New metal roof over unventable old cathedral ceiling?
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.
Richard and Stefanie
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