Snow On The Roof May Literally Weigh Tons – Here’s How Much

snow-weight-on-roof

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:

Garelick 21-Foot Aluminum Snow Roof Rake With 24-Inch Blade

 
What’s the most snow you’ve had on your roof?

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47 Comments

  1. I never design a building roof with less than 80 psf snow load capacity and metal roofing with no less than 6/12 slope for North Idaho.

    Even warmer climate areas can be surprised some winter with that dump of wet snow.

    I can not count how many of those temporary shelters (plastic or coated canvas) over vehicles and tractors that have collapsed due to snow.

    1. That’s good! Equates to handling ‘about’ 4 feet of heavy wet snow.

      I had two of those ‘shelters’ (still have one of them). One collapsed during the first winter.

      We had gone on a road trip, late winter. And woudn’t you know it – the day after we left there was a major snowstorm dumping a foot of the heavy stuff. Not being there to roof rake, when we returned, it was collapsed into a heap.

      The other one miraculously survived. Go figure…

      1. Ken, about the fith-wheel RV. I contacted an RV dealership in Vermont and was told that unless there is a weak area (i.e. cracked seal etc.) then you should be fine with snow on the roof. Or if the roof, for some reason is soft, then there may be concern for a possible cave in. Thinking about the thousands of RV’s sitting in storage throughout the winter in the north country ,then used again without a problem, unless they don’t maintain their seals/seams.
        You don’t see RV dealerships clearing off the roofs in the wintertime.
        At your own discretion of course sir. If not careful, one could cause more damage, than the good done by removing the snow.
        Pete’s RV has a good video about it on YouTube.

        1. CR, That’s a good point… Also, the roof is slightly bowed upward (helps to transfer weight to the side supports) and I know there are metal (aluminum) ceiling support/struts holding it all together.

  2. How much snow have we had on the roof ? For this area, probable close to two feet. Since the house is two store I have it shoveled off, plan on it twice a year. I had a double pole roof rake and one could reach a foot or two on the roof edge, waste of time and handling that length I felt like Carl Walenda sometimes. I have not been up there for years since I don’t bounce so well anymore.😨 We are getting the heavy rain today, had about ten inches of snow on the roof, roof edges were clear so no shoveling this time, no ice dams so draining off fast. Part of living where we live. ❄🌧🌨🥃🤞

  3. Never lived where snow weight was a concern, but the year after we built our retirement home we had a terrible ice storm. Most trees suffered loss of major limbs, and some large trees broke in half or were uprooted because of the weight of the ice. My home, barn, both cabins, and all outbuildings have metal roofs, underlayed with 3/4 inch marine plywood, with 2×4 engineered trusses. Weight of the ice posed no problems, even though it was close to 3 inches thick, but I failed to install ice dams around all the vent pipes (had never experienced such an ice storm in my lifetime up till then). When the ice started melting, it slid off the roofs shearing the vents off even with the roof.

  4. Fortunately here with the never ending wind, most of it just blows off. But it could be real problem if it ever snowed without wind. I may need to put a roof rake on my preparedness list.

    I wonder if it would work with volcano ash?

  5. My house is pretty old, multiple additions, waaayyyy too many valleys! I usually rake it off when it gets to about a foot, give or take…usually once or twice a season. The outbuildings have metal roofing, so I don’t usually worry about those, although seeing the numbers has made me change my thinking on that!

  6. I live in a stupid state, even here up north the building codes are for a 25 pound load….. yea right, NOT on my house ya don’t, built it with a 50 pound load and 100mph wind loads.
    BTW tis a flat TPO roof (1/2 in 12).
    Most snow i allow is 2 feet. Than it gets shoveled.
    BTW had a wopping 6″ of snow here this year….. total.

    1. NRP
      Read the Code – your area has a 50 lb dust load factor. haha I’ve seen pictures of them big rolling dust clouds moving in on southern areas.

          1. NRP
            Seriously, have you considered a roof-top garden with that low slope roof? Sure would keep the house cooler.

