How Much Snow Can Your Roof Handle?

Juno, Markus, and Neptune. If cold weather and snow is a part of your winter experience, you’ll recognize those as the names of winter storms that have made the 2014-2015 winter season one of the worst on record so far for many regions. Boston, Massachusetts, in particular, has gotten the brunt of each storm, resulting in the obliteration of their 30-day snowfall record of 58.8 inches by more than 31 inches.

snow roof

Their total snowfall for the season stands at just under 8 feet, a number that’s sure to increase with yet another storm brewing every week.

What makes Boston’s weather woes particularly dangerous isn’t just the low temperatures and poor road conditions. For homes and businesses, a greater threat lies in the weight of the snow piled on the roof. With steady snowfall over a short period and temperatures consistently too low to facilitate melting, the potential for a roof collapse is very real.

Weight of snow

The weight of snow is measured by determining the weight of the water content. In meteorology, a common rule of thumb for determining the Snow Water Equivalent (SWE) is the 10-to-1 ratio. That is, 10 inches of snow typically melts down to 1 inch of water, and it’s the weight of the water equivalent that determines the weight of the snow. According to the National Oceanic and Atmospheric Administration, 1 inch of water weighs 5.2 pounds per square foot (psf), so 12 inches of powdery snow will weigh just under 7 psf. Wetter, more compact snow with a higher water content can weigh as much as 20 psf.

Other points to consider are how long the snow has been there, whether any drifting occurred, or if there was any recent rainfall. Over time, snow compacts, which increases the weight, while the lower layer may have melted and refroze, becoming a layer of ice beneath the most recent snow. An inch of ice weighs just under 5 psf.

Roof strength

Building regulations will differ depending on your location, with regions prone to high snowfall amounts requiring stronger roofs, but a typical roof can hold between 30 and 40 psf of snow.

Sloped roofs are designed to allow water runoff and shed snow. Buildings with flat roofs are more at risk, both from standing snow and the pooling that occurs once the snow starts to melt, which can lead to significant water damage. Additionally, roof structures such as dormers and chimneys or HVAC equipment can lead to snowdrifts building against these structures that exceed the allowable snow psf.

Signs of damage

According to the Federal Emergency Management Agency, signs that your roof’s integrity has been compromised due to snow or ice load include:

  • Cracks in walls
  • Popping and cracking noises
  • Doors and windows that fail to close properly
  • Sagging ceilings or truss bottom chords

Contact your city’s buildings department to determine the required snow load capacity for your location and have a snow rake stored in your garage if the predicted snowfall totals exceed that amount. You can average how heavy or wet the snow is when shoveling to give you an idea of whether raking is needed.

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