When heavy snow meets fierce winds, even the best-engineered buildings can collapse. That’s why it’s important to have a roof snow management plan and to be adequately insured for this type of peril.
How snow load can damage your buildings
Snow load is the downward force on a building’s roof by the weight of accumulated snow and ice. The roof or the entire structure can fail if the snow load exceeds the weight the building was designed to shoulder, or if the building was poorly designed or constructed. It doesn’t take a blizzard to cause problems; an imbalance of drifting snow can cause one part of a roof to give, causing a domino effect.
During a heavy-snow winter, keep a close watch on structures and be prepared to move operations to safer quarters.
How much snow is too much?
Calculating snow load takes more than an educated guess. The University of Wisconsin Cooperative Extension Service says that a ballpark estimate of snow load can be made with the following formula:
Calculated Roof Loading (lb/ft2) = Depth (ft) x Density (lb/ft2 /ft depth). The approximate density (lb/ft2 /ft depth) for light snow is 5-20, packed snow 20-40, packed snow with ice 40-58, and ice 58.
For example, a roof with 3 feet of light snow has an estimated roof load of 60 pounds per square foot (3 ft depth X 20 lb/ft2/ft depth density = 60 lb/ft2). You should know the roof weight limits for your buildings and outbuildings, and rebuild or fortify them to withstand snow loads that meet local building standards.
Some failures can be prevented with careful snow removal. The University of Wisconsin Cooperative Extension Service offers the following suggestions:
- Use caution if standing on the roof, making sure to wear a safety harness and use securing ladders.
- Use a snow rake, and avoid chipping or picking away at ice as that may damage the roof.
- Remove snow in narrow strips to keep the load somewhat even.
- Not all snow needs to be removed. A thin layer of snow can protect the roof from damage while snow is being removed.
A few minutes can give you reassurance during a heavy-snow winter. Check with your Nationwide commercial agribusiness insurance agent to:
- Confirm that your property insurance covers roof or building failure due to snow load.
- Make sure your policy pays for actual replacement costs, so you’re not out in the cold if you have to rebuild.
- Verify that valuable equipment stored in a building or outbuilding is covered.
Design roof snow load
Design roof snow load is determined by the engineer designing the roof of the building. During the design, the engineer will take into account the wind blowing the snow from the windward side of the roof and drifting it onto the leeward side of the roof. In general, a roof will be designed for a snow drift of about twice the depth of the design snow depth.
Ground snow loads have been determined based on historical weather data and are published in building codes. The design roof snow load takes into account:
- How protected the roof is from wind
- If the building is heated
- What the building is used for (agriculture, residence or assembly)
- Roof slope
- Roofing material
Keep in mind that when a new, taller building is built adjacent to an existing shorter building, the shorter building will need to be reinforced for a greater snow load because the taller building will cause snow drifts to appear on the shorter building.
Roof snow management plan
A roof snow management plan needs to be developed for each structure, based on the design roof snow load that was used for the design of that structure. Some questions that need to be addressed in a roof snow management plan include:
- Who checks the roof for snow accumulation?
- On what frequency is the roof checked?
- Who has authority to order snow removal activities?
- Is any special safety training required for people doing snow removal?
- If an outside company does snow removal, do they need to be pre-approved?
- At what depth of snow on the roof do removal activities begin?
- How are snow drifts on the roof addressed?
- Who inspects the roof for damage after snow removal?
- How are ice dams at the eaves addressed?
For more information about roof snow management, we encourage you to seek the assistance of your building engineer or contractor, or contact Nationwide’s Property Engineering Group.