Winter storm Nemo caused serious mayhem in portions of the U.S. last week and many regions are still recovering. Each year, an average of 105 multi-day storms producing snow reach the continental U.S. Nearly every location in the country, including southern Florida, has seen snowfall sometime during recorded history. While snow presents its own problems, adding high winds like those experienced during Nemo greatly complicates the situation.
Heavy snow accumulation can cause a roof to collapse and ice dams allow water to leak over flashings and under shingles. Snow can slide down from skylights and sloped roofs onto the heads of pedestrians. Snowdrifts surround buildings, making it difficult for people and vehicles to enter. On windy days, snow can blow inside buildings, wetting interiors and leading to more damage.
From economics and human safety perspectives, snow loads on roofs are arguably the most troublesome snow problems. Climatic factors including snowfall amount and type, air temperature, and amount of sunshine determine snow loads. Roof thermal properties, shape, exposure, and the surrounding environment also play roles. However, the effect of wind is considered the most important variable affecting roof snow loads.
A snow-covered roof is highly susceptible to drifting because it is exposed and wind speeds are higher at the roof level than they are on the ground. During a snow storm, wind speed tends to increase. More flakes are carried in a horizontal fashion past exposed areas of the roof to rooftop areas featuring lower wind speeds. This creates snow deposits in lower levels of a multilevel roof, the downwind side of an arched or peaked roof, and in roof valleys.
During this process, some snow may blow off the shingles and fall onto the ground. However, the freezing rain that often comes with a snowstorm can create a hard, frozen surface that does not permit drifting. Though wind and snow can blow from any direction, it is possible to predict the possible shape of a drift on a roof. The National Building Code includes information on the topic and experienced commercial and residential roofing contractors bring supplemental knowledge regarding aerodynamics.
Building architects and roofers can take steps to prevent wind from enabling roof snow loads to reach unsafe levels. Commercial and residential property owners who are concerned with snow loading on their roofs should consult with these professionals. A simple, affordable solution may be available to eliminate the worry of potential damage.