Unvented roof detail with metal roofing and purlins over rigid foam
Hello! First post here, so please bear with me as I figure out the best way to format my text and paragraphs here.
I’m attempting to sort out some details of my future self-build. I’ve told myself if I can’t fully detail it in CAD in 3D, I’m not allowed to attempt to assemble it in person 🙂
My current focus is the roof assembly. I plan on having an unvented, conditioned attic in Zone 4C. The total roof span is 16 ft with a 15/12 pitch gable (8′ rafter span).
I’ve thoroughly read a few blog posts by Martin Holladay regarding this assembly and I think I have the roof assembly order mostly right:
1. Interior drywall, painted and air-sealed
2. R-35 mineral wool batts between rafters
3. Plywood sheathing, taped
4. [Optional] peel-and-stick membrane
5. 3″ rigid foam, taped
6. Felt underlayment
7. Horizontal 1×4 purlins, 24″ OC screwed through to rafters
8. Metal rib or corrugated roofing
My first attachment is what I believe the assembly would look like if I were to use a second layer of plywood instead of horizontal purlins. This detail makes more sense to me and I can visualize assembling it in my head. It seems like it would work great to direct any moisture under the roofing out and away from the house.
However, I like the purlin idea because it saves cost, weight, and labor over doing a second layer of plywood. When attempting to detail this assembly in CAD though, especially fascia and drip edges, something just rubs me the wrong way and I am a little confused.
1) Where does the eave drip edge go? See my below drawing for my comments related to this question.
– If I put the drip edge on top of the purlin, this immediately violates code, but it looks nice because the fascia can cover and protect the edge of the purlin.
– If I put the drip edge under the felt (which in turn is under the purlin) this is code-compliant, but is also a bit strange as the fascia can no longer extend all the way up to cover the purlin’s edge
– What if I pulled up the felt at the last purlin and overlapped it? This seems like it’s probably wrong, because it could allow water to pool, but wanted to throw it out there.
2) Does the rake/gable fascia come up to meet the purlins?
– With plywood instead of purlins, I assume the rake fascia comes up to cover the edge of the plywood to protect it.
– If we do this same detail with the purlins, does this not create a trough for water to collect in between the purlins?
– See my below drawing for the 2 options I’m thinking about
– Should the felt underlayment actually extend out over the fascia in Option 1 so that any moisture doesn’t drain into the crack between the fascia and the barge rafter?
– The more I look at Option 2, the more wrong it seems. The metal rake trim should do a good job protecting the edges of the purlins, no?
3) Should the eave drip edge extent all the way to the edge of the rake fascia instead of stopping at the end of the eave fascia board?
– I think this question is mainly a product of my ignorance. I might have detailed the fascia incorrectly and the eave fascia should actually come out and cover the end of the rake fascia, and the drip edge should be installed along the entire length of the eave fascia.
– See my final attachment below. The underlayment now extends over the rake fascia and down its face a couple inches. This detail feels more correct and robust. The drip edge goes all the way to the end.
All of the gaps with the purlins also make me nervous about bug/critter entry. I know you use foam closure strips at the base and ridge with metal roofing, but it doesn’t seem like that would take care of everything. There’s still a gap between the purlin and rake trim.
I’m probably way overthinking some of this, which I tend to do. I appreciate any advice and any corrections in my detail drawings.
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