Reminder about the weight of snow

With the collapse of the Metrodome roof and the number of people at work today complaining about aching muscles from snow shoveling, I thought it would be a good time to remind homeowners about estimating snowloads on their roofs.

The ratio of snow to water from the weekend storm was in the range of 14:1 to 16:1 for much of the Metro area. Given that snow amounts ranged from 12 to 20 inches, the measured water content then probably ranged from near 1 inch to about 1.40 inches. Indeed the water content at MSP Airport from the Friday-Saturday snowfall was 1.21 inches, while it was 1.12 inches at Chanhassen.

Each inch of water content contained in the snow cover represents about 5.2 lbs of weight per square foot of surface area. Thus it is possible that on a 1000 square foot roof (only 20 ft by 50 ft) if snow has drifted to a depth of 12 to 20 inches, the roof is supporting 5000 to 7500 pounds of weight.

Many roofs are designed for 20 to 30 pounds per square foot in their load capacity, but it is not good to allow that much weight to stay up there for weeks. And certainly given the weather forecast the snow cover is not going to be melting soon. Thus, it may be a good idea if such drifts are visible on portions of your roof you might use snow rakes or other devices to eliminate some of that weight.