Roof Vent with Steel Roof Over Purlins
Hello all,
I’m designing a house for SW Wisconsin, zone 6 and would like to get opinions on this concept for a vent space over a cathedral ceiling.
The idea is to use 16″ TJI joists over a 24′ span. The full 16″ cavity would be dense pack cellulose. 3/8″ CDX on top of the joists, then 2×4 spacers parallel to the TJIs and 2×4 purlins perpendicular to the spacers. A standing seam metal roof rated for use over purlins on top of it all. Vents at the eaves and ridge. It’s a simple gable roof without any valleys or dormers.
The roofing contractor is concerned that there will be condensation on the underside of the metal and this will cause moisture damage on the wood below. He recommends that the metal roof goes on wood sheathing with a waterproof membrane, with ice & water shield along the eaves.
My question is, if there’s condensation in the vent space under a metal roof, wouldn’t that also occur under wood sheathing?
I would also be interested in any other comments on this design vs putting the vent channel along the top web of the TJI.
Thanks!
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Replies
With TJIs, you can use the top flange as vent space. No need for extra lumber above. Just staple a sheet good (heavy duty house wrap, insulation mesh or fiberboard) to the bottom of the top flange to form the vent space. Dense pack the rest. 14.5" of dense pack is a pretty solid roof.
Metal roofs will always get some condensation on the underside from night time radiative cooling. This is why all manufactures require an underlayment bellow metal roof.
You need this underlayment to protect any wood bellow. This can be any of the synthetic underlayments. Doesn't need to be anything fancy like I&W, standard stuff rated for at least 240F is fine. I prefer to use one of the breathable ones to allow for a bit of additional drying to the vent channel.