Overhang bracing detail
rockies63
| Posted in Green Building Techniques on
I will have a 5′ overhang at one corner of my shed roof (at the lower eave). The roof panels are 12″ thick SIPs. I will have to support the roof overhang at the outer corner with a beam and brace back to the wall (made out of 6×12 timber).
What is the best way of detailing the brace’s connection to the wall? The wall itself is 2×6 framing with 1/2″ plywood sheathing, then the air/water barriers, 2 1/2″ Roxul Comfortboard, 3/4″ furring strips and then rabbited cedar siding.
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Replies
Scott,
Any chance of a sketch?
Scott, two things to think about: structural support and water management.
For structural support, you will want solid blocking of some sort in the wall where the brace lands. Sometimes horizontal blocking between studs is enough; sometimes it's better to install additional studs. There are both vertical and horizontal loads you need to support.
For water management, rain will run down the brace and you want to kick it out away from the wall as quickly as possible. Some builders leave this task to the WRB, but I prefer to use an additional "spline," often a scrap of peel-and-stick membrane, between the brace and the WRB and kicking out onto the siding.
I'll echo Malcolm--a sketch would help us give you more specific advice.
This is what the brace will look like underneath the overhang. The overhang is 5' from the surface of the wall sheathing to the fascia and the brace beam is 4'. I think there should be a vertical "post" as part of the brace against the wall but I'm not sure if it should be attached directly to the sheathing or I should include a piece of rigid foam between the brace and the sheathing.
In the elevation you can also see a beam on the right supporting a porch roof. I was going to run that through the sheathing and have it supported by a post hidden in the wall but I didn't want to break the siding, insulation and WRB layers so I added a post that will sit against the siding (and be bolted through to the wall framing).
I figure I can do this because the post goes right down to the porch foundation so the load is supported. The SIP roof of the porch will go through the siding and insulation and be attached directly to the sheathing but I think this won't be a major thermal bridge since the SIP panel is full of insulation.
However, with the brace the overhang load is transferred to the wall, not the foundation so I think I will have to attach it directly to the sheathing and butt the exterior Comfortboard insulation and cladding against it (unless there's a better way. With a 5' overhang there hopefully won't be too much water hitting that brace.
Scott,
Thanks for the sketches. A couple of suggestions:
- I think mounting it on the foam would work as long as you provided solid blocking at the bottom of the brace back to the sheathing. That small an area wouldn't be much of an energy penalty. A vertical leg on the brace that extends a little way past the bottom of the diagonal member would make both mounting it and trimming around it easier. Locating it in so the corner trim can be continuous might be a good idea too.
- Another alternative which might distribute the loads sufficiently to allow support to be mounted on the siding, might be to run a timber beam along the top of the wall instead of a brace. How far it would have to extend is probably largely an aesthetic decision.
Malcolm, you mean a timber beam on top of the gable wall and extending out under the overhang? That would be the simplest solution but I was reading an article by Allison Bailles called "A Sword Through the Ribs" and he talked about the thermal bridging that occurs when a timber support beam extends through the building envelope. I'm trying to avoid that.
https://www.energyvanguard.com/blog/54221/A-Sword-through-the-Ribs-of-the-Building-Envelope
Scott,
No not on top of the wall, outside the foam. Unlike the brace, with the beam there will be no lateral forces transferred to the walls, only vertical ones.
What direction are the splines running in the 12" SIPS? What snow load do you have? Did the SIP engineering require a brace in that location?
I plan on using 2x material for the edges of the panels, running down the roof and extending to the end of the overhang. The maximum ground snow load will be 88 psf.
The SIP company said that an overhang up to 24" deep usually doesn't need to be supported but mine will be 5' in one corner. I plan on doubling up the 2x material at the panel's bottom edge as well to help stiffen the panels.
Perhaps you could increase the splines at the longer overhang to make it structurally work. Then you could add the brace as a non-structural element.
The splines can be placed at 24"oc or increased to 4x material.
Seem like it should be engineered due to the snow load and that they are SIP panels. The SIP company should be required to engineer the overhang as part of the purchase agreement.