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Snow on a Roof – How much snow can safely be on a roof?

by | Dec 17, 2023 | Snow, Snow Removal

So you’ve got snow on your roof, should you be concerned?

Through the winter season snow accumulates on your roof. It’s inevitable. Depending on your home or building construction, this may be a problem, and you’re probably wondering if you should remove the snow from your roof. Factors to consider include depth of snow, heaviness of the snow, your roof design, ice damming, and potential for moisture to enter your building. Generally speaking, for a residential home, more than a foot of heavy wet snow on the roof could be a problem and should be removed.

In this post we’ll look at how to determine whether the snow build up on your roof is of a real concern. In the case that it should be removed we’ll discuss some classic roof snow removal strategies.

How heavy is snow?

Snow density averages about 20 pounds per cubic foot, or 320 kilograms per cubic meter. This depends mostly on the moisture content of the snow. Light fluffy dry snow obviously weighs less than moist dense snow.

Calculating snow weight for your roof

To calculate the weight of the snow on your roof use the following general formula. Remember, the significance of snow on your roof depends on the design of your structure.

Weight of Snow = (Length of Roof x Width of Roof) / (Degree of Slope x Snow Load) Snow load for the purposes of this formula can be found by multiplying the snow depth by the snow density. 

You can now take the weight of snow for the whole roof and divide by the total area of your roof. Typically if this value is 20 lb/ft² (100 kg/m²)  then you should remove snow from your roof.

How much snow is safe on a roof?

What is safe and what is not safe will vary greatly from building to building when it comes to snow accumulation.

If for instance, you are concerned about structural integrity of your roof system, for a wood frame residential home, likely upwards of a foot or two of heavy dense snow accumulation is cause for concern for a sloped roof.

Other commercial buildings and institutions are designed to carry more snow load.

Flat roofs, because of the nature of design, can be of more concern, because snow will not slough off naturally.

Danger levels of snow on a roof

Generally speaking dangerous levels of snow accumulation on a roof are about 2 feet of wet snow or about 4 feet of light fluffy snow.

Unseen structural damage is always a concern, and so is ice damming.

Your roof system would have been designed to support a specific snow load, and if exceeding that load, then it’s time to organize roof snow removal, or an inspection for professional advice.

What is snow load?

Snow load is a factor that engineers and roof manufacturers use to design structural roof systems. This includes wood roofs, made of wood truss systems, metal deck roofs, and even concrete roofs in commercial buildings.

Your home or building would have been designed to withstand snow loads specific to your geographic location and local building code requirements.

Snow loads from a design or engineers perspective

In Canada, and as per the National Building Code of Canada (NBC 2020), ‘specified snow load’ can be calculated using a formula found in article 4.1.6.2. where the snow and associated rain accumulation on a roof or any other building is calculated for load, variable S.

It takes into consideration importance factors, 1-in-50-year ground snow load, basic roof snow load, wind exposure, slope factor, accumulation fact, and rain loads.

Calculations like this, and others, are used to determine the structural requirements for the roof system on your home or building.

If you needed to know the specific snow loads your home or building was designed for, the information could be in your blueprints. If you don’t have access to the blueprints you can check with the municipal government office or building inspectors office, as they may have a copy of your building’s blueprints on file.

Roof pitch for snow

If you have a steep pitch on your roof you’ll be less at risk for snow accumulation on your roof. A steeper incline roof is best as it allows snow to fall more easily off of the roof. Most roof designers would recommend having a roof pitch of no less than 1:12 if you are in an area that is prone to heavy snow.

What about ice damming?

Ice damming is crucial factor to consider when assessing necessity of roof snow removal.

An ice dam can cause structural or membrane damage to your roof system over time. It’s a serious cause for concern, if ice dams sit for long periods, meltwater and moisture could penetrate into your building or home.

Ice damming occurs when heat from your roof or attic space causes snow to melt on the surface of the roof. The melted water then flows down your roof deck surface. Because the temperatures outside still are freezing, the melted water can get trapped under snow layers, near the edge of the roof, and freeze on the deck. Over time the melt water and ice build up cycle continues, until you have an ice dam situation, which worsens over time if it’s not taken care of.

