Spacing of roof rafters

Discussion in 'Building Construction' started by dynamorph@hotmail.com, May 6, 2005.

  1. Guest

    I am building an enclosed porch using native lumber. The underside
    of the roof will be visible from inside so instead of using 2x6
    and plywood, I am using using rough sawn 3x6 rafters spaced at
    30", and 1x12 pine smooth on one side for the roof decking.
    My question is will the 1x12 ( 7/8 x 11 3/4) be sufficient
    at 30" spacing? The roof will be covered with asphalt shingle.
    Thanks,

    --
    Jeff
     
    , May 6, 2005
    #1
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  2. Bob Morrison Guest

    In a previous post says...
    > I am building an enclosed porch using native lumber. The underside
    > of the roof will be visible from inside so instead of using 2x6
    > and plywood, I am using using rough sawn 3x6 rafters spaced at
    > 30", and 1x12 pine smooth on one side for the roof decking.
    > My question is will the 1x12 ( 7/8 x 11 3/4) be sufficient
    > at 30" spacing? The roof will be covered with asphalt shingle.
    > Thanks,
    >


    Jeff:

    This will depend on your snow load. You could apply a layer of 5/8"
    sheathing on top of the 1x12 just to make certain that there is enough
    strength. This would also make it possible to use a slightly longer
    roofing nail and less likely that the nail will go through the 1x12.

    --
    Bob Morrison, PE, SE
    R L Morrison Engineering Co
    Structural & Civil Engineering
    Poulsbo WA
     
    Bob Morrison, May 6, 2005
    #2
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  3. RicodJour Guest

    wrote:
    > I am building an enclosed porch using native lumber. The underside
    > of the roof will be visible from inside so instead of using 2x6
    > and plywood, I am using using rough sawn 3x6 rafters spaced at
    > 30", and 1x12 pine smooth on one side for the roof decking.
    > My question is will the 1x12 ( 7/8 x 11 3/4) be sufficient
    > at 30" spacing? The roof will be covered with asphalt shingle.
    > Thanks,


    What's the span, what's the grade of the pine, what's the pitch and
    what are the required design loads? It will most likely suffice, but
    can't be sure without additional information.

    Where did you get the lumber? Sizes that you mentioned are a little
    odd around here.

    R
     
    RicodJour, May 6, 2005
    #3
  4. Guest

    Lumber is native rough sawn white pine from
    this area (southern Vermont). Rafters are 3x6
    and roof boards 1x12 square edge. Above
    that is felt and cedar shingle. Slope is 4/12
    and there is a lot of snow.

    I realize that 30' rafter spacing is pushing it -
    24" is more common in these parts.
    But the rafters are beefier than normal
    and I am putting in some 30"
    wide skylights. That's why I wanted to go with
    the rafters on 30" to keep the spacing even
    because it will be visible from inside. But I
    guess I should go with 24-26" rafter spacing
    and just frame in around the wide skylights
    with extra rafters to the headers as is
    usually done. I am not using plywood
    anywhere. Thanks,

    --
    Jeff
     
    , May 6, 2005
    #4
  5. Hul Tytus Guest

    Calculation shows a deflection of about .1 inch for a 200 lb load on a
    1X12 30 inches long. Max stress is 125 psi. If you put a couple of scrap
    2x4's 30 in. apart on a concrete floor with a short piece of the intended
    1X12 across them, a little jumping might give you an even better
    indicator.

    Hul
     
    Hul Tytus, May 7, 2005
    #5
  6. Matt Whiting Guest

    Hul Tytus wrote:
    > Calculation shows a deflection of about .1 inch for a 200 lb load on a
    > 1X12 30 inches long. Max stress is 125 psi. If you put a couple of scrap
    > 2x4's 30 in. apart on a concrete floor with a short piece of the intended
    > 1X12 across them, a little jumping might give you an even better
    > indicator.
    >
    > Hul
    >


    Point load mid-span or distributed load? I'm figuring it must be a
    distributed load as I know that if I stand in the center of a 1x12 that
    is spanning 30" it will deflect more than 0.1" Been there, done that.


    Matt
     
    Matt Whiting, May 7, 2005
    #6
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