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Friday, February 4, 2011

A Foot of Snow Will Bend A Low Sloped or Flat Roof.

Last year a very wet foot of snow fell on Dallas for the first time. At 32°F you have the wettest snow possible. Many people called us with sags and other problems. One adjuster told me he say no structural damage from the snow and I let out a roar of a laugh. "What about all the collapsed patios and carports?" He said that that was different. I asked "how?" He said those were different. Again I asked "How?' and he gave me the blank stare.

One guy told me he kept and I on his home and the sag over his cathedral ceiling was new. The insurance said no.

A new customer has a leak on his low sloped composition roof that began last year. First the roof leaked only in harder rains and after we tried to fix the tarred up skylight, it leaked in a sprinkle. We noticed that when observing the roof from the other side that the roof looked like it had a lake on it. When standing above the roof you could not see the lake.

The roof had sagged and parts of the roof had less than the minimum magic slope of 2/12, a fall of two feet every twelve feet. In fact the surface became a collection bowl toward the center.

The adjuster saw nothing broken but bought the slope of roof and the inside damage. There was no reason to buy this slope except for the fact it was bent and he didn't pay for the unbending, reframing that is, of the section. What a dizzz. He had the gall to argue with me about the minimum roofing slope claiming the statute was 3/12. I quoted the IRC, International Residential Code (the precursor for the UBC or Uniform Building Code), the NRCA, GAF, Certainteed.... he said we could agree to disagree. I told him "No. I am right and you are wrong." The gall bladder of him.

Some of these bent and sagging roofs are starting to show more stress but the insurance companies will continue to blame defective construction methods. Funny how these methods worked until we had the first foot of snow in this epoch, and it was as wet a snow as can be. Besides, as the snow slowly melted a pool formed behind the overhangs causing more weight than the foot of wet snow. Now a small lake maybe two or three inches deep of slush and water was added to the foot of snow.

If the structure bends, the framing will never go back to where the original plane was. The result will be somewhere between 1/3 and 2/3 spring back but if you are at the minimum, you no longer are. Leave this on the roof for a week and she'll never be the same.

On top of all this, on low slope roofs the weight is placed more on the rafters and joists themselves rather than the outside walls. The steep the roof the more the load is on the walls. The less the slope, the more weight is on the middle of the roof.

Let's see how much wet snow weighs.

Ken Hellevang, from the University of North Dakota says:

"The weight of snow varies greatly. Light fluffy snow may only weigh about seven pounds per cubic foot. More average snow may weigh 15 pounds per cubic foot and drifted compacted snow may weigh 20 pounds or more.

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Or how about this answer from

That depends

It depends on how much you have!!
One shovelful of snow, for example, weighs less than the amount of snow on your driveway.
I suspect what you are really asking is not how much snow weighs but how much it weighs per cubit foot or cubic yard. Weight per unit volume is called density.
But even that is tricky with respect to snow. The density of snow varies greatly. Lightly packed powder weighs very little per cubic foot, whereas slushy, wet snow can weigh over 62 pounds per cubic foot -- about the density of water.


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And another:

A square foot of snow does not have a weight: the depth of the snow is needed to give the weight because weight relates to the volume of snow. Even if you know the depth of the snow, say a foot, giving a cubic foot of snow, there is a wide range of possible weights because snow can vary in density depending on whether it is lightly or densely packed.

The range of density of snow compared to the density of water can vary from 100:1 (for snow that is 100 times less dense or heavy than the equivalent volume of water) to 3:1.
A more common density might be around 12:1.

Water has a mass of 1kg/L so for a cubic meter (1000L) of snow, the mass could be anything in the range of 10kg to 333kg.
Keeping that in mind, a foot of snow would be about 6.75lb to 223.82lb.
The weight of snow can not be answered based on an area of a square foot of snow: the volume is required. It also depends on the kind of snow it is. Example- packing snow will have a different weight (because of its density) than other kinds of snow.

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Are we sure that there is no possibility that this newly sagged roof, that didn't sag before the snow, could not have been damaged by the melting foot of very wet snow. If you add 620 lbs. of wet snow to the middle of the roof for every roof square, and the average roof is 35 squares, then you have 21,700 extra pounds on the roof with much of it going to the center. Any preexisting sag will now become an arroyo.

The roof I've been referring to has only 625 feet of surface area on a 2/12 pitch with no framing between the ridge and fascia. That's 3,875 lbs, almost an F-150 Ford pick up, and is if you add the melted pooling water under the snow. That is if you break it down by the foot because some areas give up weight to place it in the middle of the roof.

Remember the Metrodome!

Jon Alan Wright
Jon Wright Roofing, Siding, and Windows
1915 Peters Rd., Suite 310
Irving, TX 75061
972.251.1818 Office
214.718.3748 Cell
972.554.8090 Fax
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