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New metal roof over unventable old cathedral ceiling?

Richard Feldman | Posted in Green Products and Materials on

Our 1955 house is in CZ 3C. Tomorrow its shake roof comes off, to be replaced with OSB + felt + standing seam metal. The problem area is over a cathedral ceiling: solid 2x T&G deck over 4×6 rafters, drywall below, rigid foam in between. Never vented. I bet that’s OK because the T&G and felt and shakes are sufficiently permeable. What will happen with a metal lid on it?

I wanted to ventilate at soffit and ridge, with a bit of space for airflow over & around the insulation. But each 48″ wide rafter bay is closed off by 2×6 blocks near the top, bottom, and maybe in the middle. They support a nonstructural 2x “rafter” for drywall nailing.

We see no practical way to open a ventilation path under the exisiting T&G deck.
Do we need to put spacers between that and the new OSB, and cut through the bottom deck board to create soffit vents?
If that’s an uneccessary or unhelpful cost, then OSB will go right on top of existing deck, as planned. That’s skip sheathing, except over the cathedral ceiling. It seems that a ridge vent would contribute little over the unvented cathedral ceiling, and a couple of eyebrow vents would suffice for the attic areas.

Perhaps there’s a good solution that we have overlooked. All advice welcome.
Thank you,
Richard and Stefanie

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Richard and Stefanie,
    There are a couple of issues here. One concerns the sufficiency of the insulation; the other concerns venting.

    Is the existing rigid foam insulation at the cathedral ceiling installed as a continuous layer? Or is it cut into strips and inserted between the rafters?

    How thick is the rigid foam? Do you have anything close to the minimum R-value required by code?

    If you want to include a ventilation channel, you can ventilate above the 2-by T&G decking if you want. (You would need to install flatways 2x4s above the decking, 16 in. o.c. or 24 in. o.c., running from the soffits to the ridge, to create 1 1/2-inch-deep ventilation channels. Of course you would also need to install plywood or OSB on top of the 2x4s.)

    It's hard to understand from your description whether this solution is possible or desirable.

  2. GBA Editor
    Martin Holladay | | #2

    It looks like Richard and Stefanie have decided to continue their questions on a new thread:
    https://www.greenbuildingadvisor.com/community/forum/green-products-and-materials/28756/more-metal-roof-over-unvented-cathedral-ceiling

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