The Snow is Gone, but Not Forgotten
By Keith M. Bouchard, E.I.T.
After a long winter spring is coming to New England. The weather will soon be warm, construction season will be in full swing, and you’ll be making weekend plans for the beach, mountains, or Fenway Park. Despite this change of seasons, it would be worthwhile to remember the events of this past winter if you own or manage a building. Nearly eight feet of snow fell on the Northeast this winter collapsing roofs and resulting in untold amounts of damage to residences, municipal, and commercial buildings. While not every winter is as severe, the probability of another heavy snow event occurring within the next ten years is high. Building owners and managers need to review their facilities to determine if their buildings are at risk of snow-related damage.
- Location: The design snow load on the Cape and Islands is much less than the design load of an inland city like Worcester or Springfield. Additionally, a building with a high exposure to the elements, meaning there aren’t tall trees or buildings around to shield the roof from wind, will have less chance of accumulating deep snow depths.
- Roof Construction: A roof framed with light materials will be more susceptible to snow overloads than a heavy roof. Pre-engineered metal buildings are especially at-risk as these structures are designed with little or no reserve capacity above the design load.
- Thermal Factor: Roofs over unheated spaces are also at-risk due to the large snow accumulations that can develop. Many homeowners recently installed insulation in their attics to increase energy efficiency only to end up with the unwanted result of ceiling and wall cracks from increased snow loads.
- Drifting: Snow drifts are caused by wind sweeping over roof steps and projecting equipment creating an eddy in the wind flow where snow can accumulate. A drift can be as much as 3 to 4 times the base depth of snow, leading to significant structural loads. If those loads are not accounted for, which is usually because the roof step or projecting equipment was not part of the original construction, then significant damage or even collapse may occur.
- Drainage: Functioning roof drains are critical to allow for melting snow and rain to be carried off the roof. A roof drain clogged with debris or ice may allow water to pond on the roof creating a loading condition much greater than what the structure was designed for.
The question most building owners and managers have once they understand the risk factors associated with snow loads is: how do I know when there is too much snow on my roof? The answer is not as easy as measuring the height of snow since the density of snow accumulations can vary greatly. The best method is to isolate and weigh a 12”x12” section of undisturbed snow on the roof to determine a load in pounds per square foot (psf). That load can then be compared to what the roof was designed for. A typical flat roof snow load is 31.5 psf for Boston and 38.5 psf for Worcester or Springfield, although these numbers will vary depending on the type of roof. Dealing with drift conditions is trickier. If there is any reason for concern, the safest route is to consult with a Structural Engineer to review the condition and determine the best course of action. That way you can set your mind at ease and go back to planning those summer trips to the Cape.