Overroof venting, aka a weird intersection of roofs…
Hey all, I’m building a home in the foothills of the Cascade mountains in WA. It’s listed as Zone 4, but it gets some snow, is on the edge of a rain forest that gives us very wet winters and very dry summers.
The house has two 20′ wide roof sections that intersect at a T. One is vaulted and vented using the I-joist method similar to last year’s Fine Homebuilding project house. The other roof is a more conventional raised heel truss roof. Ridge vents for both.
The conundrum I’m getting lost in is how to detail the ventilation for the little roof section that extends from the vaulted roof line over the truss roof where they intersect.
The vaulted roof will have a ridge vent. That can easily extend over the intersection to where it meets the truss roof section. But the question I still have is how to get air in there? I can cut some slots in the plywood of the truss roof similar to soffit intakes to let air in, but I wonder if that’s going to mess up the ventilation for the truss roof section. Also because of the width of the vaulted roof section and the length of the truss roof section, the truss roof section is really only pulling air in from one side.
The other possibility is that I’m worrying over nothing and that a roof built on top of a roof doesn’t need any venting at all and that the truss side will be just fine with only pulling air in from one side.
Thanks in advance for any insights!
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Andrius, in situations like that I like to vent the space under the "cricket" roof into the larger attic space, with either one large hole or two large slots in the roof sheathing. If you ignore it there is a chance it will be fine, but also a significant chance that it won't.
Thanks for the answer Michael, I've been getting some conflicting opinions but I agree with your assessment...holes in the plywood to let air in is the way to go! I've been thinking that there aren't too many downsides to venting that section.
Another follow up question is if I have a truss roofed section that because of the intersection can only have a soffit on one side, what's the best solution to making sure there's adequate air flow to keep the roof vented? Is it as easy as just doubling up the size of the soffit intake vent strip?
There are various work-arounds that people use for this situation -- Michael Maines has mentioned one of them.
I have a conservative attitude toward vented roofs, however. If you have a gable (or shed) roof with a clear shot from the soffit to the ridge, then you can vent the roof. If you have valleys, you can't vent the roof, and the best solution is to detail the roof as an unvented roof.
My conservative approach is explained in the following article: How to Build an Insulated Cathedral Ceiling.