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Rethinking roof framing

joenorm | Posted in General Questions on

Before I started reading here, I was planning low sloped parallel chord for a new build. I’ve since learned that venting a 2:12 slope is not recommended(even though I think it might prove OK in my area)

Also turns out parallel chords are pretty expensive as roofing members go. And if I was to spray foam them to go the sealed route things start to get really expensive

I am now thinking of a sealed roof, same pitch, Zone 4C so I’d put 2-3 inches of rigid foam over the roof deck and fill the stud bays with batts or cellulose.

TJI’s probably make the most sense to accommodate for moderate spans
I like exposed rafter tails and I am wondering what the technique is for stopping the TJI at the wall and then sistering on a tail?


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  1. andy_ | | #1

    Using I joists for rafters is a little involved, but in practice just becomes a matter of making up a lot of the parts and pieces ahead of time. Sistering on the rafter tails isn't really that difficult.
    The rafter tail needs to extend inside the structure (I think it's called back spanning) with generally a 2:1 ratio, so if you have a 2' overhang, you'll need 4' inside so a total of 6' 2x6.
    The I joist will already be getting packed out with a web stiffener of 3/4" plywood where it sits over the wall, so then the rafter tail is easily attached as it's now pretty much flush with the flange. You'll probably also need a little block at the wall to further support the rafter tail.

  2. Zdesign | | #2

    Consider your span with TJIs and the depth you will actually need. Any roof system we design with TJIs we run through our software and its a pain due to the plumb cuts and required hangers. Don't go by the span charts. Typically a 11-7/8" 230 can span much further than a regular 2x12.

  3. joenorm | | #3

    How bout to create the overhangs perpendicular to rafters? Where you cannot notch them like you can 2x12.

    1. Expert Member
      Michael Maines | | #4

      Joe, this document shows how to create overhangs: It also includes all of the other important details unique to TJI rafters.

      1. joenorm | | #5

        thank you!

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