How Much Snow Is Too Much On My Roof

Two feet or more of old.

How much snow is too much on my roof.

Wet snow weighs considerably more than dry or fluffy snow because as you can probably guess water weighs more than air. According to the national research council of canada a roof should be able to withstand pressure of at least 21 pounds per square foot psf this number can fluctuate based on regional building regulations but any well built roof should be able to sustain this much weight before it gets too stressed. This mainly occurs when the snow is concentrated in one area of the roof. One of the major determining factors for how much snow is too much for your roof is how heavy the snow fall is.

Packed snow however weighs more. Unfortunately that means a roof can collapse with as little as 18 inches of snow. For the twin cities metro area the roof snow load equals 35 pounds per square foot or 7 x 50. In snow country that number might be anywhere from 30 or 35 pounds per square foot psf to more than 70 psf.

A new house can typically hold around 100 pounds per square foot on the roof. But as the chart below shows packed snow with ice can easily overload a roof once it reaches about 12 off depth. You have probably heard of or seen the difference between wet snow and fluffy snow. Older homes however can only handle about 45 pounds per square foot.

Armed with that number a builder can frame the roof appropriately. These questions aren t quite so easy to answer because there are several variables. If you notice a tighter feel when you swing the doors or difficulty in closing them you may be seeing the first signs of too much snow on the roof. Cracks may occur before changes in the swing and closing of doors become evident so pay particular attention to these signs of structural duress.

The chart below courtesy of paul schimnowski p e gives some examples of snow loads. How much snow a roof can hold depends on the type of snow roofline and material among other considerations. As everyone knows cold fluffy snow is very light while wet snow can be extremely heavy. Because rafters are typically spaced 16 inches on center this amounts to 8 inches in both directions from the rafter s center line.

Ten inches of fresh snow equates to about five pounds per square foot which means your roof likely can support four feet of fresh snow. The municipal official that issues building permits can tell you what the design snow load is in your area. Look for new cracks around doors and windows another sign of stress resulting from a heavy snow accumulation on the roof will be cracks in the walls around doors and windows. The smaller the area the lighter the load each rafter carries.

So how much snow does this equal.