          2. hermit us;
            No need for extra insulation on roof.
            Remembering I built my own home, roof is a full R-38 Fiber and a full 5″ of ISO (R-25) on top under the 60mil TPO, Basically the roof is right around an R-63, the walls specked out around an R-46.
            I don’t like to be cold… HAHAHA
            Ohhh yeah, 2 wood Stoves, 2 DX unites (85 btu/2-ton & 135 btu/5-ton) AND in floor hedonic heat with a 190 btu boiler. PLUS 5 ceiling Fans…… HAHAHAHA Yes I’m NUTS!!!!

          3. NRP
            But think of the practicality and aesthetics, there is NRP standing atop his field. And all the lettuces you could grow.

    2. When i was living in Co we were framing a house in Grand Lake,
      The engineer specs were for a 250# dead load and 200mph wind load, @ the 250# per square loading, was a first for me coming from the islands where at the time dead load was 50# per sf and a 100 mph wind load,
      Co, 11-7/8” TJI @ 12” OC with 1-1/8” ply over and supports no more than 10’ OC below
      VS
      Hi 2×8 @. 24”OC w 1/2” ply over not a support between wall and ridge on a 18’ run

  7. Building my retirement home in a few months….in an area that gets no more than 4-5″ of snow at a time. That being said, I am adding a cable attached to near the peak of the roof and a safety harness when I’m up there cleaning the solar panels and solar hot water unit. It’s a two story place in the middle of nowhere and can’t afford to fall off at my old age! Any thoughts?

    1. jon dowe
      —since you are already attaching cable to peak, go all the way, and rig up a gizmo with rotating brushes. Put a sensor in, so when snow hits two inches, it starts up and clears the snow. — One should try to plan to avoid climbing on second story roofs, especially after one’s retirement, especially in the middle of no where.

      1. For what it’s worth Mary, I’m not that old…yet. My last Ironman was two years ago. Just thinking about the future…I do like your suggestion! I’ll keep it in mind!

    2. jon
      Build with a steep roof 8/12 or more slope – does not cost much more – the snow will slide off.

    3. My solar panels are ground mounted. No climbing. Just a floor push broom after a storm, except you pull – not push.

      So my suggestion if you’re building new in a snow zone, if you have the space, and don’t mind seeing them, panels at ground level.

      1. I should have explained more… the yearly total there is a little more than a foot, just the maximum individual snowfalls can be 4-5″ at a time. I would like them on the roof due to concerns about theft and vandalism. There has already been one incident of that on the property while I wasn’t there. Guess even in the middle of nowhere isn’t a total escape from the realities of the world…. :-) While it’s being built, I’ll be living on property in a travel trailer and maybe that will limit the issues. I thank all of you for your feedback!

    1. I have some of mine on the roof. No problems. I also have a couple mounted on a south west facing wall to catch the last bits of light til sundown.

  8. We have a 10/12 pitched roof, so snow accumulation is minimal.
    At the most, probably 4 inches.
    When I was a kid, and blizzards where common, I remember dad on the garage roof shoveling snow off a couple times.
    The big joke of my uncle once took a snowblower to a flat roofed church. How he got it up there, I have no idea. Would have been a site to see.

  9. I have a manufactured home that I had built for the climate up here in North Idaho. Holds the heat well. I had over 3 feet of fairly wet snow on the roof one year, with no problem. However, another snow storm was coming, and that got me very worried. Took 7 hours to shovel that stuff off, and left about 6 inches there for safety, so as not to damage the roof with a shovel. Got another foot that night. Long time ago, but still have the pictures to prove it.

    1. Where I am at, we usually get good amounts of snow every winter. If it is a snow storm of the kind that comes into our area like a storm that hits a good portion of the north, usually there is a lot of wind so there is not too much accumulation on the roof. However, if it is lake effect snow that comes off Lake Michigan, then a lot of the times there isn’t much wind. Lake effect snow can drop a huge amount locally up to a few feet to more. Years ago there was one lake effect storm where a state of emergency was declared for our area. We didn’t have a snow plow come by the house for two days. Afterward, we went shopping and noticed that about 25 miles south of us, there was only a dusting of snow of about one inch. The plows were not even really needed as vehicles blew most of the snow off the road.

