You might be very surprised to discover how much snow weighs up on your roof! Heavy snowfall accumulation will stress your roof and may potentially become a danger to roof collapse (design dependent!).
A cubic foot of snow can weigh about 7 pounds if it’s dry and fluffy. It could weigh over 20 pounds if it’s wet and compacted!
Two feet of snow on a roof with a footprint of say, 28×40 feet, could weigh 45,000 pounds! That’s more than 22 tons!
Flat roofs may be especially vulnerable to roof collapse, depending on its engineered design load. Snow won’t slide off. Though pitched roofs can also accumulate snow. Metal roofs are better than shingles for snow to slide off.
In addition to roof integrity, watch out for snow slides. If you’re in the wrong place at the wrong time, you could be seriously injured. The main roof on the outbuilding I recently built has a 9/12 pitch. When the snow/roof contact resistance ‘breaks’, it’s a sight to see a foot of snow avalanche to the ground. Sounds like an earthquake! Wouldn’t want to be underneath at that moment!
Here are a few examples of the weight of snow on a roof:
All numbers are approximate.
psf = pounds per square foot
How Much Does Snow Weigh on a Roof
28×40, Light, Fluffy Snow
1 foot of snow on the roof (8,000 lbs, 4 tons, 7 psf)
2 feet of snow on the roof (16,000 lbs, 8 tons, 14 psf)
3 feet of snow on the roof (24,000 lbs, 12 tons, 21 psf)
28×40, Heavy Wet Snow
1 foot of snow on the roof (22,000 lbs, 11 tons, 20 psf)
2 feet of snow on the roof (45,000 lbs, 22 tons, 40 psf)
3 feet of snow on the roof (67,000 lbs, 33 tons, 60 psf)
Lets say you’ve got an industrial building or outbuilding with a 40×60 footprint and 2 feet of heavy snow. That roof will be straining under 70 tons! Pretty crazy when you think about it…
Building codes take into account winter snow roof loads for the region. For example where I live, roofs are generally designed to 50-60 psf. That likely covers up to 3 feet of heavy wet snow. Though I wouldn’t want to test the integrity!
Weight of Water per square foot of rain
Now lets say you have 18 inches of snow on the roof and then you get a unseasonable rain storm that dumps 2 inches of rain on top of all that. Now how much does all that cement-like snow weigh?
(This just happened to me today – which is why I’m posting this).
1 inch of rainfall weights 5.2 pounds per square foot.
Thusly, if my 18 inches of snow on the roof weighs about 20 lbs/sq.’ (medium weight snow) and then I add another 10 pounds for 2″ of rainfall, I get about 30 pounds per square foot. Should still be safe assuming the builder designed my roof properly! Phewwww…
Still, this kind of situation will start to reveal where roof design may have been less than ideal. Will look for roof collapses on the news tonight.
Weight of snow on a RV or trailer roof
What about the trailer roof?! I have a 5th-wheel trailer which I had to roof rake today in the pouring rain. It had more than a foot of rain soaked snow on it.
I knew it could handle it though because when I first moved here several years ago I let it get away from me that winter… It had over 2 feet of snow on its roof (had it parked out of sight – forgot about the situation). Whoops! Spent a day getting it off. Thank goodness it didn’t collapse.
That said, I have no idea what the roof design parameters are for a typical trailer. It certainly is likely NOT as good as my home roof!
A Good Roof Rake!
The first year I lived up here I bought a roof rake. It broke the end of last year after several years of service. I should have bought the better one (you get what you pay for – learned once again).
I bought a new one last month and it’s much better than my old one. It’s all aluminum rather than a plastic ‘shovel’ end. That’s where it broke last year. Where the plastic shovel (scoop) hinges with the support brackets to the pole – the strain eventually cracked the plastic (surprise, surprise).
Anyway, here’s the one I just bought:
What’s the most snow you’ve had on your roof?