Snow load on roofs diminishing with melt
That sigh you hear when the house is quiet tonight may be your roof trusses relaxing a little after carrying an aching load of snow this winter.
While daily thawing and melting might cause ice dam issues, the melting snow this week is a great relief to the structural integrity of Northland roofs, engineers at the University of Minnesota Duluth said Monday.
Modern roofs are designed to carry a bad winter scenario -- based on building codes -- to support about 80 percent of the peak snow load expected on the ground in any given region.
But this year those snow loads may have exceeded those structural levels because heavy snow that fell in December never melted and was added onto by snows in February and March.
That load led to a few complete roof collapses, some structural issues for the Miller Hill Mall roof and some sagging roofs in residential homes across the region, said Rania Al-Hammoud, assistant professor of civil engineering at UMD and an expert on structural design.
Al-Hammoud said getting rid of snow is the best measure to take, even if the sun eventually will solve the problem for us. Mary Christiansen, another UMD civil engineering professor, agreed. Even if the snow gets wetter, it's losing weight with every drip off the roof.
"The transition from snow to ice (due to melting and re-freezing) can be problematic ... that's why getting the snow load off is critical,'' Christiansen said. "But the melting is good to reduce the load."
The one variable would be a heavy rain on top of roofs, which would add to the overall weight load.
If your roof is leaking, it probably means some ice got under your roofing material, and you'd better get that roofing material fixed before next winter or an ongoing leaking roof could lead to structural problems, especially with wooden building material.
But if your roof is or was sagging, Al-Hammoud said, move quickly to have the structural integrity of the roof support system inspected and fixed.
"A leaking roof is a problem. A sagging roof is a big problem,'' she said. "Some materials are designed to stretch, to sag a little, to be flexible ... that's good so they don't break. But that means something is going on'' that shouldn't be.
A cubic foot of snow averages about 15 pounds. But heavily drifted and compacted snow, or unusually wet snow, can weigh 20 pounds or more per cubic foot. That can add up to tons and tons of snow on roofs. So far this year Duluth has had about 85 inches of snow, 15 inches more than usual for this date but far short of the record 135 inches that fell in 1995-96.
Still, the lack of any melting for much of the winter left, at one point in February, 36 inches, of snow on the ground on the level at Duluth International Airport. And drifted areas on roofs could have seen deeper, heavier loads than that.
"Engineers build those drifts into account when they do the design. But if people add things on to those roofs ... that cause drifting snow that wasn't accounted for, that can be a problem,'' Al-Hammoud said.
Duluth's snowpack on the ground already had lost about 10 inches from the February peak, to about 26 inches, as of Monday.