      1. Reminds me of when I first got married here. We were newly weds and got snowed in for 21/2 days, no one around, and nothing for us to do.(lol)

  10. – Long ago in Germany, we were in the Fulda gap, actually in the village of Fulda. Noticed the (steep) half-timbered roofs, and the doors with no balcony, etc., on the second floor. Came there again that winter, we got almost 3 meters (ten feet) in one snowfall that winter. I also remember standing on the top of the canvas extension on the Tactical Operations Center (the extended top M577 APC) trying to sweep the canvas clear during the middle of the night so it wouldn’t collapse (Basically the frame was 3″ pipe). Great fun to take your turn at in the middle of the windy, COLD dark night.
    – Papa S.

    1. – Forgot to say, during the winter, they would unlock those second floor doors and that would become the new front door. The first floor became a basement, and the door just remained closed.
      – Papa

  11. Papa Smurf:

    Not trying to pry but do the words 11th Armored Cav ring a bell? If yes, allons!

  12. In some places in Alaska the ground and the roof merge. I don’t know what the specs are up there, but I have seem snow blowers on roofs up there as a normal procedure. We moved into a new house in winter and a few days later i was shoveling snow off the back deck and didn’t realize it ended and stepped off into about 4ft of snow. And i am only 5 feet!

  13. 11 years ago we saw 56 inches of snow between Christmas and New Years. Herds were being fed by Helicopter! Front loaders and dump trucks were being used to create one way tunnels through town.
    We built our home of steel. roof is 4/12, 62# snow load, 150 mph winds. WAAAY over the county specs. built our car port with same loads, wind spec etc. Many neighbors here foolishly did not, and all have suffered damages over the years. So far this year we have had 71 inches of snow since Oct 4. This is actually typical when not in drought. deepest has been 16 inches in one snow fall so far, but our really heavy dumpers (32 inches) usually happen in spring when we get was is called an Albuquerque low. Then we see some heavy snowfalls. True splendor to watch and listen to THAT load come off the roof. LOL DH and I laugh and yell AVALANCHE….but you sure would NOT want to be standing under it as it comes down!

  14. Our home has a 5/12 pitch , metal roof with snow brakes and we had 2 foot eaves built to leave a fairly protected walkway during rain and snow . We usually get 3-6 inches of snow at a time and it is gone in a week or so. I talked to several old timers in the area prior to building and this seems to be a normal pattern. After 8 years here that is the pattern we’ve experienced. A few folks talked of a 2 foot snow several years ago, but it was not a normal snowfall. This winter is different, much warmer, 1 inch of snow so far .

    1. Bluesman
      If you look into the true meaning of roof pitch, it refers to the rise at the center of the building (ridge) over the total width of the building (wall to wall). Slope is the rise at the center of the building over half of the width of the building. For example, a 40′ wide building with a 10′ rise to the ridge is a pitch of 10/40 or 1/4. The slope of the roof on the same building would be 10/20 or 1/2 (6/12 slope).
      Many have gotten away from the difference of the two and swear they are the same. How the truth can change over time. :)

      1. Yup, you are right. Pitch assumed the peak was dead center in the middle of the span. That’s why using the slope is a better description of what going on for both sides of the roof where the peak might not be dead center. Pitch probably stopped being used correctly by the general public in the past 50 years. Just use slope= rise/run to get the point across. Good catch Hermit Us, you win the “archaic engineering term of the week award”. LOL!

      2. hermit us,
        Thanks for the correction, it is never too late to learn something. I have mostly heard roof pitch instead of roof slope.Enjoy the day.

  15. GF of mine moved her business up onto the skirt of Mt Rainier quite a while back. They get 6 feet of rain a year there as well as 22 feet of snow. They don’t even bother. Roof are pitched so steep they are almost cartoonish.

  16. We have a 6/12 slope on the tiled roof cottage, and a 12/12 slope on the metal garage/canning kitchen roof. 23# snow load is county spec, we over designed and built. We supposedly average 8″ of moisture annually, but have been as low as 2.5″ in recent years. Steepest slope roof I think I have had to contend with was about 18/12, on a ski lodge at A Basin ski area in Colorado, at about 10,800′ elevation. 350″ of snow a year average, or so they claim.

    1. Minerjim
      My buildings are a combination of 12/12 and 6/12 slopes. Metal roof holds some snow on the 6/12 until temperature changes break it loose. Only problem is that I have to plow snow away from building so I can look out the windows. One year I just dug tunnels out from the windows. haha

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