Roof snow accumulations in excess of specified project design loading criteria can cause significant distress to your PEMB structural system. Snow will build up in areas around firewalls, parapet walls, valleys, dormers and on lower roof levels where a roof step occurs. Since the density of snow varies depending on weather conditions during and after a snow fall, it is not possible to determine a single value for the allowable height of snow that a building can safely support.
In addition, the underlying snow density increases due to melting from the building heat loss and as water is absorbed from the melting snow above. as weather and temperature changes continue, ice may build up under the snow layers, further increasing the building roof loading intensity. This ice build up also causes additional water back-up on the roof deck.
The most severe condition occurs when rain falls on a roof system already loaded by snow. In this case, the snow absorbs the rain water, and loads can approach the weight of water (62.4 pounds per cubic foot, or 5.2 pounds per inch of depth). This condition must be monitored with extreme caution.
The following procedure may be used as a guideline for responding to roof overload conditions due to extreme snow and ice build up conditions:
1) Visually inspect the roof system to identify unusual deflections of frames, purlins or joists. Starting in this area, remove approximately on-half of the snow depth in a pattern that does not cause an unbalanced loading condition on the frames or purlins.
2. In general, the shoveling pattern should progress from each endwall of the building towards the center. On larger roof areas, additional people working from the center of the building to the ends is recommended.
3) Along the building width, remove snow from the eave towards the ridge, sliding the snow off the roof over the gutter. On gabled buildings, remove the snow on both sides of the ridge at the same time.
4) Remove the remoaining half of the snow depth in the same manner as described above.
5) Never use metal shovels or "scrape" the roof down to the surface of the panel. Remember, the objective is to relieve the excess loading condition due too the weight of the snow, not to completely clear the roof panel of all snow and ice. Attempting to scrape the roof will result in broken fasteners and loose cinch straps, creating roof leaks.
6) Keep gutters, downspouts and roof drains open and free flowing to prevent water back up and ice build up on the roof system. Ice damming conditions are especially likely on the north side of a building and in shaded areas. Installing heat tape in the gutters and downspouts can also be used as a precautions, however, heat tapes may not be 100% effective in extremely low temperatures and should be checked regularly.
7) Watch for extreme defelections and listen for unusual noises when snow and ice build up conditions exist.
1) Always provide proper safety precautions when working on the roof.
2) Pay special attention to and be aware of Translucent Roof Panel locations. These panels are not intended to support roof foot traffic loads.
3) Be cautious of snow or ice breaking away and sliding down the roof, even on low slope buildings. Metal roof systems are extremely slipper when wet. It may be necessary to locate ladders at the end of the building to avoid sliding snow.
4) Use extreme care when working along the edge of the roof.
5) Never send one person alone on a roof to remove snow.
Gideon Steel Panel Company, LLC does not make any recommendation on when to remove snow from roofs. It is up to the individual property owner to consider the benefits and dangers of snow removal and decide their own course of action. Remember to consider the depth and relative moisture content of your snow and the capacity of your roof structure in making your decision to remove snow or not.
It is also recommended to review the "Snow Removal" section of the Metal Building Systems Manual, current version.
Also refer to the IBCEST - 2001 article 'Minimizing The adverse Effects of Snow and Ice on Roofs'.