If you live in a winter wonderland, you should learn to calculate how much of that fluffy awesomeness is on your roof. Snow is pretty lightweight, or so it would seem, but two feet of snow on your roof is roughly 19 tons of additional weight.
Most roofs are designed to take a lot of weight. In fact, building codes require that roofs in your area are built to withstand average snowfall in your area.
There may come a time, like during a freak snow-storm or blizzard, when snow will be more than average, and removing it from your roof is necessary. Following is a calculation you can use to estimate the snow load on your roof.
Snow Calculation # 1
Keep in mind that snow weight varies depending on how dry, wet or packed it gets. You can also have a layer of ice under all of that snow, so Ice plays into the equation. Saturated snow weighs about 20lb per cubic foot. One Inch of Ice weighs 5.2lb per sqft.
Here is a quick calculation you can use to approximate the weight of snow on your roof. S x 1.25 = P. When S = Depth in Inches of snow on the roof, 1.25 = The weight of 1 sq ft of snow for 1-inch of depth, P = Pounds per square foot (lbs/sqft.)
The average roof can handle 20 lb of snow per square foot (sqft.) An inch of snow weighs 1.25 lb per sqft. Sixteen inches of snow on your roof weighs approximately 20 lb per square foot and ice weighs 5.2lb per sqft.
If you have 30 inches of snow on your roof, how much would that weight per sqft?
30″ of snow x 1.25lb per sqft = 37.5 pounds per sqft of snow load.
That’s 17.5lb per sqft over the 20lb capacity of some roofs.
Warning: Ice build-up on the roof needs to be calculated into the formula.
Add 5.2lb per sqft for each 1-inch of ice depth.
The Snow Load Calculation Isn’t Perfect
This calculation ( S x 1.25 = P) is based on a 25% moisture density, which may be conservative based on your snowfall. Saturated snow weighs approximately 20 pounds per cubic foot.
However, snow’s moisture content can range from approximately 1% to 33%, which relates to snow potentially weighing from 1 pound per cubic foot to over 21 pounds per cubic foot.
How To Remove Snow From Your Roof: Snow Rake
The safest way to remove snow is from the ground with a snow rake. Before you get started, clear a path 360° around your house so you can access the roof.
How To Remove Snow From Your Roof
- Use a snow rake. They come with handles in 10′ telescoping sections and can be purchased at most big-box stores.
- To rake snow, start pulling snow from the edge of your roof and work your way towards the top of the roof.
- Leave a thin layer of snow when raking, or you will damage or lift shingles or roofing materials.
- Avoid smacking windows with your rake.
- Be careful with hanging ice and chunks of snow.
- Stay away from electrical and overhanging lines.
- Use roof melt pucks (calcium chloride) to avoid ice-damming. Pucks are thrown on your roof to melt ice, just like you do on the driveway or sidewalk.
- IF you don’t think you can remove snow safely, hire a snow removal professional.
Warning: Don’t, whatever you do, walk on your roof
How Roof Pitch Effects Snow Load
Flat roofs and roofs with less pitch are more susceptible to snow. The higher-pitched your roof, the better chance you have of snow falling off, especially if you have a well-insulated roof.
- The most common buildings to collapse from snow are unheated outbuildings like carports, garages, sheds, and barns that allow snow to accumulate.
- If your home’s roof is pitched and you have the heat on, snow will probably melt off. Focus on areas of your roof that are flat or have less slope as these areas are more likely to accumulate a snow load.
- Flat Roof: With 18″ of snow on a flat roof, it should be removed. If you can’t remove snow from the ground, hire a professional.
Indications Your Roof Is Collapsing
If you get any indication, your roof is collapsing, get out of the house immediately. Call the local fire department to assess the situation.
- Roof is sagging
- Cracks in drywall
- Popping and creaking noises
- Severe Roof leaks
- Bowing walls, rafters, and trusses
- Doors and Window Impossible to open
Analysis of Roof Snow Load Case Studies: Uniform Loads, US Army Corps of Engineers, M. Orourke, Associate Professor, Civil Engineering, P. Koch Structural Engineer and R. Redfield Research Civil Engineer, Cold Regions Research and Engineering Laboratory link