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Old 02-12-2010, 03:03 PM   #21
Tom2
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Default Re: how much snow can a roof hold?

Most roofs can hold quite a bit. Most people start clearing theres around here when theres 3ft or so.

We usually have a few roof collapses a year around here. They tend to be old barns, or flat top roofs like shops, wal-marts, etc..

We've hardly had any snow this year.. We're on pace for average snowfall, but it's just in half inch increments. I've only had to snowblow like 4 times.
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Old 02-12-2010, 05:02 PM   #22
WNYflyer
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Default Re: how much snow can a roof hold?

In your Philly area the density of a typical snow should be about 18 pounds per cubic foot. Thus for every inch of snow on your roof that would be a snow load of 1.5 psf. I would think new built roofs in your area should be able to handle at least a 25 psf uniform snow load (could be more ?)which doing the math equates to about 17 inches of snow. I believe new code says an absolute minimum of 20 psf roof uniform live (comes and goes load such as snow,wind) load regardless of what part of the country you are in but of course may have to be adjusted higher depending on the code snow amounts in your area. At code mimimum of 20 psf you would be at 14 inches of snow.

So many variables of course such as pitch on the roof, valleys, construction etc. But if you know from your building official what uniform snow load (psf) new builds should be designed for you can use the 1.5 psf per inch of snow depth to come up with the depth of snow that is code for the new builds.

Hope that helps some.
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Old 02-12-2010, 06:55 PM   #23
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Default Re: how much snow can a roof hold?

Yes, deflection is the right word. It is a property that can be used to quantify stress. Do this now while it's a given that your OK as far as supporting this quantity of snow. Measure distances between marked points on roof rafters, joists or beams and a bench mark. Record these and measure the distances at the same locations again as soon as the snow has melted. The difference is your deflection due to snow load.

distance with no load - distance with load = deflection

Example: Drive a nail into a rafter (best location for greatest measureable deflection is the center of a span), then put a laser at a marked location on a sound footing (for a dirt floor or no footing drive a rod below your frost line, its got to stay there now for good, use the top of it as your bench mark) and measure and note the perpendicular distance from that nail to the laser line as it is sitting on the footing or on the rod top. Do this again as soon as the snow has melted (the humidity and temperature difference of summer will render the results useless) and record these results. Calculate your deflection. If its say 3/8", now you know your good for that deflection whether you've got 8" of rain soaked snow or 36" of powder.

Label the nails A, B, C etc. In my garage I would only need to measure one location, from the bottom of the center of my ridge beam's span to the laser's beam while it is sitting on the footing. In my case the greatest deflection occurs here.

Now the next time your concerned take a measurement at the same location(s) and if their deflection(s) are the same or less for that given location your OK.

Barring insect or water damage, and or changes to the associated structural members, the deflection will not change for a given load (+ or - a smidge).

Play around with it, its easier done then said. If you can get a permanent bench mark below your weakest roof point, this can be as simple as a plumb line with marks on it for load and no load.

A new roof will mean a new "no load" distance will have to be measured.

This is ball park, I didn't know of any rules of thumb either!

Last edited by stewart; 02-12-2010 at 06:57 PM. Reason: typo
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Old 02-12-2010, 07:13 PM   #24
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Default Re: how much snow can a roof hold?

Here's my take... In NH, and we get (usually) a lot of snow...

If you want to be sure, you should pay a structural engineer with experience with your type of building to evaluate your roof. I've seen people blissfully ignorant of the fact that they need to remove the snow on the roof, and they are at major risk of damage or total failure. We get a a*lot* of buildings collapse each year we have heavy snow because people either don't check, don't understand how much the snow weighs, or are away and simply don't realize how deep it is.

Rules of thumb... Measure your ground cover snow. Roof snow is about 80% of that number. The weight of snow varies with the type of snow. Rain on top of snow is very bad in many cases because the snow soaks it up like a sponge. (I lost a building because of rain on top of snow a few years ago...)

My garage is rated at 75 lb per sqft. OK, so for the rough size of my building, that's 135,000 pounds! (Almost 70 tons). Heavy stuff. And, yes even with that rating we've shoveled it a few times.

Bottom line, get a snow rake if the building is not too high, lets you remove much of the roof snow, while standing on the ground. Shoveling from the roof is very dangerous, let someone else do it if at all possible!

The (I believe) USDA or Farm Bureau has some good information for snow loading on barns too. And one recommendation is to use the heating system to attempt to melt some or all the snow. If you have an uninsulated building that is very viable (not significant if your roof/ceiling is insulated...) if you can heat the building up substantially.
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Old 02-12-2010, 07:55 PM   #25
Charles (in GA)
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Default Re: how much snow can a roof hold?

How much snow can a roof hold?

Answer: NOT ENOUGH

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Old 02-12-2010, 08:04 PM   #26
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Default Re: how much snow can a roof hold?

One thing nobody has mentioned, if it should warm up and rain very much, and the snow doesn't slide off the roof, the weight will increase a lot. The weight of saturated snow is much closer to that of plain water, than to dry snow. You could easily get up to 30lbs./cubic foot of snow/water combination, and the weight per square foot of roof will skyrocket if you have a couple or three feet of snow up there. Murphy's law says this will occur during the night while you are sound asleep!
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Old 02-12-2010, 08:10 PM   #27
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Default Re: how much snow can a roof hold?

Was hoping the township or fire dept. would have a snow rake.. no such luck.. Will try to build one out of a long length of pipe I have laying around and a length of 2x6 I think. Will also call around locally to see if the hardware stores have one - it would be good to own.

On the way home today, saw the front overhang on a house ripped completely off - it was about 4 feet deep by 25 foot long overhang.... now, it was a really crappy house, poor construction, but it makes you think.
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Old 02-12-2010, 10:06 PM   #28
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Default Re: how much snow can a roof hold?

We used to run a rope the lenght of the roof and down to the ground on either end. Pull in a sawing motion down the slope. About 75% of the time by the time you got to the bottom it would slide. Just be careful as snow hits the ground hard.
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Old 02-13-2010, 07:51 AM   #29
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Default Re: how much snow can a roof hold?

Would scattering some calcium chloride (is that the correct name for the rock salt alternative?) near the edges promote melting, or cause more problems?

That rope idea is a good one
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Old 02-13-2010, 08:15 PM   #30
willymakeit
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Default Re: how much snow can a roof hold?

If you have a metal roof , I wouldnt use calcium chloride, would cause rust. They do make some citrus based de-icers and melting agents, but I have zero exp. with them. Google ctrus deicer.
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Old 02-13-2010, 08:27 PM   #31
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Default Re: how much snow can a roof hold?

I know how much my carport couldn't hold!



14" of snow and an inch of rain!
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Old 02-14-2010, 07:11 AM   #32
Daniel Dudley
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Default Re: how much snow can a roof hold?

Yeah, do what you can to get it off. It isn't a big worry IMO, but heavy rain or freezing rain added to that would be a LOT more weight. The snow would hold it like a sponge.
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Old 02-14-2010, 07:18 AM   #33
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Default Re: how much snow can a roof hold?

I got up on the flat porch roof and carefully shoveled the 4 feet near the edges on all sides, and then broke up the middle so wind and sun can help me out a bit. Based on how back breaking the work was, I'm glad I got the weight off there. The barn is just too high to get at so I'm going to have to roll the dice there right now unless the neighbor and I can figure out something creative.
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Old 02-14-2010, 07:34 AM   #34
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Default Re: how much snow can a roof hold?

I'm more concerned with hurricane force winds than snow...
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