I am working on a house design for SW Wisconsin, zone 6. Roof rafters will be 16″ TJIs, 16″ O.C., 4/12 pitch over a 24′ span. Planning on dense pack cellulose with a vent space over. It’s a gable roof with no valleys or dormers, so the vent space is a clear straight shot from the eave vent to the ridge vent. It’s a cathedral ceiling.

I was thinking of creating the vent space like this: 3/8″ or 1/2″ CDX plywood on top of the TJIs, 2×4 spacers on the CDX, parallel and on center of the TJIs, 2×4 purlins at 24″ O.C. perpendicular to the spacers, then a mechanically seamed steel roof rated for use over purlins.

The roofing contractor doesn’t like this for two reasons: 1) Concern about condensation on the underside of the steel roof wetting the wood components below. 2) Lack of redundancy should the roof develop a leak. He recommends that the roof be installed over plywood or OSB sheathing with an underlayment and ice and water shield at the eaves.

I’m pretty much ready to abandon the spacer and purlin idea and put the vent space under the top flange of the TJIs, but I wanted to check in with this (awesome) forum on those two points.

First, I’m wondering if I don’t understand something about condensation. Wouldn’t there be nearly as much on the underside of the wood sheathing as there would be under the metal? Seems like the wood wouldn’t be that much warmer.

On the second point, I don’t know about the reliability of a mechanically seamed steel roof. Can anyone comment on that for residential use?

Thanks!