Image source: https://extension.umn.edu/protecting-home-rain-and-ice/dealing-and-preventing-ice-dams

The results can be catastrophic, including structural damage, and mold or mildew buildup in the building interior. Insulation can get compromised, and the overall performance of your roof and building envelope system deteriorates.

Not only that, paint, soffit, and siding materials can be damaged. It’s best to call out a professional service to remove ice dams, as depending on the severity of the issue, allot of work is involved. After removing an ice dam, it’s ideal to get the areas affected inspected, in preparation for remediation of the areas later.

How heavy is ice?

Consider that for weight about one inch of ice is equivalent to about one foot of snow.

Roof features affect snow accumulation

Not all homes and properties are built the same. In some scenarios a home or property will have parapets, valleys, vents, stovepipes, skylights, chimneys, dormer windows and solar panels.

You can try and predict where snow might build up on your roof due to wind direction and speed.

In general the design of your home or building’s roof features should have taken into consideration snow loads, snow drift, wind, drainage routes, and building envelope considerations.

From a roof snow removal perspective, it’s best to treat roof features just the same as the rest of your roof.

So it’s time for the snow to go?

How to remove snow off a roof

In most cases you can use a roof rake to clear snow off of a roof. To use a ladder when removing snow off of a roof ensure it is secured at the top and bottom. This will ensure that when snow does inevitably come off of the roof, it doesn’t case your ladder to fall over. A roof rake allows you to stay on the ground and pull snow down. Most roof rakes have a telescoping handle, giving you adjustable reach. Any local hardware store should stock these.

As a general rule of thumb avoid the use of any chemicals and sharp tools. A sharp tool could puncture the membrane adhered to your roof sheathing or damage the shingling. Chemicals could cause leeching into the substrate and compromise the adhesion of the roofing membranes and other adhered products. Make sure to check your roof warranty for additional considerations when it comes to working on or removing snow from your roof.

If you are not confident that you can remove the snow from your roof get a professional roof snow removal company out to inspect the property, get you a quote, and take on the work on your behalf.

How to use a rope to remove snow

You can utilize nylon ropes or tensioned wires along a roof line to allow for snow to be displaced and fall off of the roof. The ropes can be pulled to disrupt the snowpack and allow it to sluff off the roof in a controlled fashion. This a great method to proactively alleviate snow load concerns prior to the snow build up becoming a hazard.

Roof snow removal with shovel

If there is not too much snow, and it’s safe to do so, removing snow with a shovel is always possible. This is only really possible where the pitch of the roof is not too steep. You will also need clean and clear access to the roof, so tie off your ladder at the top and bottom.

If you were to consider a de-icing product, use something less corrosive

If you were to consider using a de-icing product try a magnesium chloride product, as it is less corrosive than other de-icing product. This is amore suitable choice to prevent potential damage to the roof system. Check with the manufacturer of the product to determine the amount needed. It would also be wise to check with your roof system installer, and product date, to see what products might be best for your roof system.

Snow on solar panels

If you have snow on solar panels of a roof you need to use a soft brush to gently remove the snow. Don’t bother scraping ice off the panels yourself, rather leave that to a professional.

What about ice?

Consider that for weight about one inch of ice is equivalent to about one foot of snow. Use picks or other metal tools to chip away at ice build up on the roof or in gutter. Clear drainage routes of ice in preparation for the snow-melt season.

Pre-emptive measures

Snow guards or snow cleats

A snow guard or snow cleat prevents snow from suddenly sloughing off roof sides. This allows accumulated snow to intentionally collect along the edges of a roof, where it will naturally melt over time. Snow cleats are a great safety  feature that protects people and property below.

Heating cables

Consider using heating cables to remove snow from your roof and prevent ice dams from forming. Various heating cables could be run on the roof and have enough heat to melt snow build ups and prevent ice damage.

Insurance details

Most commercial insurance policies would cover damage caused by roof collapse due to snow load. It’s nonetheless wise to take steps to mitigate the chance of that happening.

Is melting snow flood damage?

Depending on the details of your insurance policy consider the fact that snow melt and intrusion into a building could be considered and argued as flood damage. Some residential home insurance policies do not cover floor damage.

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