Sizing and Framing Applied Overhangs
I’m working thru details on planning a continuous exterior insulation retrofit project in Zone 5, adding 6″ of polyiso to the roof and 3″ of polyiso to the walls (I’ve found that we’re slowly converging on basically the “Roof 1” and “Wall 1” assemblies from the MassSave DER guide (https://www.buildingscience.com/sites/default/files/migrate/pdf/GM_DER_Guide_2013-01-18.pdf). I’ve read enough articles here to know that “generous” roof overhangs are recommended, and the DER guide also mentions the necessity of overhangs. The house has very small to nonexistent overhangs depending on the area, so it’ll be handled as a “chainsaw” retrofit with not much cutting required, and overhangs tacked on later.
From what I’ve seen, there’s generally three ways to handle added-on overhangs: (1) Add overhanging 2x framing in line with the rafters, screwed into the rafters, over the roof air barrier but before applying roof insulation. Carefully cut insulation to fit between framing. (2) Add overhanging 2x framing on top of the roof foam, screwed thru to the rafters, as part of creating a ventilation channel. (3) Add just an overhanging “triangle”, “hanging” from the roof sheathing fastened thru the wall foam to wall structure.
Approach 1 adds thermal bridging and makes insulation install more difficult, and approach 2 requires a vented over-roof which I’m not planning on, so I’d prefer approach 3. This also appears to be the approach shown on page 85 of the DER guide, and in the attached detail sketch.
However, my structural help is concerned about making this type of overhang more than a few inches deep — which I doubt would be considered “sufficiently generous” by this group. The DER guide doesn’t mention dimensions that I saw, but it looks like they’re got more than the 4″ or so in my sketch. His concern is less static roof load and more along the lines of when someone steps on the edge during construction / roof maintenance, and a desire to be conservative since he’s not particularly familiar with this type of assembly.
Are there any published guidelines on the actual limits to this type of add-on overhang? Or guidelines for structural design / connections thereof? What overhang dimensions are considered “sufficiently generous” in a suburban exposure, northeast Ohio area?
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Replies
Jonny,
Be careful with this. There are a lot of details high performance builders use, from applied overhangs, thickened slab footings, or using foam under load-bearing elements, that may be good ideas and perform well, but can put the work outside the accepted solutions found in building codes, and necessitate approval by a structural engineer. That's definitely something to find out prior to going ahead - even if y0u can find examples in government publications.
Hi Malcom,
I'm planning to meet with my local building inspector soon to discuss exactly this kind of thing and find out what they're okay with or what structural info they'd need to see. Fortunately my "structural help" is my dad, who is a licensed engineer. However, he's not familiar with these types of high-performance details, so we're both learning on this project. I'm just trying to do some legwork and see if there's any more detailed references on these types of overhangs, or on what kind of overhang depth is recommended. But, in the end, if all he's comfortable stamping is a 4" overhang, or if the building department wants to see a different detail, that's just the way it'll have to be.
Jonny,
Having an in-house engineer father is a great advantage. We are in the midst of a building boom here and I made the mistake of specifying a detail that would need an engineer's approval, and after contacting six, two of who I've used before, I've been unable to find anyone to take it on.
I'm similarly puzzled by a number of the details for applied overhangs I've seen, meticulously drawn, but including long structural screws unto some unspecified (or invisible) backing. I'll be interested to see what you can find.
Jonny,
I have site-built overhangs with exterior insulation that are attached to 2 ledger boards. The upper ledger is screwed into the rafters and the lower ledger is attached to the rimboard using Fastenmaster ThruLOK though-bolts. My walls have 3” of Roxul, but I used Type II EPS foam under the ledgers because I was concerned about compression with the Roxul. You won’t have any compression issues with the polyiso.
Thanks for sharing the photo, it's helpful to see that the overhang (and the insulation under it) actually went on *before* any of the roof insulation. Did you frame up the overhang as one unit and then bolt it on, or did you attach the ledgers first and then frame in the overhang in pieces?
