Options for attaching overhangs over thick roof insulation
I have a timber frame with a 12/12 pitch built in climate zone 6A (central Maine). Atop the frame I have nominal 2×6 spruce decking followed by zip sheathing (some background details in an earlier post here: https://www.greenbuildingadvisor.com/question/wrb-choices-on-spruce-decking-with-exsulation).
On top of the roof zip I will have 8″ of polyiso consisting of multiple layers with overlapped seams and the bottommost layer sealed with tape. I then intended to place roof underlayment, then 2 x4’s on the face for strapping and then a metal roof. Because my shell is a box with no overhangs (to prevent or minimize thermal bridging) I have to build overhangs prior to placing the roof on. I am now realizing that the depth of my roof insulation will make attaching any structural portions of my overhangs to the upper parts of the walls difficult. My sidewalls have 2″ of polysio on the outside and the roof insulation will sit atop these. If I have a soffit/fascia detail with material that is 10-12″ long I can bite into the framing at the bottom but there is nothing but insulation at the top of this span. My question is how are others dealing with this wall attachment issue. Thanks for any advice,
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Not sure if this might help? https://www.youtube.com/watch?v=yTbMVdmnNlw
Great minds think alike. That was the video from Matt Risinger I was going to suggest. There's also this other video on his own house build that might be helpful.
Matt also visited a budget builder, Scott True, that had some pretty good methods.
Thanks rockies63 and canada_deck,
I had seen those videos but worth rewatching! I think my question may not have been clear enough. My roof will have a lot thicker insulation than the roof in the Risinger videos and I am specifically concerned about the edge where there will be 8 inches of exposed insulation with the 2 x 4's on top of this. My insulation layers will not extend out past the plane of the underlying roof, in other words there will be no insulation out in the overhangs. I was thinking of having some supports under the 2 x tails that stick out (these would be similar to the Risinger video but on top of all the insulation) that attach back to the side walls. However, for my situation there is nothing structural in this side wall until more than 10 inches down (below the insulation stack and the zip). It seems the only approach here is to build a pretty deep box with a side wall that can be screwed into the upper house wall down below this 10" height and then through the 2 x 4 tails into its top. Does this explanation make sense? Thanks,
Is this what you mean? Black is sheathing (OSB, Zip, or Plywood.) Orange is insulation (10" on wall, 8" on roof.) Blue is 2*4s with 8"+ screws/lags going through the insulation to the roof. Green is the metal roof.
Here are a few past discussions:
It's a tricky detail. How far out are you hoping to go with the overhang? I assume your strapping over the foam is horizontal because of the metal roof? Will you change strapping direction near the eaves so you can extend the strapping out for the eave overhang? Are you boxing the eaves?
For the rake, assuming you have horizontal strapping, you can use those as tension members, nailed (and possible glued, because it will be in shear) really well to ladder blocking built as an 'applied' overhang.
The tricky part is that the compression part of this 'beam' is into nothing but foam, unless, as you say, you make a really deep soffit. One option may be to run wall strapping (if you are using that?) up past the edge of your roof foam and tie it into the roof strapping somehow (metal angle bracket?). This wall strapping would then serve as the backing for the ladder framing compression forces, which in turn applies a bending force to the wall strapping (its a small length of strapping though). Just an idea; not sure about all the specifics of the buildability. (This is sort of what I drew in post #14 of the second linked thread, but a little different, because in hindsight, the strapping should probably go behind the ladder framing and connect solidly to the top tension member (strapping)).
Another thought is to embed something solid at the edge of the roof in place of foam. A bit of added thermal bridging, but may be worth it to avoid added fuss.
We're doing something similar at the office but without the added complexity of the wall foam and with a plywood skin to receive shingles instead of strapping for a metal roof. We're adding a solid nailer to the edge of the roof in the bottom 2-3" of foam.