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  1. #1
    Ed Lazear's Avatar
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    Default ceiling joist span

    I am remodeling a duplex where we will raise the 8 foot ceiling to 9 foot. My current ceiling is using 2x6's with a span of a little over 15 feet 24" O.C.
    Is this adequate or should I install 2x8's.

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  2. #2
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    Default Re: ceiling joist span

    Ceiling joist span;
    2x6 @ 24" oc 10'6"
    2x8 @24" oc 13'3"
    2x10 @24" oc 16'3"

    According to the span table above (from the IRC) you should use 2x10


  3. #3
    Ed Lazear's Avatar
    Ed Lazear Guest

    Default Re: ceiling joist span

    Stanley, that is what I came up with also. The house was built in 1984 and the ceiling does not sway so I am trying to figure out how they got away with this and why the ceiling has not sagged.


  4. #4
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    Default Re: ceiling joist span

    Consider the likely-hood you're referring to a bottom chord of a truss, not a mere "joist" and/or advanced framing techniques having been employed.

    Your limited descriptions don't provide the necessary factors required to design or apply. There is much more to design other than mere simplified span tables.


  5. #5
    Ed Lazear's Avatar
    Ed Lazear Guest

    Default Re: ceiling joist span

    It is a stick frame house, the 2x6's go from the perimeter wall to a beam where joist hangers are used or the joists sit on top of a wall. There are no trusses in this house.
    There is however a strong back half way across the span.


  6. #6
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    Default Re: ceiling joist span

    Quote Originally Posted by stanley frost View Post
    Ceiling joist span;
    2x6 @ 24" oc 10'6"
    2x8 @24" oc 13'3"
    2x10 @24" oc 16'3"

    According to the span table above (from the IRC) you should use 2x10
    That assumes limited attic storage. If there is no storage you can use the tables in R802.4(1) which prescribe 2x8 for #2 SPF or even 2x6 for #2 SYP.

    That strongback isn't doing double-duty to support roof purlins, is it?


  7. #7
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    Default Re: ceiling joist span

    Quote Originally Posted by Ed Lazear View Post
    It is a stick frame house, the 2x6's go from the perimeter wall to a beam where joist hangers are used or the joists sit on top of a wall. There are no trusses in this house.
    There is however a strong back half way across the span.
    If by "strong back" you are referring to purlin and brace, then you have two 8' spans. In this area strong back is used to refer to the gable end wall bracing but different areas have different names for almost everything.

    The beatings will continue until morale has improved. mgt.

  8. #8
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    Default Re: ceiling joist span

    Quote Originally Posted by Vern Heiler View Post
    If by "strong back" you are referring to purlin and brace, then you have two 8' spans. In this area strong back is used to refer to the gable end wall bracing but different areas have different names for almost everything.
    I assumed the strong back to mean a 2x4 laid flat with a 2x6 laid on edge in the middle of the span, forming an L. It's usually used to tie the joists together, reducing sag and preventing twisting. Sometime, however, people use a strong back to support a purlin and brace, which (unless engineered properly) places additional loads on the joists from the rafters.


  9. #9
    Garry Blankenship's Avatar
    Garry Blankenship Guest

    Default Re: ceiling joist span

    Definitely not a field of depth for me, but it sounds like the old ceiling is coming out. If that is correct, lots of options, including 16" centers, engineered lumber and the quality of the lumber used matters. #1 Doug fir will yield different results when compared to #2 & better Hem/Fir and so on.


  10. #10
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    Default Re: ceiling joist span

    If the joist are sitting on an exterior wall how will you raise it to 9'? Build a knee wall on one end and nail the joist to the rafters on the other end?

    One option would be to install a trey/tray ceiling.


  11. #11
    Ed Lazear's Avatar
    Ed Lazear Guest

    Default Re: ceiling joist span

    Thanks all for your incites. The strong back I have is one used for stiffening the ceiling at mid span, not to support a purlin. I did not know you could do that. My building is a rectangle, what I plan on doing is to install a gluelam, some refer to it as a power beam, down the long side slightly off center because that is where the support will be for the beam. I will hang my joists off the beam and nail it to the rafters for my 9 ft. height on one side ( the long side) and the other side will hang off the beam and sit on top of a wall on the short side. All the walls will be ponied up to the new ceiling height but the knuckle joint should not be a problem do to the gluelam. The problem I see is providing support for the roof.

    Questions I have, can a purlin be supported in any other way then to a wall? Can a roof rafter be converted to a truss for part of the span then continue on as a rafter (may need to draw this out)? This would get the support to a wall since the purlins would not be picking up the roof at the right distance up the rafter.

    I already have broken rafters, some that are sistered, and atleast one broken ceiling joist that I just found. I believe these are all do to under sized lumber for the span or lack of support.

    Any thoughts.


  12. #12
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    Default Re: ceiling joist span

    Find out what the AHJ for your project requires for your situation (taking a sketch or two along, and any supporting documentation from your P.E.), and move on from there.


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