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Community and Q&A

Exposed rafter tails, GBA CAD detail

Gordon Taylor | Posted in General Questions on

I have some elementary (for you) questions that are not elementary to me.

First, the basics: we’re going to be building a house in Oregon, Marine Zone 4c. I’m assuming a simple gable roof, shallow 3/12 pitch, California Bungalow style, with the rafter tails showing (no closed soffit). There’ll be a raised heel truss, 16″ heel to get R-49 in the attic–which will be vented. The walls will be 2×6 (Roxul R-23 mineral wool) with 2″ of foam (or Roxul board) and vented rainscreen under 1/2″ Nichiha fiber cement siding. Questions:

1. We want a big overhang (42″) on the “rafter” tails, which I assume will mean at least a 2×6 on the top chord of the trusses. I’m also assuming that the trusses will have to be 16″ o.c. Right? Will this be strong enough?

2. Is there a construction detail available at GBA for the Roof/Wall connection that I describe? (Fiber-Cement Siding Over Rigid Foam, w/ furring strips, plus Raised Heel Truss, exposed rafter ends) I’ve just spent a very tedious time going through CAD details on the site, and all I can find are drawings that show an enclosed soffit with vents. This may be such a simple detail that the GBA editors didn’t deem it worthy of inclusion, but to me it’s not easy. Or maybe I just missed it. In any case, I’d like to see it.

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Replies

  1. Expert Member
    Armando Cobo | | #1

    I've used 2x8-36" rafter tails, 1x6 T&G sheathing above the tails and 3/4" OSB the rest of the roof, with trusses at 24" spacing; your truss company should let you know size and spacing for 42" tails. Details should be the same as in the GBA's, just take the soffit framing out.

  2. user-1129975 | | #2

    Agree with Armondo - truss company will tell you spacing, and I expect it will be standard 24" despite the overhang size.

    I, too, am building exposed rafter tail structure(s) and can supply you with my SketchUp detail that is presently under review at the zoning and permitting office. I am in Zone 6, and have designed essentially the same wall and roof structure that you and Armondo describe. I can rummage around and post it here tonight when I get home, if you'd like.

    A couple of suggestions. I don't know how you plan on venting the "non"-soffit space, but that turned out to be a nettlesome detail for me. I could not get a large enough NFVA using round vents in the blocking between the rafters, which was my desired design. I did, however, come across a product from Cor-A-Vent that sits atop my blocking, beneath the 1x6 T&G, that provides 10 sq. in. per lineal foot.

    Additionally, I included a rain screen behind my fiber-cement cladding. For the cost of some ripped 3/8" plywood, it seemed like a simple and worthwhile upgrade to wall design.

    Just some thoughts.

  3. Gordon Taylor | | #3

    Kent and Armando: Thank you for your replies. Kent, I would very much like to see that detail if you get a chance. Yes, I do intend to include a rain screen behind the cladding. And ripped 3/8" plywood strips seem a lot more economical than the 1x4 furring that normally shows up in GBA drawings. Thanks for the input!!

  4. user-1129975 | | #4

    Gordon -

    I will dig up my drawing and post it tonight when I get home. Here is a resource you might find helpful if you are still in the design phase for your walls:
    http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design/

  5. user-1129975 | | #5

    Here is the wall detail as I submitted for permitting. Maybe you can get some good ideas....

  6. user-1129975 | | #6

    Here is the wall detail as I submitted for permitting. Maybe you can get some good ideas....

  7. Gordon Taylor | | #7

    Terrific. A lot to think about. Thanks so much, Kent.

  8. Gordon Taylor | | #8

    Already I've discovered something new. Never in my life have I heard of car siding.

  9. user-1129975 | | #9

    I should add that I am attaching furring strips to the top of the truss top chords to match the height of the car-siding, then putting OSB roofing sheathing over the entire matrix so as to have a deep enough structure to drive roofing nails without having them protrude through in the open rafter bays.

  10. Aj Builder, Upstate NY Zone 6a | | #10

    Kent, excellent sketchup detail sheet. I do basic 3d work on sketchup, need to get back on it and work toward your abilities. If willing to share any files I am at ajbuilderny on gmail.

  11. Gordon Taylor | | #11

    Kent, I was just wondering about that very detail. I couldn't see using 3/4" plywood for the roof decking to match the 1x6 car siding. Did you consider T1-11 siding, faced down, and with the grooves running crosswise, to get the same effect? Somewhere else on GBA I've read of someone using that detail.

  12. user-1129975 | | #12

    Gordon, no, I hadn't considered T111, though if someone liked that look, seems like a reasonable alternative to furring strips atop the truss top chord.

    I did consider beadboard panelling ripped in half and laid over my 24" rafter tail bays. Since the panelling comes in thicknesses as little as 1/8", I thought I could lay my 1/2" roofing OSB over this, and the resulting gap would likely not matter as it would be within the attic space. Plus, the stuff made for use in laundry rooms and bathrooms is moisture resistant engineered wood, so I thought it might hold up reasonably well.

    Ultimately, though, I wanted a substantial enough structure to accept a 1 1/4" roofing nail without penetrating through, which is why I went with the design I did.

  13. Gordon Taylor | | #13

    Kent, Also good ideas. Thanks.

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