Zone 6: A straight forward and affordable metal roof assembly… does it exist?
A continuation from our previous conversation “Zone 6: A straight forward and affordable wall assembly… does it exist?” We are now needing a solution for the roof. Everyone was so helpful with our previous question and we were finally able to settle on a simple & effective design for the walls (If interested, we opted for the recommended 4×8 wall assembly: vertical corrugated metal siding > Coravent or similar > Zip or LP Weather Logic > 2×8 walls > Intello > Plaster or Drywall )
But now for the roof!
Currently the plan is a standing seam black metal roof. The attic will be a vented 8/12 pitch with somewhere between R40 & R50 blown in insulation. A vapor smart membrane (Intello) will be installed behind the finished ceiling. For the roof, do we need a rainscreen like the siding walls or would it be enough to use something like Dorken’s Delta Trella? The uninsulated Zip or LP Weather Logic sheathing would also be used for the roof. I’ve seen roofing details where people do sheathing, insulation board, sheathing. I’ve also seen full rain screens installed under the metal roofing but again, in our region, contractors are not familiar with this way of building so as straight forward as I can make it, the better.
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Replies
Emil,
With a vented attic the roof sheathing has a good drying path to the underside. There is no advantage (and some disadvantages) to strapping under the metal panels.
I'm also not sure the Intello adds much unless you are using it as the primary air-barrier. You will never get any (nor do you want) drying to the interior when it opens up.
So a well vented attic, adequately thick blown-in insulation on the ceiling drywall, and a metal roof on an impermeable synthetic underlayment attached directly to the roof sheathing.
On your walls: If you are mounting your corrugated metal siding directly to the sheathing, you can't use Cor-a Vent at the bottom as it doesn't conform to the panel's profile. You will need to use perforated flashing.