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February 25, 2010 10:30 AM   Subscribe

Walloped by a snowstorm. Currently, a foot of wet snow has accumulated on the roof, with an additional foot possibly up the pipeline. How much snow can an average snowcountry roof handle without structural damage?

My house is of recent construction, and was built with the proper materials and insulation to rough it through heavy snowstorms.

But the last 48 hours of our "snowmageddon"-prone year have been a first in terms of accumulation.

I've inspected the roofs, and they seem to be holding. No ice dams as of yet. But in a worse-case scenario, in which two feet or more accumulate on the roof and gutters, what possible structural damage might I be facing? Could a roof (again, of recent build) sag or sway under this quantity of snow? What's my general margin of error in terms of inches before I need to be concerned?

Are snow rakes useful in these situations? (No luck in buying one now, but for future reference . . . ) Are there any other measures I can take?
posted by Gordion Knott to Home & Garden (11 answers total)
I wouldn't start worrying about it until 3ft or more.
posted by sanka at 10:42 AM on February 25, 2010

Roofs in theory are designed to handle maximum expected snow loads in their location with a healthy margin of error. Some of the high mountain places around here are designed for several metres of accumulated snow. Trussed roofs will meet that theory better than site built roofs. If the snow you are getting is unprecedented in your area then you can start to worry but if not it shouldn't be a problem. The worst risk is usually from rain after heavy snow.

A quick call to the building inspector in your area would garner a lot more accurate assessment than guesses from the internet.

PS: a push broom clamped to a 2X4 with conduit clamps can make a decent improvised snow rake that won't damage your roof if you keep the bristles down.
posted by Mitheral at 10:49 AM on February 25, 2010

If your city has their local building codes online you should be able to quickly search for allowable snow loads for your area. You might even do a google search for that and come up with an answer somewhere. It's probably expressed in pounds per square foot and your roof and structure will have been designed to accommodate that.

You could also call your insurance company who does your property insurance. They probably have a metric about snow damage, etc.
posted by amanda at 10:56 AM on February 25, 2010

Just for reference, my 85 year old, non-snowcountry DC home just survived 50 inches without a hitch (good old gal!). A neighbor did lose a flat rooved porch during our recent unpleasantries, but we unprepared luddites in a 1920's bungalo-filled streetcar suburb weathered the storm with nary a snow rake to be found.
posted by Pollomacho at 11:10 AM on February 25, 2010

Your location isn't in your profile, but here in Sherbrooke, Quebec (45 minutes north of the Vermont border) what you're describing is what we call "Tuesday," and isn't something to worry about in particular.

A snow rake is always a good idea, and Mitheral's suggestion is a very good one. Before I got a steel roof, I used to just go up and shovel, being very careful and tying myself off to the far side of the house and shoveling the opposite roof section, then reversing the process.

Even clearing the edges with a snow rake, 2x4, etc. will give you some play for snow to slide off with. If you have a slanted roof, gravity is your friend; the snow will want to slide off rather than stick on, especially when you have a bit of compressed snow as a slippery "undercoating," and having clear edges will help that. Be hella careful as you clear, though, and be ready to jump back if the accumulation decides to come down all at once.
posted by Shepherd at 11:11 AM on February 25, 2010

Are snow rakes useful in these situations? (No luck in buying one now, but for future reference . . . ) Are there any other measures I can take?

Yes, not only for removing the load of snow on your roof, but to avoid ice buildup (and dangerous icicles), snow falling on people outside and generally to avoid the potential for leaks. We've used one for years and it's a great tool.

That said, if a foot of snow collapsed roofs, those of us in Atlantic Canada wouldn't have survived a single winter. You're safe for a couple more feet as long as your roof is structurally sound.
posted by Hiker at 11:17 AM on February 25, 2010

One of the problems with snow on roofs is ice dams. This is when the roof over the heated portion of the house melts some of the snow on the bottome and the water runs down to the eaves and refreezes. The subsuqent water runoff starts pooling behind this 'dam' and works up under the shingles and starts leaking into the house. This is bad and will cause structural damage. the worst part is you may not have any idea this is going on until the water appears inside the house, usually by coming through a drywall cieling. The best way to prevent this is to use the snow rake or install heating strips along the eaves and gutters. BTW if you have gutters it is much more likely you will get ice dams.

The structual problem from just the weight of snow is a little tougher. In general most houses with a sloped roof can support at least 2 feet of snow without problems. The steeper the pitch the more it can support safely and the less it is likely to accumulate without sliding off. The bad part of this is it is hard to say, without a detail analysis or the original engineered plans, exactly how much a roof can support. My rule of thumb was to let any less than 2' pile up, but anymore than that i was up on the roof with a shovel and broom clearing it off. If you do this be very careful, tie off so you don't fall all the way down and make sure someone else is around to call the ambulance if you do fall off.

At the least, I would rig up someway to clear the eaves of snow to prevent ice dams and this will also lessen the overall snow load on the roof. The eaves can often be cleared without climbing on the roof from either the ground or a short ladder. Good luck
posted by bartonlong at 11:20 AM on February 25, 2010

This snow is only likely to stay on the roof for a few days because of the daytime temps and the overnight temps are not going to dip terribly much below freezing (if at all) for the next week or so. I think the likelyhood of ice damming is very slim in this case. You get ice damming in the above mentioned cases when you have cold temperatures and accumulation that sits for a long time. In other words the water has to freeze to make the dam and the runoff from your roof is not going to.
posted by Pollomacho at 11:37 AM on February 25, 2010

Just for a sort of sobering counterexample here -- I'm in definite snowcountry (southern Maine) and last winter a house near me lost its roof and another one (by the same builder) had to be reinforced with steel cables.

The construction details of the house will tell you a lot about how much to worry. Modern open-plan and loft-type structures tend to be more vulnerable, because they often have a less balanced structure supporting the roof, and fewer collar ties and reinforcements. A standard box-with-pitched-roof-on-top type house is pretty bombproof.
posted by rusty at 1:47 PM on February 25, 2010

I have about 2 feet of snow on my roof right now and my house was built by drunken monkeys and it's fine. I did reinforce the rafters after I bought it because they were beginning to sag, but they were 50 years old at that point.

Ice-damming is a pita but you will see the ice form and it generally takes fairly cold temps and a while for it to be a problem to the point of leaking. If you have ice/ water shield or any other kind of water tight barrier under the shingles in your roof (which you might because it's new) then you're probably fine even if significant ice dos form. You can go up in your attic and see if you have ice in the eaves if you're really worried about it.
posted by fshgrl at 2:04 PM on February 25, 2010

Heed the advice about ice dams explained above. Also, Mitheral is right about inspections being a good resource, but the code usually defines standard or minimum, not maximum loads or requirements.

Someone at inspections will finally tell you the local requirement. You may have to be persistent because they can be a grumpy lot, depending.

If you find out the designed for snow load you can do a simple experiment to weigh your particular snowfall on a home scale to get an idea of what you are facing.
posted by mightshould at 2:25 PM on February 25, 2010

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