Roof snow loads

[SMF]

Howdy,

Since it seems the fright of roof collapses around my area is done for winter (I hope) and the frenzy to find a roof rake has passed I have a question if anyone can help.

Without hiring a structural engineer to come check out your home, is there a formula to determine how much snow weight a roof can hold based on pitch by spacing of rafters by slope of the roof. I would think this would be easy to find but I have searched far and wide and have not found anything easy to use.

It makes me think there was really no need to clean off the residential roofs in my area that are sloped and this was just a bunch of hype. I don't recall any home collapses but instead dozens of homeowners in the ER from falling off instead. I'm in Connecticut.

Many thanks!

[RobertPatrick]

SMF wrote in news: [e-mail]:

Myself and others in my neighborhood use a roof rake to prevent ice dams at the edge of the roof. It's nothing to do with weight of snow since

99.9% of the buildings were made to withstand a lot of snow.

[trader4]

[e-mail]:

All I know is that of the occasional roof failures that I've heard about or seen all have been on flat roofs, usually on large commercial type buildings or schools. I don't know of a single snow failure on a normal, pitched residential roof.

And I would agree that it's more dangerous to be up there trying to shovel it off. Also, it would obviously depend on the composition of the snow, ie how much it weighes per foot.

[hrhofmann]

[e-mail]:

Somewhere there must be an architectural type who knows where tables of acceptable roof weights are located.

[Keith]

I'm in Connecticut too. I've never figured out the Courant's archive bit, but a few weeks ago they had a photo series on their site about collapsed roofs, and all in the pictures were pitched. A lot were farm buildings, many were commercial, there was at least one fire station, and the rest were all residential. I believe the destruction to the homes was severe enough that all had to be razed. They didn't look like particularly old homes in the pictures, although one of them had lasted through 200 winters.

Different parts of the state had different amounts of snow, too. I'm in the CT River valley and we tend to get off pretty easy, but the three largest storms tracked right up that big sheet of ice. For weeks we could only see out the top halves of our downstairs windows.

[Erma1ina]

Here's a pretty good Courant link on the subject of this season's roof collapses in Connecticut:

[Erma1ina]

Check this out:

[Erma1ina]

This shows a couple "normal, pitched residential roofs" that collapsed in Connecticut during late January and early February:

[ktos]

LSMFT wrote in news:j%E9p.29928$ [e-mail]:

How stupid is that!! Who in their right mind clears snow off the roof after each snow? We get snow several times a week or more. Really stupid to clear the roof. Nobody's roof caves in. They are made well. You must have a wrongly made house/roof.

[HeyBub]

Snow weighs - give or take - about twenty pounds per cubic foot.

I'm in southeastern Texas. It snows about 1/4" every eight years. Call me an expert.

[Ed Pawlowski]

wrote

I know, I know Pick me -- Pick me.

Senexet Road in Woodstock CT.

Not only is the roof gone, the house walls are bowed out and a dozer is parked up agains one wall to hold it in place. Owner managed to get their belongings out.

A few houses in CT had damage, many outbuildings such as sheds and barns wend down or hat at least a partial roof collapse. One industrial building went down with a pitched roof and wood trusses

[trader4]

Here's a decent answer I found that references the BOCA code:

It has the MINIMUM allowed design load as a function of roof pitch:

0 - 19 deg 20lbs/sqft 19 - 45 16 45+ 12

Snow weight varies greatly, so that could be several feet of light snow or less than a foot of very heavy wet snow. I would bet that in most of those cases shown of roof failures in CT, the roofs were not designed to that spec.

[trader4]

After I posted the above, I read more of the link. They go on to say that snow is considered in addition to the above "live load". So, the roof is supposed to withstand the above plus an additional load for anticipated snow. It says in the northeast, that amount is from 15 to 100 pounds and that amount can be derated the more pitched the roof is. So, I guess the answer is it can vary greatly and only your local code official knows the correct answer.

[ktos]

"Ed Pawlowski" wrote in news:3q2dnXP1jOFUu_rQnZ2dnUVZ [e-mail]:

Proof of shoddy building.

[Don Phillipson]

It depends where you live. Where snow accumulation is normal, most governments maintain building research laboratories (so as to produce building safety codes etc.) If you cannot locate yours, ask the municipal office responsible for building permits and codes.

[bud--]

You could use the size of the rafters, rafter length, and spacing between them and use a floor joist structural strength table to get the possible load. The roof load is the roof deck, shingles, and snow. It would work for a roof with a simple ridge. Wouldn't work for more complicated roof, like a hip roof.

It is also possible for the connection of rafters to joists below to fail so the roof triangle flattens.

Someone posted this previously. I haven't looked at it. I notice "snowload" is in the URL

(the URL has to be reconstructed into a single line)

A news station here talked to a structural engineer about snow load. The design for snow load in Minneapolis is 35 psf. That is, according to the engineer, about 6 feet of snow. Further north in MN the design snow load is higher.

If there is a roof collapse it is big news because it is uncommon. How many roofs have collapsed near you? Roof raking is probably more important for preventing ice dams. (There are safer ways to prevent ice dams.)

[Higgs Boson]

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.

Reactions?

[Jules Richardson]

I'm in northern MN. I just checked our place - rafters are on 12" spacings and use 2x6's on the north side and 2x4's on the south side (with vertical braces halfway down the south side to the ceiling joists below). I was surprised at the mixture, but I suppose the south side gets the sun and never does have as much snow on it as the north - plus it's a steeper pitch than on the north side.

We've got some sections of flat roof, too - near as I can estimate (without drilling a hole in a ceiling ;-) those are done with 8" high joists, but I don't know the width or spacings.

Ours have been bad this year on the north side - probably around 10" high until I got rid of them the other day now that things are starting to melt a little.

Oh, when we moved to our current place there was no gutter on the north side of the house, but there were supports for one made from wood and attached to the house with hinges - the hinges just seemed strange, and I couldn't think why they'd made these big chunky wooden cradles for the gutter, either. Then other day my wife realised that the previous owners probably took the entire gutter down at the start of each winter and swung the supports back against the house beneath the eaves so that any ice/snow slides from the roof wouldn't tear the entire lot down. I've never heard of that being done anywhere before, but it's an interesting idea. Bet the darn thing never sealed properly at the joints, though!

cheers

Jules

[SMF]

Thanks so much for your reply. Still hard to figure out since my home was build in 1960 and I'm sure the code was different, possibly better.

[SMF]

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