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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