Jonny,
The architect had specified it to be built in 8' sections and then raised into place, but my framers preferred to build and hang the triangles one at a time. One person made all the cuts, another person nailed the triangle sections together, two guys working in the cherry-picker hung the ledgers first and then nailed the triangles in place. For the gable ends, they built 12" ladders that just required 1 screw every 16". I don't remember the exact dimensions but the hypotenuse was about 3' so about 1' is secured to the furring strips that were used to hold the roof polyiso in place.
If I were doing this today, I would see if I could get a truss manufacturer to fabricate this.
Thanks for the additional details and sizes! I was thinking that prefabricated on the ground and lifted into place might make it easier to end up with a rigid, level, and square structure.
If you made it longer, would you also keep the horizontal soffit (making the effective beam depth larger)? Because that may help.
If you look at the DER guide, it is out further but also significantly deeper. Partly due to the slope. Extending it also places your main compressive force through only 3" of foam (and then into the wall) instead of the end of the roof foam, which is effectively much more foam to possibly 'creep.'
I'm not an engineer, but off the cuff, things I might consider: Making the depth deeper if aesthetically acceptable; make sure your plywood is beefy enough, or possibly add in some 2x blocking behind the 24"oc blocking to further distribute compression loads; if still squeamish, perhaps add in 2x's as tension members in the last layer of foam (which does complicate the foam installation for the last layer, but less than if you placed the 2x's all the way down at the roof deck. I'm not actually sure if this gets you much since I imagine decent plywood in tension is sufficient, especially if well-fastened with a good broken joint).
This house has two different roof slopes -- the detail I attached is the low-slope portion, about 10 degrees, while the higher slope portion is about 33 degrees. For the higher slope part, it's easier to get a good strong triangle structure without the fascia being abnormally thick. I think the low slope portion is the more challenging structurally, but perhaps as you suggested some solid blocking back to the wall would help. Or, maybe get funky and do upswept soffits?
Upswept soffits: https://hays-ewing.com/project/stony-point-house/
Yeah upswept, or braces if the aesthetic allows.
I agree with the sentiments that this detail is tricky and not commonly lace, but the physics are there to make it work for a good bit of distance I would wager.
With the sheathing in tension, the bottom portion in compression, the sheathing just requires good fastening* and enough length of plywood. The compression portion I think is the trickier part given the foam. It requires interfaces to be tight (to keep the tension members in tension and not bending) and to have adequate pressure distribution for the foam psi rating.
Getting the compression member as low as possible reduces the effective psi, as does distributing it. I agree, low slope makes this harder, but I imagine (without doing any actual calcs) that you could still build a 'decent' overhang of 12+" with careful construction and good compressive distribution (even with a normal sized fascia). Of course running some actual numbers might be wise if someone is able.
* I suppose with all that foam to screw threw that's nothing to scoff at either
If you are building an unvented roof, I don't see the point of adding the overhangs on top of the structure. Just wrap the zip panels around the overhangs instead.
Even with a vented roof, I would just frame a 2 inch opening on top of the overhangs. Much simpler in my opinion than trying to bolt some ladder framing to the structure.
Alex, I'm not sure what you mean -- Are you saying that I should build overhangs first and then carry my exterior insulation up the wall, out and around the overhang, and then over the roof? (For the record, I'm just using layers of rigid foam on its own, not zip panels)
I wasn't thinking about insulation but about your air/water control layer but yes that's the way I would do it if you are doing the wall at the same time. Would cost a bit more foam but you wouldn't have to cut your trusses.
Check out this article :
https://www.buildingscience.com/documents/insights/bsi-063-over-roofing
That article does more the OP's detail then the one you describe, no?
This one maybe sorta? https://www.buildingscience.com/documents/insights/bsi-046-dam-ice-dam
It does seem a decent idea. I wonder if you could even insulate the soffit, if really concerned about thermal bridging? Surely someone's gone down this road...
Another diagram to stare at:
https://foursevenfive.com/content/product/smart_enclosure/wood_retrofit/475_Wood_Frame_Retrofit_eBook_Aug_2019_Pages.pdf
PDF Pg 60 (doc pg. 55)