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Snow Loading and Roof Failures

Alert 17: ISSN 1195-5228
Produced by Professional and Specialized Services in consultation with the Industrial Health and Safety Program
Issued: February 1996

Hazard Summary:

Recently, heavy snow has caused roof failures at a department store in Thunder Bay, a mall and an industrial building in Sault Ste. Marie and a greenhouse in Wawa. Also, structural faults due to snow have been found in two schools and a paper mill in Northern Ontario. The roofs of many other buildings in Ontario may be carrying a greater weight of snow than is allowed for in their structural designs.

A cubic foot of snow can weigh from seven pounds for snow that is new and dry to 30 pounds for old, compacted snow. Rain falling on accumulated snow will add even more weight. Drifting snow may put excessive loads on the areas where it piles up, for example against equipment or penthouses or at walls between roof levels.

When snow removal is necessary, it should be remembered that unsafe procedures may cause a collapse. It should also be remembered that workers on a roof must have adequate fall protection and that workers and others nearby can be injured by snow being dumped from a roof.

Locations and Sectors:

All areas where snow has been accumulating. All sectors.

Recommended Precautions:

The Occupational Health and Safety Act requires an employer to ensure that a roof "is capable of supporting all loads to which it may be subjected without causing the materials therein to be stressed beyond the allowable unit stresses established under the Building Code Act" [s. 25(1)(e)]. The Regulations for Industrial Establishments require that materials be moved in such a way and with such precautions and safeguards that the safety of workers is not endangered [R.R.O. 1990, Reg. 851, s. 45].

The Ministry of Labour makes the following recommendations:

• Owners of workplaces and/or employers at workplaces where there is snow on the roof of buildings should have the roofs assessed by a professional engineer to determine whether
  • the snow load is significant, or
  • there are any visible signs of structural distress, for example, twisting, bending or cracking.
• If snow is excessive or a roof shows signs of distress, the owner or employer should implement a safe snow removal procedure.
• A snow removal operation should avoid producing any uneven or concentrated loading on the roof.
• Areas onto which snow will be dumped from a roof should be secured to prevent access.

Also, workers on a roof must use fall-arrest or travel-restraint equipment in accordance with the fall-protection requirements of the Regulations for Industrial Establishments [R.R.O. 1990, Reg. 851, s. 85].

A civil or structural engineer should be consulted: (1) to determine whether snow loads are excessive; (2) to determine whether there are signs of structural distress; (3) to obtain a removal procedure that will not cause more structural problems; or (4) to reinforce a structure that is overstressed.