Its snowing outside and you are probably wondering, should I shovel snow off my flat roof. Here is a simple rule of thumb you can use to help if you are concerned: shovel snow off your roof if it exceeds two feet in depth. If you want to play it safe, do not let the snow exceed a foot or a foot and a half. Two feet of snow weighs about 30 lbs per square foot (more details below), which is about how much the average house’s roof can withstand. Some stronger roofs in areas with great snowfall can withstand over 40 pounds of snow per square foot.
The earlier calculation of 30 lbs is based on a 25% moisture density. On average, saturated snow weighs approximately 20 pounds per cubic foot. The moisture content of snow ranges from 1% to 33%, which means that snow can weigh from 1 pound per cubic foot to over 21 pounds per cubic foot.
Calculation: S x 1.25 = P
S = inches of snow on the roof (depth)
1.25 = weight of 1 sq ft of snow for each 1 inch of depth
P = Pounds per square foot (lbs/sq ft)
Example: If the snow on your roof is 20-inches deep, should you shovel it off?
20-inch roof snow depth x 1.25 lbs/sq ft = 25 lbs per sq ft of snow load
In this example, you should shovel it off since it is very close to the roof’s limit.
Important: If there is any ice build-up on the roof, you need to account for this in the formula. Add 5.2 lbs for each 1-inch of ice depth.
Remember you can always check your roof snow load by cutting a one foot by one foot square of snow from your roof and weigh it in a plastic bag to determine is getting close. Don’t forget to account for ice build-up as well.
Please be aware of the potential dangers of shoveling or raking snow from a roof. Besides the potential damage to your roof, there are other factors to worry about such as a person sliding off the roof or falling off the ladder.
If you want more information on what your roof can withstand, Seva Call can help you find a local roofer to consult with.