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Roof Trusses 4′ oc?

etting | Posted in Building Code Questions on

Would roof trusses spaced 4′ oc fall outside prescribed methods for stud construction of a house in the 2012 IRC and therefore require approval from an engineer (other than the engineers at the truss manufacturer)?

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  1. Expert Member

    I can give you a parenthetically useful answer as I live in Canada. The spacing would be fine with the truss engineer's stamp. The only sheathing that would meet our code without engineering would be 2x t & g. The ceiling would have to be strapped with 2x material.

  2. etting | | #2

    Thank you, Malcolm. I should have explained that I want to use purlins and steel panel roofing atop the trusses.

  3. user-757117 | | #3

    I'm also Canada.
    As far as I know 4' spacing of trusses is normally a feature of agricultural buildings...
    For residential construction where roof load requirements may be higher, savings from using fewer trusses may be offset by requiring those trusses to be beefier.
    Also, from experience, 4' spacing is a pain to work in.

  4. Expert Member

    Jeff, Depending on your snow loading (and perhaps local wind or seismic requirements) it is probably quite doable. However under our code you would need an engineer to specify the appropriate spacing of the purlins and gauge of the roofing as it is beyond the proscribed maximums.
    The only time you commonly see 4 ft oc is on timber frame or steel construction, using t &g or steel decking. Is there a particular design benefit that you see in using it?

  5. etting | | #5

    4' spacing will make it easier for me to install the 3' wide roofing panels from underneath, which will feel much safer for me than climbing around on top. For post-frame buildings, including residences, trusses can be spaced as far apart as 10', but the whole building has to be engineered. I'm hoping the 2012 IRC, as used in my town in the southwestern US, will allow 4' truss spacing with stud construction without engineering beyond that of the truss manufacturers. I haven't found anything in the 2012 IRC that answers that question one way or the other.

  6. user-757117 | | #6

    Yeah, sorry don't know the IRC well enough to help you there.
    But again, from experience, unless you have unusually long arms you will probably find fastening the far edge of those panels not-so-much-fun.
    In any case, if that's your plan, I would still recommend 24" spacing for the trusses - 48" is an uncomfortably wide straddle if you need leverage that way (and I've got pretty long legs).
    Not sure how long your panels are but I would say your best bet is to haul them up top edge first over the eave rather than try to weave them through the trusses from below.
    If your roof pitch is 4/12 or less I wouldn't worry too much about finding sure footing even on secured steel panels - of course you should wear a harness in any case.

  7. Expert Member

    In the venerable GBA tradition of giving advice on a question that wasn't asked... any reason you are going with 3 ft roofing panels? If you switched to a 12" or 16" snap-lock, the installation would be much easier, and for the life of your roof you would never have to worry about the fasteners leaking.

  8. etting | | #8

    Thank you, Lucas and Malcolm. I had ruled out standing seam because of the cost, but I see that snap-lock are supposed to be less expensive, and I'm getting a few quotes on the materials. The sun here might destroy the seals on exposed fasteners, and a shorter reach on a narrower panel would be welcome. I installed 3'-wide panels once before from below, but the trusses there weren't as high, so if I keep considering a 3' panel, I'll need to do a simulation to make sure I can reach on the current project.

  9. wjrobinson | | #9

    First, stick to 24" spacing.
    Next, switch to hidden fastener.
    I Roof over felt over Plywood.
    As to working on a steep Metal Roof, buy a ridge Hook for your ladder.
    And do wear fall Protection.

    Simple affordable and your code people will pass it.

  10. Expert Member
    MALCOLM TAYLOR | | #10

    Jeff, I think you will be pleasantly surprised by how simple installing snap-lock panels is. Just before Christmas I did a 28ft x28 ft shed roof with all the trim singlehandedly in four hours.

  11. etting | | #11

    I found the answer to my code question in the 2015 Wood Frame Construction Manual, which has a very nice table of prescribed limits. The entire manual is available for free at The 2012 manual isn't free, but two supplementary documents are, and the Workbook has essentially the same table. Maximum prescribed truss spacing is 24".

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