How Much Is Too Much Snow On My Roof?

It is the start of another snow season, and a lot of people will probably spend time wondering how much snow they should allow to accumulate on their roofs before it becomes a genuine source of concern. I know I will.

In Maine, it is normal to see people wielding long-handled roof rakes the day after a storm. When the snow gets really deep, it's not unusual to see whole families up on the roof with snow shovels.

My wife and I rake the valleys over the front and back doors, and when the snow really piles up I go out and rake the shop roof, too. By the time I've worked my way around the building, hip deep in snow, my shoulders are burning with the effort. Then I must shovel the mountains of roof snow away from the doors. It's a lot of work.

It turns out I probably have less to worry about than I think.

My friend Joseph Lanza, a designer and builder in Duxbury, Massachusetts, says a roof that meets current building codes can handle snow loads typical for that area--without shoveling or raking.

The municipal official that issues building permits can tell you what the design snow load is in your area. In snow country, that number might be anywhere from 30 or 35 pounds per square foot (psf) to more than 70 psf.

Armed with that number, a builder can frame the roof appropriately. For example, where the design load is 50 psf, 2x10 No. 2 spruce rafters can handle a maximum span of about 15 ft. If the design snow loads are higher, the framing has to be adjusted to accommodate them.

So how do you know when the amount of snow on the roof is reaching the load for which the roof was designed?

That's harder to answer.

The weight of snow depends on how much water it contains. A cubic foot of water weighs about 62 pounds. Melting down a cubic foot of snow tells you how much water it holds, and consequently how much the snow weighs.

If that sounds a little involved, the National Research Council of Canada estimates that 1 in. of snow equals about 1 to 1 1/2 psf. So even a foot of fresh snow adds only 12 psf to 18 psf to the roof.

There are some caveats. Wet snow weighs more than that, and a storm that brings snow and then rain certainly puts more strain on the roof. Old houses, built long before there were such things as building codes, may have roof framing far less robust than what you'd expect today.

And so there is no perfect answer to the question of what snow depth equals a dangerous overload.

If there are obvious signs of distress--rafters that are suddenly bowed or cracked, for example--or if the snow is unusually deep or wet, I guess I'll do something about it. Otherwise, I'm going to try worrying a little less about my roof this winter.