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Way off topic -- How much snow can a roof support?

Post in 'DIY and General non-hearth advice' started by ControlFreak, Feb 1, 2011.

  1. ControlFreak

    ControlFreak Feeling the Heat

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    Our roof is an 8-pitch roof, how much snow is too much?

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  2. webbie

    webbie Seasoned Moderator Staff Member

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    There is no real answer to that since the density of snow varies so much.

    We have a 12/12 and I really doubt any amount of snow could hurt it due to the fact that some would likely slide off.

    An 8 pitch is pretty steep too.

    It seems to me the biggest danger is to gutters and also having ice dams end up making your roof leak. However, newer houses - and probably built older ones - have protection against this. When I built my garage addition, they used 3 foot of roll roofing along the bottom under the shingles so that water would have to work a LONG way up....never had yet!

    I think flat and low slope roofs are the ones to worry about. I don't recall many instances of roofs with 8 and over pitch failing in any way.
  3. Cowboy Billy

    Cowboy Billy Minister of Fire

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    It depends on the building code in your area. In Michigan snow load code go from 25 to 60 lb/sqf depending on where its built.

    Billy
  4. nate379

    nate379 Guest

    No way to tell without knowing so many details and even then still.

    If new house and built to code for area I would not let more than 4-5ft build up just to play it safe. More so if temps get warm or it looks like it will rain... snow will get HEAVY then.
  5. LLigetfa

    LLigetfa Minister of Fire

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    I have a 12/12 pitch roof and if it were not for the valleys, my answer would be "lots" but alas I pull the snow down before I get the answer to how much it cannot support.
  6. peakbagger

    peakbagger Minister of Fire

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    Its right on topic considering how many ice dam questions are out there. Unfortunately it depends on construction of the roof. IMHO usually roofs fail due to poor construction details before the beams snap which would be considered a classical roof failure. Dependent upon the age of your house, it should have been built to a building code which would have specified a design snow load. As mentioned in one of the other threads the snow load can range from 10 pounds per square foot to 110 PSF. Once you know that value you can convert the snow depth to weight per square foot on the roof. I hope your reference to 8 pitch is and 8/12 pitch (8" rise in elevation every 12 inches of horizontal). Assuming an 8/12 pitch thats a real steep roof and usually snow just cant build up too high. 1 inch of water one foot square is 5.2 PSF. Snow usually ranges from about 12" per inch (powder)of water to 6" per inch (wet snow) so at 30 psf, a square foot area can have from 69 inches of powder to 35" of wet snow. Obviously anywhere that liquid water can pond up is 5.2 PSF times the depth of water.

    In order to come up with an estimation of what your roof is good for, an engineer would need to know the size of your rafters and how far apart they are spaced. From there, making an assumption on what type of wood, a roof load capacity can be calculated and from there its just math to figure out if the weight of snow exceeds the capacity. Of course somewhere there is a factor of safety that depends on what is underneath the roof. If its storage, the safety factor is low, if its you family, it will be a lot higher.

    Some general comments

    Most roofs older than the mid seventies had lower ratings as it was assumed that the heat in the house would melt off the roof. I know of flat industrial roofs in Berlin NH that were designed for 10 PSF as they were heated and had no insulation. Generally these older roofs are the most in danger as the insulation has been beefed up and the ventilation has been installed to keep the roof cold to keep ice dams from forming. Good for ice dams but bad for snow loading. Of course an old roof on a vacant buildign that used to have heat but no longer does is also subject to problems.

    The codes require that any roof near an adjacent higher roof has to have an increased load rating, it may have changed but I think 30% more, as snow can build up in those spots. These areas are a good place to keep and eye on as the snow is usually deeper.

    Many roofs are designed to a minimum deflection (amount the roof beam sags) rather than ultimate load. If you want to hang plaster 1" in 360 inches of beam length keeps the plaster from cracking. There is additional load capacity for the roof past this point before things break. Generally 1" inch in 240" is the minimum recomended for spaces that dont have finishes. So if you measure the length of the roof rafter and its deflected less than 1 in 240, the rafters probably wont fail. Of course my observation of the contstruction details failing comes into effect. Many pitched roofs fail at the point where the rafter tails are nailed to the top of the walls. Sometimes the rafters are just toe nailed into the top of the wall and not tied into the floor joists of the attic. If there is evidence of this, it probably too late to do anything now except shovel and hope. A lot of folks dont realize that dependent upon the pitch, the rafter tail is subject to two major loads, one load is downwards and the other is outwards. Down is not a problem but unless the rafters are kept in place by floor joists running from one side of the house to the other, there is a lot of sidewards load on a couple of nails driven at a 45 degree angle. This can happen with truss type roofs where the homeowner decides to cut the bottom chord of the truss.

    Ultimately if things are creaking and groaning, its time to call in professionals. The only person that can give you a hard answer on safety is a professional engineer with a license in your state and qualified to do an analysis. A fire chief or building inspector can declare the building unsafe, but they are pushing it if they deem it "safe" beyond stating that it appears to be built to code.
  7. bogydave

    bogydave Minister of Fire

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    Varies allot, trusses now are engineered & rated for different areas by average snow fall.
    The type of snow effects weight too.

    When in doubt, shovel, be on the safe side, because being wrong is disastrous & expensive.

