Venting a metal hip roof
One issue our office (architect) continues to encounter is providing at least code compliant ventilation requirements for unconditioned attics. Yes, some of the designs have a hard time meeting the code minimums. In general, we use a combination of ridge/soffit vents and add “static” ones as necessary for compliance with 2009 IRC. I realize this forum is for higher goals, but some clients are looking for minimums, except of course when it comes to finishes, which is story for another day. Construction is plywood deck on 2x rafters.
This method meets “code”, however, we have a project that has mostly hipped metal roofs. Providing adequate soffit vents is easy, but how to address the ridge is a concern. The project is climate zone 2 (mid-way between Houston-San Antonio, hot-humid) on a river. We typically use ice/water shield under metal roofs as a matter of practice, so now the waterproof membrane would have to be cut to accommodate a hip vent to help with ventilation. This is not a pyramid roof so we do have some ridge, but not sufficient to meet the code minimums. One local manufacturer seems to not like any vents, http://www.berridge.com/Ridge%20Vent%20Considerations.pdf)
We could use “battens” and vent under the metal roof, but the attic would not be vented. Making a conditioned attic is an option, although that would most likely require foam (or similar method) and not likely in the budget. We could use as much soffit vents as possible along with a hip and ridge vents and say that is the best we can do even though, I dare say, we don’t meet code. I have never seen a metal “static” vent nor do I want to pepper the roof with them as we do on the asphalt shingles. A vented cupola is not an option either.
I realize there are numerous other methods to address this issue and this is not a “green” home, but please keep responses/solutions to a ventilated attic design.
Any thoughts are appreciated.
Thanks
Glen
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Replies
I'm assuming you are using standing seam, not 5-V because the 5-V is pretty much self venting and all you need to do is keep the bugs out. Either way, cut your 5/8 plywood back at the hip and ridge sufficient to provide 3/4" to 1" of air flow on either side of the ridge, run your Shark Skin or peel and stick cut back in these spots, cover with 1/8" galvanized hardware cloth, and cap with a standard metal roll-ridge, holding the metal 3/4" above the roof panels. Occasionally, rain (and powdery snow) will get blown in but that's been happening for years and it will dry out in a ventilated attic.
Alternatively you could cut cor-a-vent to fit between the standing seams and resist wind blown rain and snow better than the hardware cloth detail. (Hardware cloth, AKA Ratwire, goes under the roofing but on top of the tar paper or Shark Skin on the assumption that the water will stick to it and run down between the metal and the building paper.)
Hopefully you are not posting responses at 1:30 due to Irene since I know NC was in her path. If so, I think there are more important issues along the east coast these days then my venting question.
Yes, standing seam. We have used the 5-V before, but "technically" that would not provide a vented attic condition but a "vented" roof. I have yet to find anything that keeps the bugs out permanently, at least in this part of the country.
I like the hardware cloth since it is easier then cutting the cor-a-vent, but wonder if the metal might have an adverse impact on the metal roof. My do some research and see what products may be substituted for the hardware cloth. It is probably not a concern since so little is actually in contact with each other. I think the cor-a-vent is the best method particularly where snow is an issue, which obviously is not an issue here, but eliminates the metal to metal.
Appreciate your comments and enjoy the FHB articles. Thanks.
Glen I think that so long as you have galvanized against galvanized the hardware cloth won't pose a problem w/ corrosion. Thanks for the kind words I'm not up due to Irene, we got lucky there, just trying to squeeze out some writing time after a busy day running the biz, finishing up a piece on David Hill's great talk on duct design at Building Science Summer Camp.