Snow Loading and Roof Collapse Safety

By Mike Rongione, Managing Structural Engineer at Pare

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Scenic icicles and snow-capped rooftops can be striking and charming, but their elegant beauty can be cause for concern. Snow can possess significant weight and becomes heavier when combined with rain, ice and sleet. Ice and snow on roofs are often forgotten until a leak develops – or worse.

With the numerous snowstorms we have experienced recently, excessive ice and snow loads can overload a building’s structural members and sometimes even cause roof collapse. Drifting snow conditions can also cause excessive snow loads. Generally, these drifts are common at pitched roofs, roof valleys, lower levels of multilevel roof areas, and on roofs adjacent to projections. Snow drifts are usually the result of wind-blown snow piling up in discrete areas. Larger roofs are more prone to excessive snow drifting because there is more snow available to form the drifts.

As snow might continue to accumulate or heavy rains and melting could add to the weight on buildings and houses, you need to be aware of the potential hazards and important safety measures you can take. If you suspect imminent roof failure, evacuate the building and call 911.

THINGS TO LOOK FOR

Generally, buildings will show signs of distress prior to roof collapse. The following are some warning signs of roof distress that should be recognized:

  • Loud popping, cracking and creaking sounds from the structure.
  • Sagging ceiling tiles.
  • Severe roof leaks indicating potential torn roof membranes and/or ice damming.
  • Cracked or split wood members.
  • Bends or ripples in metal supports.
  • Cracks in walls or masonry.
  • New cracks in wall or ceiling drywall/plaster.
  • Sprinkler heads pushed down below ceiling tiles.
  • Water ponding in areas it never did before.
  • Doors that pop open.
  • Doors or windows that are difficult or no longer can be opened/closed.
  • Bowed utility pipes or conduit attached at ceiling.
  • Cracks in welds of steel construction.
  • Sheared off screws from steel frames.
  • Sagging roof members including steel bar joists, metal decking, wood rafters, wood trusses and plywood sheathing – visually deformed.

REMOVING SNOW FROM ROOFS

When snow removal is necessary, remember that unsafe procedures may cause a collapse and injuries. Anyone working on a roof must have adequate fall protection and keep in mind that workers and others nearby can be injured by snow or ice being dumped from a roof.
Once it has been determined that the snow must be removed, there are several options for snow removal:

  • Consider hiring insured professionals to do the job.
  • Be aware of large accumulations of snow build-up or snowdrifts on your roofs.
  • If snow can be removed with the use of a snow rake (available at most hardware stores), do so.
  • Try to avoid working from ladders, as ladder rungs tend to ice up. Snow and ice collect on boot soles and metal ladders.
  • Use caution when removing snow off roofs to avoid damage to roof membranes or shingles. Use plastic shovels or wooden rakes to avoid damage. Consider leaving a few inches of snow on the roof instead of scraping the roof clean.
  • Flat roofs can be shoveled clear, but only if it is determined that the roof is safe to stand on. Start from the edge and work your way towards the center of the roof. Do not pile snow in local areas.
  • Remove snow evenly from both sides to avoid unbalanced roof loading conditions.
  • Flat roof drainage systems should be kept clear to minimize the risk of excess roof ponding in the event of subsequent heavy rainfall or melting.
  • Exercise care when on the roof to avoid potentially dangerous falls.
  • Large icicles can form on roof overhangs, but do not necessarily mean ice damming is occurring. Icicles overhanging doorways and walkways can be dangerous and should be carefully removed.

Professional structural engineers can assist with building assessments if you believe your structure is at risk.  Local building officials and your local fire department are also familiar with snow-loaded roofs and the signs of a potentially overloaded structure.  If you have concerns, they can answer many of your questions before taking the next step and hiring a professional.

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