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Do I need a rafter tail?

BDohrn | Posted in Construction Techniques on June 3, 2008 08:33am

I am building a house which will have an 18″ and 24″ roof overhang (and in another spot, 36″). The roof sandwich, from bottom up, is 2×10 24″ 0c, 3/4″ sheathing, 8″ poliso brd insul., 1/2 (or 3/4 if nec) nail base, metal roof panels.

Question is: Origianl design called for @ 5″ of the rafter (tails) to protrude as support for the overhanging part. Do I need that rafter tails to continue outside the envelope, or will the sandwich take care if itself in terms of staying flat, perhaps if I insert blocking where the insulation was?

If it is possible, would that method work for the 36″ overhang too?

A PDF is attached, if you’re interested enough to look and help, I’d be very very appreciative.

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Replies

  1. User avater
    Gene_Davis | Jun 03, 2008 09:40pm | #1

    Well, that would depend, wouldn't it?

    On, for example, what kind of stuff might be piled on the roof edge as live load.

    Your engineer might be a good person to ask.

     

    View Image

    "A stripe is just as real as a goddamn flower."

    Gene Davis        1920-1985

    1. BDohrn | Jun 03, 2008 11:18pm | #2

      We don't get snow here, (Eugene Oregon) and no dancing on the roof. I was kind of assuming that load would not be an issue, so much as whether the materials are likely to warp and wave. Am I wrong? Do tell

  2. Dave45 | Jun 04, 2008 01:46am | #3

    From your drawing, it looks like the tails are really "fakes" attached to the underside of the roof.  If so, I can't imagine how they would give you much support.

  3. User avater
    Dinosaur | Jun 04, 2008 02:52am | #4

    Do I need that rafter tails to continue outside the envelope,

    Yes.

    will the sandwich take care if itself in terms of staying flat, perhaps if I insert blocking where the insulation was?

    No.

    would that method work for the 36" overhang too?

    Definitely not.

     

    I took a look at your drawings, and have never seen a rafter detail like that before. It appears that the 2x10 rafter is sitting on a ledger bolted to the inside of the building wall, and some sort of fill structure is built on top of the top plate to bridge the gap between it and the underside of your foam. And it looks like the 'rafter tails' are not rafter tails at all, but soffit joists nailed onto the outside of the building. Unless I'm misinterpreting your sketch....

    The rafter should sit directly on the wall top plate and continue past it to the end of your eaves overhang. If you want a reduced-depth rafter tail outside the building wall that's fine. You don't need 9½" of depth to support your 'sandwich', but you most certainly do need more than ¾" of plywood. The 4½" depth of rafter tail indicated on your sketch seems okay, but the engineer or architect who designed it is the one who has to state that.

    Polyiso foam is fairly stiff stuff, but it is not structural. Steel roofing is heavy, and although the ribs in it keep it from flopping longitudinally like a wet rag, they do not hold it really straight. It needs to be supported or it will sag. 

    Now, IF you built a rigid structural panel out of ply and foam, it would probably support itself and the overhang of a zero-snow-load steel roof. But each structural panel would have to be sheathed on both sides as well as the top and bottom with plywood. To my mind, that's a helluva lot more trouble and expense than rafter tails.

    Dinosaur

    How now, Mighty Sauron, that thou art not brought
    low by this? For thine evil pales before that which
    foolish men call Justice....

    1. dovetail97128 | Jun 04, 2008 05:14am | #5

      I think that what we are looking at in the drawing is a cross section through blocking between rafters. I think the actual rafter is sitting on the wall and the tail simply cut down to a smaller profile. I am guessing of course but that is what I read. Still , I agree tails are needed. However you may be able to eliminate some of them by using the purloin that are shown in the solution part of the drawing to span between tails. Or a stout fascia supported by every 3rd. or 4th. tail with blocking between the two 1/2 layers of roof sheathing conceivably would also work. Be one tall piece of fascia though.
      They can't get your Goat if you don't tell them where it is hidden.

      1. BDohrn | Jun 04, 2008 07:20am | #6

        Thanks for the help. You are right... I posted the section through the area between rafters by accident and didn't want to bother everyone with a correction.... The reason for eliminating the tails is to keep the trim a little slimmer... (Right now, almost 14") But will need to keep the tails, or angle them. Thanks again...

      2. User avater
        Dinosaur | Jun 05, 2008 01:14am | #7

        Be one tall piece of fascia though.

        You don't run a fascia that high in one piece; it's got to be cascaded in two sectons--one fascia for the foam above the roof deck; another one set back about 3" or so for the rafter section below the roof deck. Otherwise it'll look like hell. 

        But your basic idea isn't half bad at all. He could run a 4¾"-deep rafter tail say every third rafter -- for a fascia span of +/- 6' -- and put a 2x6 subfascia on those. Wrap the subfascia with copper or cap it with scalloped 1x7, and finish the soffits with ¾" T&G.

        Then put some nice scroll-worked knees under the eaves to hold it all up Did you see the article on how to make those knees in the current FHB?

        Dinosaur

        How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

        1. dovetail97128 | Jun 05, 2008 01:48am | #8

          I have built that eave system before.
          Some years Back I remodeled a house with beam ceiling and T&G decking, detailed the roof pretty much as described. Wide fascia isn't always bad, in my case we kept the roof and 6" gutters one color and painted the fascia below so as to create a different color band . Yes I saw saw article.
          They can't get your Goat if you don't tell them where it is hidden.

          1. User avater
            Dinosaur | Jun 05, 2008 02:59am | #10

            Here's a detail I designed for a roof-over with insulation. The tricky part of this one was that I needed to ventilate through the rake as well as the eaves.

            View Image

            Sorry about the fuzzy quality of the jpeg conversion. The hardware around here doesn't seem to recognise .dwf files.

            Dinosaur

            How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

  4. DanH | Jun 05, 2008 02:38am | #9

    I think it would work if you ran the blocking in the other direction, and carried it back over the wall a couple of feet. The way you have the blocking drawn, though, it does no good.

    It is an ironic habit of human beings to run faster when we have lost our way. --Rollo May

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