Very little in life is lighter than a snowflake. But snowflakes do not retain that feather-weight quality when thousands of them are massed together. That's why a roof snow load formula came into existence. Using the formula, you can figure out just how heavy a load your roof is carrying.

Chimney roof house under winter snow
credit: Jasmin Merdan/Moment/GettyImages
How to Calculate Roof Snow Loads

Roof Snow Load Calculations

As a homeowner living in an area with white winters, you need to know how much snowfall your roof can take before it collapses in on you. Or more precisely, you have to be sure that your roof is able to carry the pounds-per-square-foot weight of your region's typical snowfall. The snow pressure will differ depending on whether your roof is flat or pitched.

Flat Roof Snow Load Formula

If your roof is flat, it is more likely to have snow-load problems than if it is pitched. You need to use a snow load formula for flat roofs. First, figure out how deep the snow is piled up there. The easiest way is to push a yardstick into the snow on the roof, selecting an area that looks typical of the overall snow depth. Express the result in feet rather than inches. For example, 18 inches would be 1.5 feet.

Next, gather snow from the ground into a container that is 1 foot high, wide and deep. Weigh the container before you fill it with snow, then weigh it afterward. The weight will differ depending on whether the snow is dry or packed. A cubic foot of dry snow weighs about 6 to 8 pounds, while 1 cubic foot of packed snow can weight 20 pounds.

To figure out the load on your roof, take the depth of snow in feet and multiply it by the weight of a cubic foot of snow. If the snow weighs 10 pounds per cubic foot and there are 1.5 feet on the roof, each square foot of the roof is getting 15 pounds of pressure. If your roof is 1,000 square feet, the total snow load is 15,000 pounds of snow.

Pitched Roof Snow Load Calculator

Ask local authorities about the maximum snow expected to fall in your town. Then, calculate the pitch of your roof, meaning how much it slants. To do this, you need to measure the rise, which is the vertical distance between the peak of the roof and the edge, and the run, which is the horizontal distance from the peak of your roof to the edge. Divide the rise by the run, then convert the fraction to a ratio of 12. For example, if the rise of your roof is 15 feet and the run is 30 feet, then the pitch equals 15 feet divided 30 feet, which is 6:12. This roof rises 1 foot for every 2 feet of horizontal distance from the edge.

After that, you'll need to use a snow load calculator. It's quite easy to find one online. Enter the information requested – like the rise and the roof, ground snow load for your city, exposure, roof type etc. – then follow the instructions. The calculator will tell you the pounds per square foot of snow.