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Snow loads on roofs


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The snow is up to my knees these days when inspecting exteriors. A newspaper article today warns of the possibility of roof collapse.

A local home inspector is quoted in the paper. "Once you start getting over a foot of snow, that's where you are beyond the design of most roofs". Any truth to that statement? There are a lot of houses around here with the typical 2 x 6 overspanned rafters.

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There are charts and calculations for this, but most of them have insufficient information. For one thing, how "wet" is the snow?

The collapse(s) around here usually happen when there's a big snow, then a thaw, then a soft wet snow or rain following. The snow holds the moisture, the roof comes down.

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Last week a 70 yr old guy in Jefferson fell off his roof when clearing snow; he had to get flown out because of his injuries. I can just see it now, people climbing up onto the roofs clearing snow because of this-

(newspaper article in part)

“Once you start getting over a foot, that’s where you are beyond the design of most roofs,â€

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I shoveled knee deep heavy snow pack off my porch roof yesterday. It's 2x4 rafters and almost, but not quite, 2/12 pitch spanning 8' and the previous owners shingled it. It ice dams and I get water dripping through it and my back porch turns into a skating rink. If it weren't for the ice I'd just let the snow pile up, those 100+ year old 2x4s aren't going anywhere.

I think I'd want a more credible source than an unnamed HI for design snow loads. I know there's a lot of snow up and down the East coast, but this isn't the first time NJ has had this much snow. How heavy is a foot of snow anyways?

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There's no point in removing an ice dam. Melting a hole in them is plenty good enough. I used to hook a garden hose up to my grandmother's water heater, haul the 40 footer into her backyard, and melt a few 3-4' wide holes in the ice dams that would occasionally form on her roof. Problem solved.

We see plenty of hatchet marks in asphalt shingles on the low edges of roofs here in MA. As often as not, there's an assprint in the ground directly below, where some hatchet-wielding idiot fell in a fit of ice dam destroying frenzy.

It's a wonder more people don't die that way.

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Yes, hatchet marks and ass prints. Pretty common. There seems to be a couple deaths every year from some poor old person knocking snow off their roof.

Darren's quoted article is exactly the sort of thing that gives HI's a bad name. A bunch of folks relating information gleaned from their franchise marketing operation, their uncle, or something they read somewhere.

Folks have a hundred or more year old house, it's been through countless snow loads and ice dams, they read an article by a "home expert", so they go out and unintentionally kill themselves thinking they have to do something.

And, this referral to professionals that has come to be standard boilerplate for HI's.......are there snow removal professionals? Would you let one of them on your roof?

Do they use hatchets?

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I don't see much of a reason to get excited about it. If a roof fails around here, it's usually because of the reasons Kurt posted and more likely to happen to a roof that already had a problem.

I've had at least a foot of snow on my roof since the beginning of the month and it's not crying uncle. I can't see the point of shoveling free insulation from the roof.

This happens every year around here. If a roof does come down, it's as big of deal here as it is anywhere else.

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And, this referral to professionals that has come to be standard boilerplate for HI's.......are there snow removal professionals? Would you let one of them on your roof?

Do they use hatchets?

Don't you read JLC? The latest issue has an article about how adding roof shoveling and ice dam removal services generates a revenue stream now as well as referrals for spring time work, complete with how to advice and a fee schedule.

The hysteria is not just for home owners anymore.

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Last I heard, they're predicting another 12-20" in Boston from Tuesday PM through Wednesday AM. Ho-hum.

I have an inspection scheduled Wednesday AM and won't postpone it unless they do. My Jeep can climb a tree in a horizontal ice storm. Matter of fact, so can I.

Despite my being jaded, I am in humble awe of the 2x5 rafters I see at 30+" OC in some 100 year old bungalows. No structural engineer with any sort of grip on his senses would put his stamp on them. Yet, they survived the hurricane of '35, the blizzard of '78, and countless other unnamed tests of endurance that a home built to modern minimum standards never could.

I'm not saying: 'We don't build 'em like we used to.' I'm glad we don't, because most of 'em burnt down.' I guess I'm only lending my voice to the oft-heard lament that book-smarts are exponentially more useful when tempered with an equal amount of boots-on-the-ground smarts.

And...ice dam stained interiors aren't the worst thing that can happen to a homeowner. You can paint it in the spring. But that spinal column...?

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Hi,

Del Greco's statements notwithstanding, I doubt if there is any "real" danger of collapse with most older northeast homes. They have very steeply pitched roofs for a reason; so when the snow builds up to a certain point it will slide off the roof. One is more likely to get hurt by ice sickles that break loose from the eaves and fall than the roof collapsing. Still, there are a lot of post-war houses out there with lesser sloped roofs that might be in danger of some damage. I think if those NJ inspectors had thought about what they'd said before they'd said it, they could have presented the issue in such a way that most homeowners of older houses wouldn't have been too worried about it and it would have been the folks with lesser pitched roofs worrying about. I dunno about the rest of you, but when I walk on a snow-covered roof it's usually a pretty solid walk unless the snow is too thin to pack under my feet. When it's too thin to pack, I won't walk on it - then it's like stepping on a sheet of Vaseline.

ONE TEAM - ONE FIGHT!!!

Mike

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While watching the news last night my wife and I were amused by a story about a woman that was praising her dog for warning her that her garage roof was about to collapse.

She explained in the story that after the dog kept barking for an unknown reason she went into the garage and heard unusual creaking noises. She become concerned about the structure and rushed to pull her cars out right before the roof fell and flattened the space below.

I mentioned to my wife that she was only seconds away from receiving a Darwin Award for caring more about her cars than her own life!

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One is more likely to get hurt by ice sickles that break loose from the eaves and fall than the roof collapsing.

I wish I had the camera with me the other day. I was driving by a really nice old home that had bays on the front.

An icicle that had to be about 8' long and about a foot in diameter had dropped straight down, stuck in the lawn, and laid itself back through one of the bay windows. I don't think anyone was home when it happened.

How would you like that surprise when you got home from work?

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I'd love to know why so many in the last two weeks?

This is not an unusual winter. There's got to be a good explanation for the very unusual number that have caved in. Houses don't cave in around here. Commercial buildings do.

Some of these places were truss built and some rafter built. Some hip, some gable. There dosen't seem to be any kind of a pattern with the age of these homes.

I'm wondering if years of smaller ice dams have slowly deteriorated rafter tails and top plates to where they finally let go.

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I'm wondering if years of smaller ice dams have slowly deteriorated rafter tails and top plates to where they finally let go.

Which brings up the question...What's the dominant failure mode with excessive snow on the roof? Rafter spread? Rafter failure?

Marc

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