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Hot Roof assembly, furring strips or full sheathing over foam?

Wunderbar | Posted in General Questions on

Sorry in advance if some version of this has been asked and answered before…

I’m working on my roof assembly now and am debating a few options.

Ive got 2×6 T&G which i’ve installed plywood over the top and taped the edges where it meets the wall sheathing. im getting ready to do 2 layers of 3.5″ polyiso over the top. Should I tape all the seams of that plywood before the polyiso?

On top of the polyiso:

Option A.
I was going to do zip sheathing on the whole roof, taped seams and then metal roof.

Option B
Is it better practice to do furring strips on top of the polyiso to give it ventilation and then metal roof on top? Do you install roofing underlayment on top of the furring strips?

 zone 4 (Oregon countryside)

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  1. Expert Member


    The easiest primary air-barrier is the plywood, so that's what I would tape.

    Metal roofing can be installed over either strapping or a solid substrate. I prefer a solid substrate (Zip or plywood) for the following reasons:

    - Strapping makes the panels rather than the substrate structural, meaning it's more likely to be damaged by debris, or workers on the roof.
    - The air-spaces between the strapping can collect moist air and experience condensation. No air-space under the roofing means no moist air to condense.
    - The strapping will not align with the slotted fastener holes in the metal panels, limiting the metal's ability to expand and contract.
    - A solid substrate provides backing for the fasteners used on trim and penetrations wherever you need them.

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