Snow loads – when to shovel your roof

Question 1:

What are signs of heavy snow load on my roof?

Answer to question 1:

If you’re concerned about the weight of snowfall on a roof, look and listen for signs of stress. These signs can include unusual sounds from the roof or supporting walls, visible movement of walls or sagging of ceiling and cracking of drywall or plaster. Other signs may include doors that jam or water leaks that show up after a big snow fall.
The configuration of your roof could be a factor. Pay special attention to areas on your roof where local snow accumulation occurs on a portion of a roof; for example, near dormers, valleys or on shed roofs. Roofs with a low slope or roofs covered with rough roofing material tend to accumulate more snow than steeper pitches or slippery roof finishes.

Question 2:

What can I do myself?

Answer to question 2:

You should never clear a roof yourself. Instead, opt for a qualified and insured snow removal company. Where the roof is accessible from the ground, you can reduce some load by using a snow rake to safely pull the snow off the roof from ground level, but even then you need to be careful.

Question 3:

How much snow can my roof hold?

Answer to question 3:

That’s hard to answer – because it’s not the depth of the snow, but really the weight of the snow that matters. The weight of snow can differ quite a bit; for example, compare lifting a heaping shovel of deep fluffy snow with a half-empty shovel of slushy snow. The building code requirements are based on snow load and geographic location as described in question 4. However, the same requirements may not apply to small accessory buildings such as sheds. You should monitor their snow loads as well and, if you can, rake them more regularly from the ground outside.

Question 4:

What are the building code requirements?

Answer to question 4:

The building code requirements are based on snowfall observations converted to a snow load. The roof is designed for a combination of snow and rain load according to a table of locations in the National Building Code (NBC). The roof is then designed to support a minimum of 1 kilo Pascal (21 pounds per square foot) or higher.
Roofs on today’s large buildings are designed for 1-in-50 year snow load events and take into account factors such as roof shape and accumulation. The trusses for today’s houses and smaller buildings built under Part 9 of the NBC are designed according to a simplified snow load equation. This is because of the relatively smaller size and historical performance of these types of buildings.

Question 5:

Does the age of my house matter?

Answer to question 5:

Code requirements and design methods have changed over time, as have the construction materials used. Roof trusses have been popular for several decades due to ease of construction and flexibility in design, while older roofs were typically hand-framed with rafters. That’s not to say that having a 40 year old roof is cause for concern; in fact, it has demonstrated its performance over 40 winters.

Question 6:

What’s the best way to deal with all of this?

Answer to question 6:

The best way to assess the snow on your roof and to answer all your questions is an on-site visit by a local expert. You can contact a building science specialist in your area. These are typically architects or consulting engineers. In some cases home inspectors also have this expertise.

Question 7:

Are there other resources?

Answer to question 7:

If in doubt, you can check with your local building department to determine to which requirements your house was built and whether the current snow accumulation would exceed that design load.

Here are some online resources:

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