find the distance in width as well as length.

Multiply your roof's length by the width to find the total surface area. Record the area for later calculations. Since you have a flat roof, you must find the area of a square; however, if you had a traditional roof, you would need to measure the area of both roof faces.

Climb up to the roof and mark off an area measuring one square foot — 12 inches wide by 12 inches long. Clear away the edges around the marked area to ensure only 1 square foot remains.

Scoop up the snow in the area you marked. Shovel it into a large plastic garbage bag. Remove only the snow in the square foot, clearing the spot until you reach bare roof.

Weigh the garbage bag full of snow immediately. Record the results. This represents the snow weight placed on one square foot of your roof's surface.

Multiply the weight per square foot you obtained by the total square footage of your roof. For instance, if the snow in 1 square foot weighs 10 pounds and your roof is 800 square feet, the total weight of the snow on your roof is 8,000 pounds. There is no need to convert this total to pounds per square feet or cubic feet. The total weight is independent of the depth.

Use the amount of snow in one square foot of your yard and substitute for the roof snow if necessary. However, this amount may vary from the roof due to factors such as drifting, melting or nearby blocks that prevent the snow from accumulating.