Question about snow on roofs

[FTL_Ian]

Julia and I have seen people scraping snow off of their roof.  She asked one of her New Hampshire native coworkers about this practice and he told her that if too much snow builds up it can damage the structural integrity of the roof.

It would seem to me that roofs in New Hampshire should be designed to hold a lot of snow.  Is there any truth to what the New Hampshire native said?

[anthonybpugh]

I think the chances of falling off the roof and killing yourself are a lot higher than the chances of your roof collapsing because of snow. 

[Pat McCotter]

Ice dams can form along the edge of the roof. This will allow water to pool and get under the shingles and behind your walls.

Ice Dams

[Pat McCotter]

You can see some houses with metal flashing at the lower edges of the roof. This is to melt the snow there as the sun warms it.

Some folks also put heat tracing along the lower edge.

[Mike Barskey]

There's a short description of this in the NH Underground Moving Guide:

If your roof is poorly insulated or poorly ventilated, it will tend to form ice dams. What happens is the heat flowing to the shingles melts the snow, and flows down to the edge. At the edge, it contacts the open air and freezes, forming an ice dam. This dam backs the water that is still flowing down up under the shingles, and can soak into the roof interior, ruining insulation, sheet rock, etc. This snow will melt a lot around chimneys, sky lights, and above where can lighting is mounted in the ceiling of the top floor.

If this happens, you need to get a snow rake with long extendable handles and rake the snow off the roof.

Also, remove the gutters in the fall, as they will tend to fill up with ice and exacerbate the problem.

Related to potential house damage from snow, they also said:

Yes, if you live in a house with plumbing through outside walls, or with poor insulation, you should leave your faucets open a crack to drip. This isn't to keep the water flowing, it is to allow any ice building up to have some place to push the water. Ice forming in the pipe will take up more space than the water it is made from, so it will expand linearly along the pipe and cause water pressure to build up if you don't give that trapped water some place to flow. It isn't the ice that breaks the pipes, it is the pressurized water.

[FTL_Ian]

Yes, if you live in a house with plumbing through outside walls, or with poor insulation, you should leave your faucets open a crack to drip. This isn't to keep the water flowing, it is to allow any ice building up to have some place to push the water. Ice forming in the pipe will take up more space than the water it is made from, so it will expand linearly along the pipe and cause water pressure to build up if you don't give that trapped water some place to flow. It isn't the ice that breaks the pipes, it is the pressurized water.

Anyone know how cold it should be outside before one does this?

[John Edward Mercier]

A few years ago (I believe March 2001), several roofs collapsed.
Doesn't need a specific temperature, just use a roof rake or if shoveling tie off, and don't damage the shingles.

[srqrebel]

You can see some houses with metal flashing at the lower edges of the roof. This is to melt the snow there as the sun warms it.

Some folks also put heat tracing along the lower edge.

Without it, massive icicles would damage the shingles along the lower edge -- or so I've heard.

[Puke]

You can see some houses with metal flashing at the lower edges of the roof. This is to melt the snow there as the sun warms it.

I was curious about this. Now I know. 
Though it is more expensive, I like the all metal roofed houses. I think it looks better than shingles.

[dalebert]

One section of our roof that's above the door and under one of the upstairs windows has a huge snowdrift building up on it. It's much thicker than other parts of the roof. The snow seems to have blown down from the higher roof. I wonder if that's potentially a problem.

[John Edward Mercier]

It would depend on the flashing between the lower roof and any sidewalls. On newer construction these areas would be protected by ice shield run up the roof and under the siding.

[Russell Kanning]

Anyone know how cold it should be outside before one does this?

depends on the situation

you could post pictures of your roof for guys to guess what is going on. Do you have any leaking inside the roof? Do icicles cause problems? I like getting snow of the roof if it is easy ... why not? In most cases I doubt you need to. Your house has been ok so far. Some old guys in NH have nothing better to do than pull snow off ... it might not be important.

[Russell Kanning]

One section of our roof that's above the door and under one of the upstairs windows has a huge snowdrift building up on it. It's much thicker than other parts of the roof. The snow seems to have blown down from the higher roof. I wonder if that's potentially a problem.

roofs around here should be strong enough to hold many feet of snow .... especially along the top edge like that

[Tom Sawyer]

I think the worst case situation is if there is a lot of snow on the roof that then gets rained on. The saturated snow can be quite heavy.

When it stays nice and cold the snow stays fluffy and blows off/evaporates over time.

It is easiest to remove the snow before it goes through a freeze thaw cycle and becomes layers of snow and ice.

I think whether to remove some of the snow is a gamble/judgement call on what the future weather is likely to be.

The steeper the roof is the less likely a problem.

One of the those roof rakes allow you to pull snow down from the eaves without climbing on the roof. If you do get on the roof to shovel snow, I leave a barrier of snow along the edge of the roof to help prevent myself from sliding off the roof if I slip.

[John Edward Mercier]

One section of our roof that's above the door and under one of the upstairs windows has a huge snowdrift building up on it. It's much thicker than other parts of the roof. The snow seems to have blown down from the higher roof. I wonder if that's potentially a problem.

roofs around here should be strong enough to hold many feet of snow .... especially along the top edge like that

Code is 40lbs, but it sounds like he's describing a dormer. If its constructed properly shouldn't be any problem. If not the heat escape at the seam between the roof and sidewall will create an ice dam.