In this month's installment of Field Notes, Scott Bowe of Kemp Station discusses snow load and how it impacts our homes.
The snowfall this year has been incredible! We set a record in February with 57.5 inches of snow in the Rhinelander area, and we are on our way to setting an annual record for the 18/19 winter season. When I recoded this broadcast last week, we were at 97 inches, about a foot short of the 13/14 season record. And, I’m sure we added to this amount last weekend. This much snow is fantastic for winter enthusiasts. Skiing and snowmobiling has been fantastic, but plowing snow at Kemp Station has been problematic. We are running out of places to pile snow. To add to these problems, there have been a number of roof collapses in the area. What I would like to talk about today are our home roofs. How much is snow is too much snow?
First, most homes are not at risk of snow load failure. More often than not, attempting to remove snow from a roof is more hazardous than beneficial. Homeowners can damage the shingles or other roofing materials and run the risk of personal injury or even death by falling.
When looking up at your home roof, there are two key factors to consider. How much does the snow weigh and how much can your roof support. The problem is that not all snow and not all roofs are the same. Please consider the following:
Snow comes in many forms and is affected by several atmospheric and geographic conditions. Regional differences in season, altitude, humidity, and other variations result in a range of snow densities. For example, the weight of 1 foot of snow in Utah does not necessarily equal the weight of 1 foot of snow in Wisconsin. The weight of 1 foot of fresh snow ranges from 3 pounds per square foot for light, dry snow to 21 pounds per square foot for wet, heavy snow. One inch of ice weighs a little less than 5 pounds per square foot, and 1 foot of ice weighs approximately 57 pounds per square foot.
Snow load may not be uniform. Unbalanced snow loading is when snow accumulates at different depths in different locations on a roof. Snow drifting and sliding snow creates an unbalanced snow load, which poses a greater risk to the roof’s structure than a uniform snow load.
In Wisconsin, we use the Uniform Dwelling Code for residential buildings. For Oneida county in northern Wisconsin, the minimum roof snow load design is 40 pounds per square foot. For Dane county in southern Wisconsin, the minimum roof snow load design is 30 pounds per square foot. As you can see, two feet of heavy wet snow can exceed the design limits required by code.
The tricky part is that all residential buildings in use don’t meet today’s code. The house I live in on Kemp Station was built back in the 1930s, well before we had a uniform building code. The main roof is steep, hand framed with full 2x4 members, and sheeted with 1 inch pine boards. I don’t worry about this roof and suspect it could withstand the impact of a rather large meteor. On the other hand, my porch is nearly flat, and may not have as much integrity as the pitched roof. One thing that a homeowner can do is inspect the structure prior to the snow season. Look for areas of weakness such as signs of decay, weak connection points, and broken structural member. During the winter months, you can watch for sagging ceiling tiles or boards, popping, cracking, and creaking noises, sagging roof members, bowing truss members, cracked or split wood members, and stuck doors or windows that can no longer be opened or closed.
It is important to note that agricultural buildings are exempt from the Uniform Dwelling Code snow load requirements. I noticed several pole barn collapses while traveling in the Chippewa Falls area last weekend. In many cases, pole barns are built with trusses 8 feet apart. The wood purlin structure between the trusses will not support the same load as a residential structure. Freezing rain frozen to these metal roofs may be preventing the snow from sliding off as it has in previous years.
Remember what I said at the beginning, most homes are not at risk of snow load failure. More often than not, attempting to remove snow from a roof is more hazardous than beneficial. Be safe and enjoy the rest of the winter.
For Field Notes, this is Scott Bowe from the University of Wisconsin-Madison’s Kemp Natural Resources Station.