As the owner of a commercial building in the Greater St. Louis area–which is known for snowy winters–with a metal roof, we want you to be aware of what Robert Haddock, Director of the Metal Roof Advisory Group, has to say on the subject. Mr. Haddock explains that since metal roofs are widely used in areas where snow is a natural winter phenomenon, unfortunately, one extremely important consideration is often overlooked in the design of metal roofs used in cold areas and that is a snow retention system.
How Snow and Ice Depth Translate to Weight on Roofs
According to FEMA’s Snow Load Guide, one foot of fresh, light, dry snow weighs roughly three pounds per square foot. One pound of heavy, wet snow is equal to about 21 pounds per square foot. Because ice doesn’t vary in density in the way that snow does, the weight of ice is more straightforward.
A one-inch layer of ice on a rooftop weighs a little less than five pounds per square foot. A foot of ice, on the other hand, would be equal to 57 pounds per square foot. The reason to think about the weight of ice on a rooftop is quite simple. As snow gradually melts, colder temperatures freeze that water, causing ice dams to form. Ice dams are as dangerous as snow accumulations on a rooftop.
The Dangers of Unbalanced Snow Loads
The phenomenon of unbalanced snow loads is a serious problem for all roofs. It wasn’t included in any building code until 1988 when the Uniform Building Code added it. Drifting and sliding snow are the leading causes of unbalanced snow loads. An unbalanced snow load occurs when snowdrifts or slides to different parts of the roof. When more snow accumulates in one area of a roof, that area is forced to bear the burden of the weight of that snow.
Drifting Snow is the technical term that explains what happens when the snow blows snow to different parts of the roof. Drifts typically occur where there are obstructions from rooftop equipment or structural parts of the building. They may also occur along eaves or another part of the roof that is protected from the wind. Sloped roofs are susceptible to sliding snow. The steeper the roof’s slope, the greater the risk of snow slides.
Ice dams pose another problem that is directly linked to drifting and sliding snow. Rapid temperature changes can cause melting in the areas where snow drifted, or where snow winds up after sliding off a steep slope. A combination of changes in outdoor temperature and contact with the warmer part of the roof causes the melting. When temperatures start to drop, water from the melted snow will freeze again. You can always tell when a building has an ice dam problem because you will see huge icicles dangling from the edge of the roof.
Mike Huber, PE, explains that metal roofs should be designed to support the weight of snow accumulations. The design should also provide a safe way for snow and ice to escape the roof. Concentrations of snow will accumulate on roofs with steep or fewer steep slopes. The design of your snow retention system should consider the following factors:
- The estimated weight of snow load per square foot
- How steep the pitch of the roof is because the greater the slope, the heavier the snow load the snow guards will have to support.
- How large the weight burden snowdrifts are imposing on the roof is.
- The Vector Force – or how much the snow load weighs in relation to the roof’s slope.
Snow guards are supposed to provide a barrier to keep snow from sliding off a pitched roof, and destroying property below, or hurting people in the path of the snow. There are two methods for attaching snow guards to a roof – adhering them or fastening them. A snow fence is an alternative to snow guards. Snow fences are clamped or fastened to roofs.
Snow rail, fence, guards, and shoes are all types of snow retention systems that prevent large sheets of snow or ice from falling off a pitched roof. The systems hold the snow on the roof so it can gradually melt off.
Low-slope roofs with parapet walls naturally keep snow and ice on the roof. On a sloped roof, however, large chunks of snow and ice can slide off as the snow melts. Avalanching snow is probable on roofing systems with a steep enough slope and/or a low coefficient of friction. This is especially dangerous for pedestrians walking below.
As you can easily see after digesting the above information, if you own a commercial building in the Greater St. Louis area and it has a metal roof, you seriously need to have some sort of snow retention system. RRSA St. Louis can install that for you, no problem! Call us today to set up an appointment for your FREE commercial roof inspection and let our expert help you decide on the best snow retention system for your building.
Serving Greater St. Louis, Chesterfield, St. Charles, O’Fallon