How to Calculate Snow Load on Roof

It can be difficult to gauge exactly when one must take action and clear the snow off a roof. After all, there are many things to consider when calculating just how much weight a roof can withstand.

Thanks to our detailed article, you'll better understand what you need to think about when it comes to clearing snow off your roof.

Calculating the Weight of Snow on a Roof

How much weight can a roof withstand?

Source: Canva

To a degree, snow can be a positive thing. It provides additional insulation and somewhat protects your home from the elements.

However, anything over 20 pounds per square foot is considered unsafe. In fact, according to the Insurance Institute for Business & Home Safety, if your home isn't damaged or rundown, it should withstand that weight without structural impact. But, a few things need to be stated to paint a more accurate picture, as both roofing styles and snow conditions have an impact. 

Depending on the home, roofs vary in shape and design. In Quebec, one can notably find: 

• Gable roofs (pitched);

• Single-pitch roofs (shed);

• Roofs with valleys and cornices;

• Hip or multi-slope roofs (2 or 4);

• Roof ponds;

• Flat roofs.

Normally, the type of roofing is determined by a local engineer based on the location of your home and various other factors. Depending on the area and environment, roofing materials and structures will vary to withstand the region's changing snow conditions. 

If your home's structure has changed over the years, your roof may be more vulnerable to the elements.

Here’s a list of precipitation types that influence the snow mass on a roof:

The Insurance Institute for Business & Home Safety (IBHS) has issued general rules of thumb when evaluating the snow load on a roof.

1) For fresh snow

Ten to 12 inches of fresh snow is worth about an inch of water, or about five pounds per square foot of roofing. As a result, the roof can withstand a maximum load of four feet before it becomes at risk. 

2) Packed snow

Three to five inches of compacted snow is equivalent to about an inch of water, or about five pounds per square foot of roofing. So, the roof can withstand a maximum of 12 inches of snow before the roof is subjected to structural strain. 

3) For ice

Ice is much heavier than the rest: an inch of ice is equivalent to 12 inches of fresh snow. Therefore, it’s vital to keep a close eye on ice build-up. 

In Quebec, the temperature is extremely volatile in winter. As a result, several layers of snow can soon accumulate on your rooftop. Rain, sleet, or freeze-thaw weathering can easily create alternating layers of ice and compacted snow.

As a result, your home can sustain water infiltration and serious damage. To prevent these problems, inspect your roof on a regular basis. 

Thinking of renovating during the cold months, but you're still reluctant to do so? Check out our article 5 Reasons Why You Should Renovate During the Winter.

Things to Consider When Clearing Snow Off a Roof

Source: Canva

As you've no doubt realized, there are many things to consider before removing snow from your roof. First and foremost, the obvious factor is the amount of snow on your rooftop. 

Then, your roof's covering and slope will need to be factored into the snow removal process. In fact, a metal covering will make the snow removal process much easier than on a shingled roof, for example. The same goes for the slope: if it's weak (less than 3/12), the melting snow won't easily run off and will therefore create icicles. If your house is old, the weight-bearing requirements are different. Precipitation and temperature fluctuations will also have an impact on snow conditions and density.

To adequately protect your home, you must clear your roof of snow and call in the services of experts. That way, you'll be spared many setbacks. 

It's especially important to be aware of the dangers of clearing snow off your roof. If it isn't done properly, the consequences can be dire. In fact, it isn't uncommon to see irreversible damage done to a roof and its materials. It’s equally possible to slip and fall off a roof. So never climb atop your roof unattended without first knowing how to proceed with the task at hand. When snow slides off your roof, it's like an avalanche, and it can be fatal to whoever is standing beneath it. 

Carefully consider where the snow will fall and secure the area from any possible foot traffic. Don't attempt to remove all the snow: the ice layer adhered to the roof protects it. In such a case, you're more likely to damage the roof than be proactive. 

Roof Snow Load Calculator

Source: Sindre Fs - Pexels

Calculating the snow load requires one to take into account not only the depth of the snow but also its density across your roof. 

For a quick basic calculation, consider the following formula:

W = (D x 1.25 Ibs +  (I x 5.2 Ibs)

With: 

• D: Depth of snow measured in inches  

• l: Ice thickness in inches

W is the weight in pounds per square foot.

Keep in mind that rainfall changes the weight of snow on your roof. And, snow absorbs water and increases its volume weight. If you want to use a more accurate calculator to determine the weight of snow on your roof, there's the OMNI calculator.

This calculator takes into account the length, width, and slope of your roof. Depending on the amount and type of snow found, the calculator will also determine the value of the snow’s weight. 

Are you looking for experts to tackle your roofing project?

Fill out our form to receive quotes from certified contractors!

Signs that Point to an Excessive Snow Load on Roof 

Source: Kelly - Pexels

Ice build-up on a roof is incredibly dangerous. Ice weighs five times as much as snow and can also contribute to major damage inside your roof. During the winter, snow only melts in two instances: in direct sunlight, or if the heat generated from inside your home is seeping through the covering.

Certain warning signs are unmistakable in terms of snow overload on your roof. For instance, if your doors no longer close with ease as they once did, if they drag on the floor, or if you notice cracks in your walls, it's more than time to act. Unusual and repetitive crackling noises can also be a sign that too much snow has accumulated on your roof. 

Likewise, if your ceiling is buckling, you need to act right away. If too many of these warning signs are apparent, evacuate your home and take immediate measures to have the accumulated snow removed. 

As a homeowner, you're first and foremost responsible for the overall level of safety of your home, not only for yourself and your family, but also for anyone else who may come in close proximity to it. If you pay close attention to the warning signs mentioned above and avoid snow build-up, it’ll prevent you from facing disastrous consequences, such as a snow avalanche, or worse: your roof caving in.

When in doubt, contact local and professional roofers to confirm the maximum snow load. 

Curious about the cost of your roofing project? Try our cost calculator.

Source de l'image de couverture: Shawn Dearn - Unsplash