Collapse from snow is rare for Haliburton County residential homes
By Darren Lum
Feb. 7, 2017
The strength of your roof is on the inside, said a local construction company owner.
Gary Burtch has been in the construction industry for 40 years, 33 of which as the owner of G. J. Burtch Construction Enterprises Ltd., a company based in Haliburton. He believes if a home is built to code using a truss system then it is capable of supporting the accumulation of snow we get in the area.
Contemporary truss roofs, he said, are built to specifications consistent with the snow received in the area. As long as the roof was built to provincial code it will be fine.
“Should. Underline should. In construction we [say it] always should be fine,” he said.
The National Research Council of Canada’s website (www.nrc.cnrc.gc.ca/eng/solutions/advisory/codes_centre/faq/snowloads.html) offers several answers to commonly asked questions related to this topic offers several answers to commonly asked questions related to this topic such as what are the signs of heavy snow load on the roof; what can a person do; how much snow can a roof hold; what are the building code requirements; does age of the roof matter; best way to deal with it and other resources.
Any concerns for the weight of snow on a roof can be identified from signs of stress, whether by sight or sound. Check for the movement of walls or a sagging ceiling and cracking of drywall or plaster, including doors that jam or water leaks after a snowfall, the website said.
Also pay attention to the roof type and where the snow accumulates on it. A low sloped roof will have more than a steep pitched or slippery finished roof.
The government discourages people from going on their roofs and recommends people to hire “insured snow removal” companies to perform the work on the roof. Where the roof is accessible from the ground it says people can use a “snow rake” to reduce the load.
When it comes to how much a roof can hold the website said the weight is determined by its makeup and not by its depth. It compares the difference between how fluffy snow is lighter than slushy snow regardless of depth. Although building code requirements apply to dwellings, “small accessory buildings such as sheds” do not adhere to the same standards.
Building code requirements are based on snowfall observations converted to a snow load. Roofs are designed according to a table of locations in the National Building Code (NBC). Roofs in Canada are designed to support a minimum of one kilopascal, which 21 is pounds per square foot or higher. For example, the table provided said for Mississauga, Ont., the requirement is 1.06 kilopascal or 22 pounds per square foot while Banff, Alta’s is 2.26 kilopascal or 47 pounds per square foot and Cape Harrison, Nfld. is 4.20 kilopascal or 87 pounds per square foot.
“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,” it states.
Age is not necessarily a determinant in assessing a roof’s strength even though code requirements have changed over time. Roof trusses are now more common than the hand-framed rafters of the past, but a 40-year-old roof is not necessarily weaker.
The best way to make an assessment about a roof is to have a local expert visit your home. It recommends contacting “building scientists,” which are architects, consulting engineers and in some cases home inspectors.
The other resources for information come from the Canadian Mortgage and Housing Corporation and the Institute for Catastrophic Loss Reductions linked by the National Research Council website.
Burtch wouldn’t get on the roof now, but he admits to earning some cash in his late-20s by clearing roofs. He doesn’t offer the service through his construction company, but knows there are people available to hire for such a service in Haliburton County. Lightening the load of snow from your roof can’t hurt it if done properly.
For a layperson there are distinctive visual cues between a truss and the rafter roof.
The truss roof is easily identified by its “webbing” or the “w” shape while the rafter roof includes rafters and collar ties or ceiling joists to form a pattern that appears to look like a triangle. He said there should be two levels of support for the rafter roof. One low to the ceiling and another close to the peak of the roof, the collar tie.
“If you just have rafters and nothing else you have a problem,” he said.
More and more people are breaking out the toolbox and working on projects to change or simply add living space in the attics of homes and cottages by removing the collar ties. This is where rafter style of roofs can become compromised.
“Some people do and that’s when you run into troubles. That’s a rafter system with a collar tie that gives the roof structural strength. They get in there an knock the collar ties out to make a loft or something you just lost your structural strength,” he said. “I always say to people if it’s a trussed roof rated for this area you [should not] have a problem. Anything else you have to look at each particular spot [to assess].”
He cannot remember hearing about any structures suffering a snow collapse, but admits this doesn’t mean they haven’t happened in the county.
The OPP say since 2005 there was one report of a residence with a roof collapse in 2014.
Building inspector Pam Sayne of Know Your Home Inspections Inc. referred to the government websites as the best source of information, but reiterated what was stated on the website.
“Snow becoming more dense and often to ice is much heavier adding to risk. A combination of weather conditions and the warm moist air leakage from the house to the attic area can add to heavy ice and the weight of the snow load,” she wrote in an email.
One measure that will not only prevent ice, but also save on heating cost is to make sure the roof area is not letting in air, she said.
“Reducing air leakage through various air sealing methods is important too. It not only reduces ice loads on the roof, but also saves on fuel costs. I have seen some homes with warm moist air entering the attic in the winter making the attic look like a refrigerator that requires defrosting. Conditions this bad can lead to deterioration of wood members in the attic over time and mould growths due to excessive moisture,” she said.
Sayne adds using the blower door test, which is part of an energy evaluation, can help home owners identify air leakages.
She said another concern is the snow coming off metal roofs.
“It can sound like a freight train as it begins to move. Without warning the snow may fall leading to serious injury of people and pets below. It is important to have snow guards on metal roofs to help reduce this threat. Also, the metal guards must be well secured to the rafter area to be effective,” she said.
The trend of new buildings replacing old ones is on the rise and is attributed to the low collapse rates, Burtch said.
“More and more of the old stock is getting replaced by the new stock, which has trussed roofs,” he said.
This has reduced the incidents of roofs collapsing under the weight of snow. Also, public awareness has contributed to the reduction because people clear them.
Often overlooked are the structures not built with the snow in mind, like sheds. Sometimes, he said, sheds are suscepitble to a weak structure, but could easily be cleared with a snow rake.
For all of this, Burtch acknowledges there are examples of structures on seasonally used properties that defy logic.
“I have seen roofs that should have collapsed and never collapse and be there for 40 or 50 years. If you went in there in the wintertime you probably couldn’t open and close the door. Come spring when [a cottager comes up] in May and everything works fine. It’s just one of those things,” he said.