There's a certain unpredictability involved when framing up supports for the roof on new home construction. Not that the methods are simply a guess, it's just that the goal of roof framing is to be as lightweight as possible, but all the while still accommodating unknown loads from the weather, specifically snow and ice.
Building codes are put into place to avoid snow-related catastrophes. Snow and ice buildup can cause sagging in roof that’s not strong enough, which will then lead to pooling water when the rainy seasons come around. If the roof isn't framed up to code, snow buildup could cause it to collapse entirely.
If the roof is constructed to code, there should be almost no way it risks collapsing because the recommendations definitely err on the side of caution. In states like those in New England, the Midwest, and Mountain West especially, which are prone to large amounts of snow in a short amount of time, contractors will even support the roof above and beyond the recommended code.
Snow Load Codes and Snow Danger Zones
Most residential roofs across the U.S. should be able to support 20 pounds of snow per square foot (psf) before the structure starts succumbing to stress. This number is generally based off new snow on a new roof, however. Obviously if the roof trusses or the sheathing have been exposed to leaks, water, and rot the integrity of the building will suffer. Plus, fresh snow will weigh less than ice or snow that has been packed down. This is why 20 psf is considered a minimum for snow load support.
How Snow Loads Are Calculated and Accommodated
Roof snow loads need to accommodate a lot of variables because not every winter weather event is soft, powdery snow. Sleet can turn to ice on the roof and a wetter snow is going to be much heavier than the fluffy stuff. Those who live in climates where wintry mixes are common may opt to increase their snow load protection beyond the 20 psf recommendations – and may be required to per local codes. A look at how the different types of winter weather affect snow load:
Fresh Snow – A foot of fresh snow correlates to about an inch of water, which accounts for 5 psf of load. Thus you can receive up to four feet of snow until the 20 psf load threshold is met.
Packed Snow – It's important to remove snow shortly after a heavy storm because the packed weight increases tremendously. Only about 3 to 5 inches of packed snow correlate to an inch of water / 5 psf of load. Your roof might safely accommodate up to 2 feet of packed snow, but anything beyond that is very risky.
Ice – Ice has a straightforward 1 to 1 (5 psf) of ice-to-water ratio. Thus your home can support up to 4 inches of ice before it reaches the 20 psf danger zone.
Where things get dangerous is when you start mixing these different types of winter weather accumulations. An inch of ice for example topped with 2 feet of packed snow and a foot of fresh snow can carry a load of 50 psf, which is well beyond the 20-psf load code.
Building Methods to Protect against Snow Accumulation
Roof pitch may have the most apparent effect on snow accumulation. It's quite obvious that flat or low slope roofs will collect more snow than ones with a steep pitch. Even roofs that are pitched as low as 10 degrees have shown to shed snow and rain at least a little bit. Homes in heavy snowfall regions are likely to have roofs with a much steeper pitch for no other reason than to combat winter weather.
Roofing materials are an effective way to counter snow load issues. Tactile materials like asphalt shingles will actually grab onto the snow and catch it whereas something sleek like metal will assist in evacuating the winter mix off the roof.
Adequate insulation in the attic is important. And although it won’t prevent snow accumulation, it will avoid problems that may otherwise arise because of the accumulation. You definitely want adequate insulation to combat high heating and air-conditioning prices, and a well-insulated home will not allow heat to escape in the winter. Escaping heat would actually melt the snow off of the roof, which may seem to be a good thing. However, that melting refreezes, causing a buildup of ice, and that buildup causes “ice dams” in which the water from melting snow higher on the roof builds up because it can’t get past the ice buildup down-slope and backs up underneath shingles causing leaks in the roof. A combination of adequate insulation and ice shields under the roofing (put on at the same time the roof is shingled) will avoid these problems.
Building features such as dormers, chimneys, and roof vents can assist in providing a spot for snow to drift. Therefore, in heavy snowfall areas these features are either removed completely or placed strategically.
Truss materials also determine how well a structure stands up to various winter weather types. Wood rafter construction, for example, may be susceptible to damage whereas trusses often prove to be stronger. Heavy timber construction provides more resistance, as does various steel construction methods in extreme cases.
Although you can't predict catastrophic winter events, you can make sure your home is built to code specific to your area. You can also have a preseason inspection from a roofing contractor to identify areas of concern in the gutters, downspouts, flashing, soffit, trusses, bracing, sheathing, interior ceilings, and any other components that may be showing signs of fatigue or damage. It's better to have a sound peace of mind in late October than to find your roof is ripe for damage in early February.
Footnote: the lead image for this article illustrates snow loads and roof construction. Clockwise from top left: snow buildup on a complicated roof (for more information, click here); building a roof to withstand snow accumulation (for more information, click here); snow buildup at the roof’s peak, ready to be removed (for more information, click here).