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Blog: Measuring Snow Loads and Preventing Ice Dams

In early 2016, a huge winter storm swept up the Atlantic coast, dropping up to 3 feet of snow on cities such as Washington, D.C., and New York City. Following these kinds of storms typically are reports of building roof collapses due to excessive snow loads, leading you to wonder how safe the roofs are over homes and commercial properties. 

Furthermore, the constant freeze/thaw cycles of a snow load on a roof can lead to the build up of ice dams, creating a completely different series of problems for the owners. 

Designing for snow loads

Builders and contractors should be familiar with the “ASCE Minimum Design Loads for Buildings and Other Structures” by the American Society of Civil Engineers (ASCE) and the International Building Code (IBC). The building codes found in this document provide the requirements and guidelines needed to design roofs to withstand snow loads of various weights. 

In areas of light and moderate snow, ground snow loads can be found in a map within the ASCE code. For mountainous areas of higher snow loads, local authorities provide information about the required snow loads for that locality. 

Snow loads are defined as pounds per square foot (psf) of snow on the ground. With that basic information, you can calculate how much snow weight a roof must be able to carry. Local building codes dictate design, however, a roof with significant pitch can alter load calculations because snow slides off the roof. Here’s an example of how this works.

Sun Valley, Idaho, is nestled in the Sawtooth Mountain range, an area that can receive a lot of snow. Liv Jensen, a structural engineer and owner of Liv Jensen P.E. Structural Engineering in Hailey, Idaho, says the local building department states ground snow loads in the valley can be up to 120 psf. 

The flat roof snow loads calculated from that ground snow load can range between 76 and 111 psf, depending on thermal and wind exposure factors; however, the code requires a minimum 100 psf flat roof snow load be used. 

If a residential or commercial property roof is under stress from a heavy snow load, the following signs could indicate structural roof problems and should be addressed immediately. 

The signs include sagging roofs, cracked or split wood members, bent supports, sheared off screws, doors or windows that are difficult to open, and finally, creaking, cracking or popping sounds.

Ice dams

Poor roof designs can cause ice dams to develop, but aside from that, the two most common reasons that ice forms on roofs have to do with improper insulating practices and the location of rain gutters.

Adequate ceiling insulation in attic spaces must extend to the exterior wall line of a building. Insulation that is blown into this location often stops short of exterior walls and prevents soffit vents from functioning. 

Heat from the house can then melt snow on the roof and turn it into ice at those locations. Extending insulation to exterior wall lines can prevent heat from escaping. 

Jensen says the code in her area requires approximately 12 inches of blown-in insulation with adequate air space above for ventilation from the soffits to the gable or ridge vents. Newer super-insulated systems consist of spray-applied rigid insulation that completely fills a minimum of 7 inches at the top of the roof joist space, where ventilation is not required due to the very high R-values that are achieved.   

Ice can also form at the roof edges when rain gutters are located higher than an inch or so below the edge of the roofline, causing snow to build up and turn into ice.

Obvious signs of ice dams forming include large icicles hanging over the edge of a roof indicating a constant freeze/thaw cycle of the snow on the roof. The larger the icicles, the larger the ice dams. 

Less obvious signs of ice dams include smaller icicles coming out of eave vents, or even behind siding boards. If this happens, water has gotten into the attic or has dripped down behind the siding, typically indicating a more severe problem. 

Also, the appearance of water stains along the corner between the ceiling and an exterior wall. This is a clear indication water has gotten into the attic, wetting the insulation and ultimately trickling down to the drywall. 

Ice dams can easily tear off gutters, loosen shingles, and cause water to back up and pour into a property. If the latter occurs, it can lead to peeling paint, warped floors, stained and/or sagging ceilings. Furthermore, insulation in the ceiling can be infected with moisture causing it to lose its R-value and ultimately be a source of mold and mildew.


Joe Nasvik is a writer and editor serving the construction and concrete industries. He has 18 years’ experience as a concrete contractor. Contact him at jnasvik@sbcglobal.net.

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