Snow load -- go brace your coop and run, BEFORE they are flattened

Discussion in 'Coop & Run - Design, Construction, & Maintenance' started by patandchickens, Oct 30, 2009.

  1. patandchickens

    patandchickens Flock Mistress

    12,521
    86
    341
    Apr 20, 2007
    Ontario, Canada
    I am posting this as a public service announcement on accounta at least one BYC member has *already* posted about their coop/run being flattened by snow, and it is not even November yet. (Thus, you have time to do it while it's still decent working weather, but ought to do it before you too get a freak wet snowstorm that tests your setup's engineering)

    (edited to add, since horsejody's post below made me realize what i forgot to mention, the following is really aimed at those with new(ish) coops [​IMG])

    The weight of snow varies depending how dry/powdery vs wet/slushy it is, and will change as it compacts with drifting or settling, but a reasonable rule of thumb is that snow accumulating on a roof will weigh in the vicinity of 5-20 lb per cubic foot, up to 50 lb per cubic foot if there is rain involved too. If you care about knowing actual numbers to be engineering for, your municipal zoning office should be able to quote you a snow load number (note that this will represent what can be expected from a particularly bad storm, not from a typical winter)

    Realistically most people are not going to go into structural tables to figure out what snowload their coop is good for, however (tho frankly it is kind of entertaining to work out, if you like that sort of thing [​IMG]). I would at least suggest that you take one of the folllowing two approaches: either a) ensure that your structure is built with the same post size/spacing, beam size, rafter size/spacing, and (if applicable) purlin size/spacing as a number of other buildings in your area that have been standing for at least a decade; or b) go beef it up as much as you realistically are gonna do.

    One thing that will really strengthen a raftered roof is to use collar ties (google if you are not sure what I'm talking about), a piece of 2x4 or 2x6 that runs horizontally between the two rafters of a pair, a few feet below the peak fo the roof. This prevents them from "doing the splits" and letting the roof collapse.

    Another thing that will strengthen a building is to add diagonal bracing, since another common mechanism of collapse is for a box-shaped building to become parallelogram-shaped and then flat. If you have heavy plywood siding well-screwed on, you are prolly ok; but with anything else, especially just wire mesh or tongue-and-groove boards, it is really worth running a coupla 2x4s diagonally on (or in) each wall, screwed to all the studs. (there are less conspicuous ways to do it too)

    You think your run can't collapse because the top is mesh, and snow will jsut fall through it? Think again. (Or go browse last year's coop design section, in which you will see pics of a lot of flattened mesh-top runs). Unless it is a dead-dry powdery snow, it easily starts to accumulate on the wires, and frequently becomes a solid snowpack up there, with all the weight issues that go along with it. Freezing rain can be a bad problem in some areas too.

    Netting will come down under stress from snow or ice, hopefully just pulling/ripping free of its supports; chickenwire will stretch; but wire can also easily *pull down* its supporting structure, sometimes causing a lot of hard-to-fix damage.

    One easy fix is to add support posts in the middle of your run, under the 'rafters' that support the mesh top. Rotate the 'rafters' if necessary so they are narrow edge up, btw, not wide edge up the way many people install them - this is much stronger. Don't just prop the post there, actually screw it (directly or using galvanized fixings) into the structure atop it, and peg it very securely into the ground, so it will not simply fall over under snow load. (Yes, they do).

    Diagonal braces help too, as described above.

    And think verrrrry carefully before putting roofing sheets or a tarp onto a minimally-supported run top. You may *think* all the snow should slide off or that you will go out and knock all the snow off before it accumulates too much, but realistically this scenario seems to result in a lot of damaged or flattened runs.

    Finally, those in southern or coastal climates, where snow is rare (but does happen sometimes), should probably take heed at *least* as much as more northerly or -inland people. Because when it DOES snow in 'borderline winter' climates like that, the snow is almost always very wet, often mixed with rain or freezing rain, and it takes a lot less of that to mash down your coop or run than it takes -20F powder snow [​IMG]

    Really, think about those numbers -- are you SURE your run top and roof can handle them, both in terms of point loading and in terms of the support for the whole shootin' match? If not, NOW is the time to beef things up. It's a lot easier than *picking* things up later.

    We now return you to your regularly scheduled "warm wet late October weather" (at least that's what we've got here, just rain),

    Pat
     
    Last edited: Oct 30, 2009
  2. Monk

    Monk Chillin' With My Peeps

    520
    0
    139
    May 10, 2009
    It's great advice. Where I live this has always been in the back of my mind and a big consideration in my coop design.
    [​IMG]
     
  3. ColoradoMike

    ColoradoMike Chillin' With My Peeps

    340
    3
    111
    Jun 12, 2009
    Northern Colorado
    Great post as usual, Pat.

    And I like the winter pic of your coop, Monk. I'll have to add my pic from yesterday morning. My coop was holding up well with ~12" of snow on it... Officially, we got ~21" of snow out of this storm and the girls came through just fine. We even got three eggs on both Wednesday and Thursday (from our three hens) so they don't seem fazed by our early winter at all!
     
  4. horsejody

    horsejody Squeaky Wheel

    6,923
    26
    271
    Feb 11, 2008
    Waterloo, Nebraska
    I think I'm safe. My coop was built in the 1930's and it hasn't caved yet!! Thank God!!!
     
