Snow & Roof Overload
WVU Extension Service Agricultural Engineering Specialist
The recent snowstorms have impacted residents statewide in numerous ways. While focusing on the road conditions, school closings, and other challenges, we often forget about how damaging excessive snow loads can be to our home and farm structures. A number of recent roof collapses in the more heavily hit areas draw attention to this issue.
All snow is not created equal. Snow can have a wide range of moisture contents, and this moisture content directly controls the weight the snow.
‘Dry’ snow, that light fluffy powdery stuff, is much lighter than the dense and heavy ‘wet’ snow. For example, two feet of a typical ‘dry’ snow will exert a load of just under 10 lb/ft2 on a flat surface, while the same depth of a ‘wet’ snow can exert over 40 lb/ft2.
Much of the snow we recently received was on the ‘wet’ end of the spectrum, and with reports of over three feet of snow in some areas, maximum estimated ground snow loads approached 70 lb/ft2. Even so, the actual snow load on a roof is often less than the load on the ground. This is because winter winds can blow some of the snow off of the building; however, if snow drifts occur on the roof, loads can significantly increase. Typically, as roof slope increases, the snow load decreases because snow tends to slide off instead of build up.
There are a number of reasons why roofs can fail. If the actual snow load on the roof is greater than the design load, then there is a danger of collapse. Improper building design and faulty construction can also result in failure, as can older buildings that have been subjected to decay or damage. In addition, buildings are typically only designed to hold the design loads for about a month before structural fatigue weakens the roof and collapse can occur.
If you believe there is too much snow on your building, the best thing to do is to remove it. Shovels, snow rakes, and brooms work well for this. This can be a potentially dangerous task and should only be performed using the utmost caution and safety procedures. There is often ice under the snow, and heed should be given to any overhead electrical lines. For larger buildings, properly trained individuals should be consulted as removing the snow in an unbalanced manner can result in unequal point loads.
Wood buildings often exhibit certain signs before they fail. Sounds such as creaking, cracking, and moaning of building components can indicate potential failure. Any bowing of structural members should also be viewed as a warning sign. If you notice any of these signs, safely and quickly evacuate the building.
Every fall, and before big storms, it is also a good idea make sure all gutters and downspouts can flow freely to help prevent ice buildup and properly drain melting snow off of roofs.