    If you really need to know, call the fire department, they may come by & check it out.
    Have a house inspector come by & inspect the trusses, they will need access to the attic. (get a few bids)
  8. nate379

    nate379 Guest

    Would need to use a roof rake. Wouldn't be safe to walk an 8/12 roof unless you are tied off. Dunno about you, but not my idea of fun to fall off a roof again. I'm still mending from a broken back from doing that 3 months ago and that was only 12ft from the ground.
  9. loon

    loon Minister of Fire

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  10. PapaDave

    PapaDave Minister of Fire

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    I got engineered trusses for my pole barn. 5/12, I believe, and they're rated @52lbs/sqft., unless my memory has failed. Northern Mi.
    I have no idea how much any particular type of snow weighs.
  11. maverick06

    maverick06 Minister of Fire

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    Ditto the above.

    A rule of thumb, I heard was, if the joists are 16" centers, you are probably good for 3'. Mine was darn close to that last year, but no creaking or anything and my roof is really shallow! (Maybe 20degree slope).

    I am always scared about it. Aside from that, this is all theory, who knows what condition your house is it, termites? water damage? was it built perfectly or were they sloppy and miss a nail or 2? how heavy is the snow? Uniformly loaded? how strong are the beams? hardwood/softwood?
  12. nate379

    nate379 Guest

    Sounds about right. The drawing I have for the trusses I built for a shed show 54.5 lbs sq/ft. Funny cause we hardly get snow here too, only about 48" avg a winter.

  13. ironpony

    ironpony Minister of Fire

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    it is to much when the roof collapses
  14. peakbagger

    peakbagger Minister of Fire

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    Be careful when looking at roof truss ratings. There is the snow load and then there is an overall roof load which includes the weight of the roofing and decking. A 52 or 54.5 may be good for 40 psf snow and 12 to 14 psf for the roof deck.
  15. Black Jaque Janaviac

    Black Jaque Janaviac Feeling the Heat

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    I've lived in various parts of north central Wisconsin all my life. I almost never observe anyone concerned about it. Yes I'll see people scrape the bottom edges of their roofs - but that's mainly to address ice damming. I've never heard of anyone's roof collapsing on account of excessive snow weight.

    There are other problems that may be more real. Improper ventilation causing ice dams is generally the biggest problem.

    The other disaster to worry about is the slippery conditions caused by snow on the roof. I've shovelled snow from roofs before and that little film of snow that the shovel leaves behind is a widow-maker. Particularly because as you shovel you work your way towards the edge.

    I think I'd rather let the roof collapse than fall off the roof trying to prevent it.
  16. webbie

    webbie Seasoned Moderator Staff Member

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    In the wider pic here you can see that the snow fell off (12/12) from it's own weight.

    The other pic shows fairly deep, but that is because those valleys come together - still, a bunch fell off, plus that is an especially strong part of the roof.

    No worries here, but a LOT of roofs throughout the state have been collapsing according to the news. I suspect mostly flatter ones, commercial, etc.

    Attached Files:

  17. PapaDave

    PapaDave Minister of Fire

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    Good point peakbagger. Something also not always taken into consideration is what might be put in the attic space or hung from the bottom chord.
    I should see if I can find the packet that came with mine. Has all the details.
  18. DAKSY

    DAKSY Patriot Guard Rider Staff Member

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    Well, we have a 24/12 pitch on our A-frame & the only
    thing that ever stayed on the roof, was the roofer (me).
    Everything else falls off - REALLY fast...
  19. SolarAndWood

    SolarAndWood Minister of Fire

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    How does that work? No floor space or lots of headroom? I have a section of 16/12. Felt like I was sheeting and roofing a wall.
  20. peakbagger

    peakbagger Minister of Fire

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    I have seen a few flat roofs collapse and Maine had a run of school roofs that were on the edge a few years back. Very rarely have I seen pitched roofs completely collapse, usually its a weak point that fails where someone did something during the life of the roof to damage its load rating, like cutting a hatch in ceiling and not reinforcing the opening or cutting in skylights without reinforcing the hole.

    I suspect that there is far more damage from ice damming and I expect from an insurance claim basis it a lot higher than roof collapse. IMHO People inherently take more action to avoid spectacular but rarely occuring events than dealing with far more likely but not news worthy events. The media shows up when a roof collapses and does a lot of stories, while they dont when someone has to replace all the sheetrock and insulation in the exterior walls due to water damage and a subsequent risk of mold. Luckilly when someone shovels a roof, they reduce ice damming so I dont think the insuracne industry is going to point out the difference.
  21. firefighterjake

    firefighterjake Minister of Fire

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    In what has to be one of the dumbest moves ever made . . . this is the auditorium in the city where I work . . . bear in mind that I am in Maine which tends to have a high amount of snowfall.

    Attached Files:

  22. SolarAndWood

    SolarAndWood Minister of Fire

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    I bet someone is pretty proud of themselves for coming up with a roof design for that big of a building with only one valley :lol:
  23. DAKSY

    DAKSY Patriot Guard Rider Staff Member

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    Well, the structure is an equilateral triangle - (3) 60 degree angles...Roof rafters are 32 ft 2x12...
    Great room has a 16 ft cathedral ceiling & has 12 ft of attic height above that...
    Where I framed an 8ft wall to the underside of the rafters, I can stand behind it. That area is mostly used for storage.
    Where I framed 4ft walls, that space is wasted...
    35 square of roofing & 10 square of vinyl siding

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