  5. Omran

    Omran Chillin' With My Peeps

    Jul 26, 2008
    Bagdad KY
    Thank you Pat for this good and very important Info, I must say I really respect your openions and like to listen to your advise.
    Thank you again.

    Omran
     
  6. Monk

    Monk Chillin' With My Peeps

    520
    0
    139
    May 10, 2009
    Quote:The pic was from our first dusting this year, I'll update pics as the season progresses. Colorado has been on the news quite a bit about the snow! Can't wait to see the pics.
     
  7. gsim

    gsim Chillin' With My Peeps

    1,993
    20
    176
    Jun 18, 2009
    East Tennessee
    One very good and strong and quick addition to strengthen a coop or run against "racking" to one side under a heavy roof-load or wind load is called "coil-bracing". it is available at home improvement stores and lumber yards. It is 2" wide steel strapping around 10 ft long, perforated at 2" intervals with holes just right for 8D nails or screws. It is common in home construction and used to make a "shear-wall". I did it down the center wall of my house when I built it in the mid-90's. You could add it inside of a coop, even with interior walls covered. Just nail it right over the existing wall diagonally, hitting every wall stud . Better yet is to "X" it in 8 ft sections. Screws are stronger still. If you have to remove nest boxes/roosts/etc, it is still worth it. You could have a lot of productive chooks killed in a collapse, or just as bad, be easy prey for preds. If looks are no concern, then install it on outside of your coop.
     
  8. LynneP

    LynneP Chillin' With My Peeps

    Another structural asset is 'hurricane hangers'. They steady and secure the tips of rafters and can steady a building under blizzard wind force. We used them on the run, realizing that wind might get under the vinyl roof. But they can be used in any part of your coop and are rated for wind speed. We used joist hangers too and various other metal connectors. There is one every foot.

    The big thing about snow load, as Pat describes, is that once a corner gives way the whole thing goes 'splat' and that includes the animals underneath. Happened to a horse barn near here, an add-on to an indoor riding ring. One horse badly injured the others trapped for hours. Well you can imagine. Also, if you have platforms or shelves in your coop or run, make them strong so that in case of a disaster the birds have shelter.

    Of course everything you add increases costs, which is unfortunate, but if you think of it as prevention, and of the danger to yourself and your animals during a collapse, it's easier to bear.

    [​IMG]

    This is PalRUF with a skiff of snow on top. It holds well if you support it adequately. Some light gets through, and we have a snow rake for this, our barn and our house in case of a huge buildup.

    [​IMG]



    Snow boards will add stability to your run, and as the snow builds up on the outside they add insulation to your setup. Remember that your drainage has to be open, so check it before installing them. Some folks use 5-6 ml vinyl sheeting.

    [​IMG]
     
    Last edited: Nov 7, 2009
  9. Sandrachx

    Sandrachx Chillin' With My Peeps

    257
    1
    144
    Oct 16, 2007
    Chelsea, MI
    monk,
    i really, really like your coop. sturdy - in an alpine kind of way.
     
  10. Ridgerunner

    Ridgerunner Chicken Obsessed

    20,139
    3,350
    496
    Feb 2, 2009
    Northwest Arkansas
    I'm a retired structural engineer, so I'll add a bit to this. First and foremost, I fully agree with everything Pat said.

    Joplin, Missouri's design snow load is 30 pounds per square foot. You might not think that is much, but if you have a 4' x 8' coop, that is (4' x 8' x 30 psf) 960 pounds. And if you have an overhang, say 12" all around, that becomes (6'x 10' x 30 psf) 1800 pounds. It adds up in a hurry.

    In structural design, the triangle is a tremendously stable shape. Those rafter collars Pat mentions are a good example of that. Another good use of the triangle is to put a diagonal brace under the rafters, running from one corner across to the opposite corner or, as Pat mentioned, in the walls.

    Occasionally a beam will just break because it is overloaded. But most of the time, there is more to it than that. Usually, the failure is more due to deflection than straight load. Technically it is called secondary forces. A rafter starts to twist, a building starts to rack, or a column starts to bend and the forces on it are magnified because of the way they are distributed. That's why the diagonals under the rafters really help. They stop the whole building from racking and the individual rafters from twisting. And any siding or roofing will help stiffen the building against racking as long as it is firmly attached to each stud or rafter. You might think that the siding does not need to be nailed or screwed to a stud to get the wind load to go straigth to the stud, and that is correct. But if you connect the stud and siding (or rafter and roofing) you keep the stud or rafter from deflecting out of the way under load. It really does strengthen the whole building.

    Another usual failure mode is that the connections fail. So use nails or screws sufficient to do the job both at corners and joints and to attach the siding or roofing. As an example, the previous owner built a shed here and used 2-1/2" smooth nails to attach the purlins to the rafters. I had a 70 mph straight line wind come through and lift 36 feet of roof off that shed. There was only 1" of smooth nail holding the purlins to the rafters. When I rebuilt the roof, I used 3-1/2" ribbed nails. I'm not really worried about the new roof coming off.

    I think Pat gave excellent advise. If you look at old buildings that have stood for a while in your area and use those building techniques, odds are you will do fine.
     

BackYard Chickens is proudly sponsored by