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Overframing a Roof for Large Overhangs

Sabotcat | Posted in GBA Pro Help on

Hey!
I’ve written in before regarding our upcoming house in California’s high desert.  We’ve headed into structural and I’m finding I’m having a heck of a time laying out the roof.   Without getting overly granular, we’re just trying to frame a 25×65 foot box (with one 3ft bump-out on one end) with 4′ overhangs.  I’d like to apply a layer of external insulation, furring strips and a standing seam roof over the whole thing.

I was thinking the house would be a great candidate for “overframing” the roof…meaning we could end the TJI joists at the walls, the roof and wall sheathing would meet at a taped seam and a second set of roof members could be affixed through the sheathing.  I found examples of this in Passive House Details by Corner, Fillinger and Kwok.   And sent the engineers scans of those and screen grabs from Matt Risinger’s videos on framing his own roof in Texas.

The structural engineers weren’t crazy about the idea, principally because they didn’t know an effective way to affix the rafter tails to TJI’s below.  I know Matt Risinger just dropped in long (expensive) screws through the rafter tails..but is there enough meat in a TJI to make this viable?  What is the SOP for this…and on a house this size am I asking too much with a 3 or 4 foot overhang?

Finally, in your expert opinions, would conventional framing and metal supports to bolster the corners be a way to address these problems?  I see endless examples of fairly deep overhangs being framed with what seems to be 2×4 and 2×6 dimensional lumber in the area.  What are they getting that I’m not.  Can someone help me out with this?  What am I missing?

here’s a detail from the book, Passive House Details.   Also, we’re in climate zone 3b, but at 4500 ft, we have a snow load of 30 psf and temps in the 20s in the winter.  So I might cheat us toward Mixed/Dry.

Thanks so much for your help…
Mark

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Replies

  1. Daniel Allen | | #1

    How about putting blocking between the below TJI's, then use Simpson A35 clips nailed to side of rafters, with either nails or screws through the bottom leg of the A35 / sheathing / into the blocking?

  2. Expert Member
    Armando Cobo | | #2

    Sorry, I erased my comment since I just realized this maybe a double wall/roof assemblies! You could use LVL rim boards to attach the overhangs.

    1. Sabotcat | | #4

      Armando ,
      Thanks for the follow up and correction. Are you envisioning a more massive LVL rim board to add some structural support behind or beneath the rafter/overhang? Or would the more substantial LVL rim board serve as a ledger in our model?

  3. Expert Member
    Akos | | #3

    This is way too complicated.

    Building high performance, you have to simplify your design and stick to the essence of what is needed. Simple high performance roofs should be vented and need a lot of insulation, the simplest is tall I-joists or trusses either dense packed or filled with a lot of loose fill.

    Transitioning an exterior sheathing air barrier to an interior ceiling air barrier can be done with either a wide section of flashing tape or a piece of CDX over the wall plates. This detail is much simpler than trying to figure out the overhangs.

    This is close enough to standard construction, has great R value, easy to air seal and hard to mess up.

    This obsession with monopoly house with bolt on eaves makes no sense to me. Adds a lot of complication for next to no benefit. About the only spot it works is for an unvented roof where you want zero overhangs.

  4. Sabotcat | | #5

    Akos,
    "This obsession with monopoly house with bolt on eaves makes no sense to me. Adds a lot of complication for next to no benefit. About the only spot it works is for an unvented roof where you want zero overhangs."

    Understood. And you can find folks who say that not having to seal the penetrations made my rafters for an overhang makes figuring out how to "add" overhangs to the system is a more straightforward choice. And the same time you can find folks who wish they had just taped around each penetration and been done with it.

    Another dumb question based on what you said. Maybe obvious, but I'd honestly appreciate an answer. No one has suggested using trusses. For a low slope 1:12 roof, could a predominantly rectangular truss be set on an appropriately angled top plate (I guess it would be a trapezoid), preserving the sloping ceiling inside the house. I believe we've arrived at a moment in California where some external insulation is preferred. But my calculations indicated that around a 24" deep truss would do well here. Am I correct?

  5. trystanherriott | | #6

    We framed the shed roof on my house with single pitch parallel cord trusses and have cathedral ceilings throughout our house. The trusses are 36” deep, making it easy to frame at 24” on center with 50 lbs/sq ft snow load and clear spanning 30’. It’s tougher to insulate with cellulose later if the trusses are too much shallower. Peak and drip eave overhangs are up to ~54” long with 2x6 top cords on the trusses. “Gable” end overhangs are hand-framed with 2x6s…the final two trusses at each end wall are dropped the depth of those 2x6s. Our air barrier is sheathing at the walls and transfers across the top plates to 1/2” CDX at the ceiling.

    All this is very simple and effective. Also, my wall top plates are not beveled; the truss manufacture built flats into the trusses where they land on the walls, but I’d imagine this can go either way depending on how the truss folks like to build this style of truss.

    1. Sabotcat | | #9

      Hey! Trystanherriott!
      I'm running into resistance here in southern California. But holy smoke, after doing the homework and calling around, rereading your post is amazing...like a blueprint. Who did your trusses? Were they in California? And if they weren't, I'd at least like to reach out to them for some guidance.

      I hope you see this...it's been weeks.

      Mark

  6. Tim R | | #7

    Hi,
    A truss would work fine - it can be engineered to what ever is needed. The porch/ eave lumber could be set at 4-6 ft oc and be screwed from below, getting the pullout embedment in the 4x6 outlookers. The eaves could have 3x t&g as the planking spanning the 4 to 6 ft spacing.
    Or If TJI's were wanted, the eave locations at 4'to 6' oc could use a GLB or solid engineered lumber to attach the outlooker with the big dollar screws.

  7. Sabotcat | | #8

    Trystanherriott and Tim
    Amazing answers and super helpful. Thanks! And thanks for the picture. I may reach out if have more questions. But you guys rock!

  8. Walter Ahlgrim | | #10

    Seems to me with large overhang and a lower pitch to the roof “overframing” does not give you much if any benefits. As all of the soffit will be above the top of walls and you have lots of room over the walls for cheap fluffy insulation on the flat ceiling.

    When we built my 5-12 hip roof with 4 foot overhangs with trusses each corner was built on the ground and craned into place.

    Walta

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