Roof snow loads

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wrote:

Thanks a lot for finding that calculator for the loads. It was what I was hoping to find.
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SMF wrote:

And that would be for materials that meets standards, in good condition, and built to code. All 2x6's are not created equal.
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I don't live in snow country, so maybe this is inapropos, but:
If your area is prone to HEAVY, continuous snows, could you run some kind of electric wires along the (flat) roof , being careful to shield them from tar or shingle roof material, and activate them when heavy snow falls. The idea being to keep the snow from accumulating by melting it.
There was a recent thread about ice dams, in which an electrical solution was suggested, so am extrapolating to (flat) roofs.
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On 2/25/2011 1:03 PM, Higgs Boson wrote:

Run the math to melt that much snow.
It's largely worked out here:
http://www.tesmar.com/html/snowmelt_design.html
~750 BTU to melt 8" deep square foot of snow.
Multiply that by 2000 or 3000 for every snowfall and you have one big electric bill.
Jeff
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AWESOME! I learned a lot. Thanks.
HB
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Ordinarily I won't go up on my roof to shovel the snow off it.
The exception is when you know that a freek warm spell is coming - and bringing lots of rain with it.
A foot or two of snow on your roof can be VERY HEAVY.
Now imagine that snow acting like a sponge when a good rain is a few hours away.
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Nobody around here shovels snow off the roof. We just wait for the spring thaw. Nothing falls down.
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This was an exceptional year in New England. Lots of heavy snow with no thaw in between. The some rain that was just absorbed into the snow rather than runoff. It was rare to clear a roof, but this years was an exception.
Some big flat roof buildings had problems. On a 40,000 sq ft building, an inch of rain adds 250,000 pounds.
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Same here as every year. I heard on the news a portion of a building in a nearby town collapsed. I still say they cave in because they aren't built right. Our schools have flat roofs and they don't fall in. Where they fall down is where some idiot cut corners when putting up the building.
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But this years was the heaviest load in 100 years. You'd have to know what the design was and what the code is to make a statement about cutting corners.
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wrote

Well, you were saying it was about flat roofs and I said our schools have flat roofs. (as well as many stores). Now you ignore that and allude to design. Your buildings have to be sub-par to have roofs caving in.
I found this site: http://modernsurvivalblog.com/weather-preparedness/total-inches-of-snow-in - boston-so-far/
It shows a map of the US. We have the same snow depths as NE. Our normal is several times that of Boston. And at least half of what we get is heavy lake snows. When you say you have the most snow in 100 years it makes me wonder if that is the same as Boston, which really is next to nothing. Your snow is exceptional this year. That is our normal and yet we don't have roofs falling down. That's because ours are built to withstand the weather. Now, if you have 15 or 20 feet of snow fallen, then I'd be really wowed.
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Note: I did not way it was the most snow in 100 years, I said it was the heaviest load. Most times, the snow is pretty well cleared a couple of times during the season, but this year it just kept coming with no relief. Then we had rain that was just absorbed into the snow, adding weight, instead of running off.
Both pitched and flat roof caved in.
You cannot say they are sub par unless you know the design and construction of each. I'm sure that some may be, others probably met code. With a couple of hundred buildings down, no one can make a blanket statement that covers them all, but any statement can probably apply to at least a few. Some of the buildings were old and possibly weakened from abuse or neglect. Some were old and built before any codes existed.
Of course, you may have actual load data between the snow here and the snow in your area for a real comparison. If you have it, let us know.
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On Sat, 26 Feb 2011 00:59:00 -0500, "Ed Pawlowski"

A local bar had their roof collapse last year. Fortunately it was on a sunday when the bar is closed, or there could have been some severe injuries or deaths.
Last month I saw on the news where a club for children similar to the YMCA had their entire roof fall inside from snow. Luckily it happened during the night when no one was inside.
Last week a local business had a huge pile of icy snow slide off the tin roof. Someone's car was parked in back of the building. The car was literally crushed, even to the point that the axles or frame bent, because the tires/wheels were bent outward on the bottom.
And while this is in a whole different category, look at the Vikings Metrodome in Minneapolis. That roof collapsed this winter.
Snow and ice is a lot heavier than most people think. But think how heavy one shovel full of wet snow is, and know there are hundreds or thousands of those shovel fulls on a roof.
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Hype - sure. Same as snow predictions. They bump up the predicted amount - at least that's the way it usually seems to turn out - to keep people tuning in (and exposed to advertising dollars).
As far as the roof loads, all of that is spelled out in your building code. In the beginning of the chapter on design they usually start with the standardized IRC and incorporate state and local code modifications.
Simply knowing the design snow load might give someone some peace of mind, or terrify them, depending, but it won't address the roof and structure as built as a whole. Wind load is on top of snow load, and one side of the roof might have a positive wind load and the other a negative. The steeper the roof, the better for snow load, but the worse for wind load. Construction details, collar ties, all sorts of things affect the strength and stability of a roof.
Generally, if your house is in good condition, doesn't have more than one re-roofing, hasn't experienced any roof leaks of note and there's no rotting of rafters, and it's been standing for a while, there's little point in worrying about your roof caving in.
R
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