Screw engagement requirements – insulated roof design
Hello, I’m working on figuring out some of the remaining details of my design for an insulated roof assembly retrofit to an old cabin. The home is in Zone 5a and currently has no insulation at all in the roof so there is a condensation issue on the underside of the roof decking. I’m looking to address that plus improve my thermal performance by adding roof insulation. The roof is a gambrel with one side having a porch overhang (the drawing included shows this side).
My current assembly is as follows
Interior (from inside to outside)
-Pine T&G Paneling
-Intello smart air barrier over rafters/Rockwool
-2×4 Rafters on 16″ filled with R15 Rockwool
Exterior (from inside to outside)
-1×12 original roof decking boards
-Membrane (475 Supply Adhero 3000)
-EPS Foamboard 1.5″ + 4″ Staggered and taped seams
-19/32 Plywood
-Membrane (475 Supply Adhero 3000)
-Standing Seam Metal Roof
I’ve included a drawing of my intended assembly stack, the details are not exact, this is mostly just for illustration purposes. Also included a rough sketch of the original idea I had which was to build framing with 2×6 on the original roof deck that I would fill in with foamboard layers. I’d build both a perimeter and then supports every 8′ to support the plywood edges going on top. The included drawing of this is incomplete, it’s just got the basics of what I was thinking.
The two main questions I have are as follows
(Questions #1)
My assembly stack depth is 7″
-Original Roof Decking (1″)
-Foam Layers/2×6 (5.5″)
-19/32 Plywood (.578)
The plan is to drive the long Spax lags through the layers directly into the rafters. My options here are either 8″ or 10″ Spax lags, which gives me either 1″ (8″ length lag) or 3″ (10″ length lag). I was hoping to get 1.5-2″ of engagement into the rafters, but that doesn’t seem possible. 1″ engagement into the rafter seems a tad short, and 3″ into a 2×4 seems like too much (plus hard to keep from walking and maybe concerning given the age of the rafter 2x4s as I don’t want to split them).
Was thinking of 12 screws per 4×8 sheet of plywood, with 2 (and sometimes 3) sides of a sheet attached through the 2x6s into the rafters.
Looking for any feedback or suggestions on thread engagement.
(Question #2)
My original intent with the 2×6 framing on the perimeter was to seal the foam and protect the whole assembly from rodents since I have a lot of those in the woods and I don’t want them making homes in the insulation foam. The 2×6 internal framing was meant to provide more strength for the plywood and ensure a straight and even finished roof deck for the metal roof to sit on. I had debated only having the perimeter of 2×6 but was concerned with the EPS shrinkage/compression over time possible causing the middle of the new roof deck to sag and cause deformations in the metal roof.
I realize I’m creating more thermal bridges with this additional 2×6 framing, but with the tradeoff of ensuring a rigid and flat roof deck it seemed worth it. Wondering others thoughts/feedback on that plan.
Thanks
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Replies
You don't need the interior 2x6 framing but you definitely want the exterior one. That should be as tight as possible with any gaps sealed with stainless wool pushed into canned foam so rodents can't chew through it. Foam doesn't shrink much in thickness only in length and width a small amount.
You only really need one air barrier and that should be the layer under the rigid. Since you have a lot of exterior rigid, you don't need a warm side vapor retarder. Doesn't hurt but not a must. The important part is tie the roof air barrier to your walls. This usually means a bit of spray foam in the soffit area above each wall plate to the roof deck. You can also do it similar to a rim joist with pieces of rigid insulation sealed in place with canned foam.
The 2nd layer of peel and sticker under the metal is not needed. Metal roofs are liquid tight, regular synthetic underlayment is good enough.
Around me roofing polyiso tends to be the best value for rigid, I would check your local commercial roofing supplier. You can also sometimes get it second hand for a significant discount.
For screw pattern, I would look at the roofing nail base panels like Hunter. They attach directly to the roof deck bellow not into the rafters. Much simpler install. For screws again look at commercial roofing suppliers for roofing deck screws. These are sold by the pail and significantly cheaper then other long fasteners.
Thanks Akos,
Good to know on the compression and it not being as much of a concern for EPS. I think my worry had been that over time with repeated years of snow weight that the foam would compress slightly, or just enough to show up in the metal roof, but it sounds like that's not an issue.
I'll plan to add those sealing/insulation details you mentioned in the soffit and rim joist areas. And I'll look at substituting the 2nd peel and stick layer for a regular synthetic underlayment.
I had looked into used polyiso and found some decent prices, and had originally planned on using polyiso for the foam, but then I got deterred by the lower performance in cold weather and loss of R value over time. So I switched gears to the EPS since I was able to get a decent deal on it from a local supplier.
Thanks for the tip on looking at the Hunter configuration, I'll check that out.