What is the maximum snow load for a roof? There is no single depth that is the answer. It depends not only on snow depth but also on snow weight, your roof's construction, and the design of snow loads for your area. The depth of light, fresh snow is not a major concern, but the depth of wet, rain-soaked snow is, as it places the same roof under much more stress. This guide by Helpful Guys explains how much snow can a roof hold, how to tell when it is a warning sign, and when to contact a professional.
Key Takeaways
It is the weight of the snow and not its depth that is important.
Wet snow, ice, and rain on snow quickly pile up.
Roofs are rated for regional snow loads, but over time, renovations and drifting decrease the margin.
Warning signs include sticking doors, new cracks and creaking noises.
Clear snow from roofs with a snow rake or rake it off the roof; avoid walking on snow-covered roofs.
What Is The Capacity Of A Roof To Hold Snow? Why Weight Matters More Than Depth
Snow consists mainly of air and water, and therefore its weight varies depending on the volume of water that it contains. NASA Space Math estimates dry snow at approximately 50 kg/m³ and wet snow at approximately 200 kg/m³. This is approximately four times the weight at the same depth. A fire department roof snow load guide indicates that the water content of snow can vary from approximately 3% for very dry snow to approximately 33% for wet, heavy snow, and almost 100% for ice.
Fresh Snow
The air in fresh snow makes it light. A layer 30 cm thick weighs approximately 15 kg per square metre, at 50 kg per cubic metre. This is the easiest type to clear, but it can accumulate with repeated storms, and wind can cause it to form ridges or mounds.
Packed Snow
When snow settles, it hardens, and as the wind and temperature change, it hardens even more. The weight of the same depth on your roof in February can be quite a bit heavier than in December.
Wet Snow and Ice
The density of wet snow is close to 200 kg/m³, and that of ice is close to the density of water. The water that falls on an existing snowpack goes in and adds weight, but not depth, so the roof can appear the same and be much heavier.
How Canadian Building Codes Set Roof Snow Loads
Roof snow loads are determined from climatic design data for each location according to the National Building Code of Canada. The design load is based on the ground snow load in the area, plus rain falling on snow. It is then adjusted to account for roof shape, exposure and drifting. This is why regions such as the Prairies, the BC coast, Ontario, Quebec, and Atlantic Canada are designed for different loads.
Codes establish minimums, and buildings are designed to provide safety margins. Older buildings were constructed according to the building codes of their time, and many have undergone changes over the years. Any modification to a roof, for instance, the addition of solar panels, skylights or changes in structure, can alter the roof's snow resistance. If you are unsure about a building, have a structural engineer look at it.
How to Estimate the Snow Load on Your Roof
This is an approximate guide only. It is not a structural evaluation.
Measure the snow depth on a flat area at ground level, in metres.
Pick a density. Apply approximately 50 kg/m³ for dry snow and approximately 200 kg/m³ for wet snow.
Multiply depth by density. For example, 40 cm of dry snow is approximately 20 kg/m², and 40 cm of wet snow is approximately 80 kg/m².
To obtain a better reading, press a straight-sided container deep into the snow, melt the sample inside and take a reading of the water level. One litre of water over one square metre is equal to 1 kg.
This is only an indication of the load. It won’t tell you if your roof will support it, and drift loads can increase the actual load on the roof. Only an engineer can compare snow loads against the actual capacity of a roof.
Warning Signs of Too Much Snow on Roofs
Inside the Building
Sticking or jamming doors or windows
New cracks in drywall or plaster, particularly around door frames
A ceiling that droops or is crooked
Creaking or popping noises
New leaks or stains
Outside the Building
A sagging ridge or roofline that is visible from the outside
Bent or bowing posts and supports on garages and carports
Eavestroughs pulling away from the structure
Ice accumulation on the edge of the roof
Ice dams and blocked drains increase weight on the edge and push meltwater under shingles. Our eavestrough and drain page explains how to keep water moving.
If you see several of these symptoms, evacuate the area and contact a structural engineer.
Roofs at Higher Risk
Flat Roof Snow Load
Unlike steep roofs, flat and low-slope roofs do not shed snow, which leads to increased snow accumulation. Drifts can build against walls and rooftop equipment, and meltwater can refreeze. Commercial owners can read about our commercial snow and ice services. For any large flat roof, it is advisable to have a written snow plan, including action points.
Older, Renovated or Extended Buildings
The reserve capacity may be less for older homes, and the distribution of loads may be affected by renovations. Snow is prone to drifting at the step where new framing meets old framing.
Garages, Sheds or Carports
An Insurance Bureau of Canada spokesperson has said that outbuildings such as barns and sheds are some of the most susceptible to collapse. They are often light-framed and unheated, so snow remains rather than melting off.
When to Remove Snow From Roofs
There is no set depth at which it should automatically be removed. Consider the circumstances instead:
After heavy or wet snowfall
If snow falls when there is already snow on the roof
If drifts form or several storms occur in succession without a thaw
Any time you see the warning signs above
Roof Snow Removal Safety: How to Remove Snow Safely
The safest way is from the ground with a roof rake that has a telescoping handle:
Remove snow in layers, starting at the edge of the roof and working evenly across it.
Do not clear only one area of the roof. Partial removal may put stress on the remaining roof, potentially leading to a partial roof collapse.
Leave a thin layer of snow (a few centimetres) to protect shingles. A depth of approximately 5 cm is recommended by the Insurance Bureau of Canada in insurer guidance.
Avoid using salt, as it can corrode roofing materials, and avoid metal tools.
Stay clear of falling snow, and keep the rake away from power lines.
A rake can only reach the bottom edge of the roof. For anything beyond its reach, you’re better off hiring a professional.
When to Call a Professional or Structural Engineer
Knowing when to call a professional or structural engineer.Do not walk on a roof covered with snow. Provincial occupational health and safety regulations require fall protection when working at height, and edges, skylights and vents may be hidden in the snow. The Insurance Bureau of Canada recommends not removing snow from a roof alone. Seek professional help if:
Snow is out of reach of a rake.
The roof is flat or low-slope.
Ice accumulations are visible.
If you notice the roof sagging, new cracks or creaking noises, contact a structural engineer. Once the snow has been cleared, a roof repair and roof inspection visit can be used to check for roof damage. For commercial owners, it's important to have a snow removal plan in place before winter.
Frequently Asked Questions
How much snow is too much for a roof?
No standard number exists. This depends on the type of snow, roof design, roof age and the design load for your area. Be aware of warning signs and have your roof checked by an engineer if you have any doubts.
How heavy is snow on a roof of one cubic metre?
Dry snow weighs approximately 50 kg and wet snow approximately 200 kg, but ice is much heavier, with a density close to that of water.
Should I remove snow from my roof after every storm?
Usually not. Watch for heavy snow, rain on snow, drifting, or signs of stress.
Can snow collapse a garage roof?
Yes. Garages, sheds, and carports are among the most susceptible structures because they are often unheated and lightly framed.
Can I shovel my own roof?
It is not recommended. Falls pose a serious risk of injury. Rake snow from the roof while standing on the ground, or work with a professional.
Does roof snow cause ice dams?
Yes. Any cold surface which loses heat causes snow to melt, flow to the cold edge, refreeze, and contribute to the loading of the ice.
Conclusion
Roof snow load is not only affected by the depth of the snow, but it is also influenced by other factors such as snow conditions and the roof design. Look for warning signs, avoid hazards from severe storms and leave climbing to the pros. Our residential snow removal services are a solution for those who prefer not to worry about snow this winter. Contact Helpful Guys anytime, and we are ready to help you clear your roof!
