Corrugated metal directly over foil faced polyisof foam board?

Jesse Petty
11/14/2017
Hi, I'm building a small cabin with 1/2" plywood on top of the rafters and a mat - 40 vapor barrier over the plywood. The roof is steep and unvented. I'm going to put 2" foil faced polyiso foam board on top of that. My original plan was to put 1x4 purlins diagonally on top of the foam board so as to not create a dam where condensed water could puddle on the foam board. I am wondering if I can skip the purlins and put my 7/8" corrugated steel Roofing directly on the foam board and screw through the foam board into the plywood (and into rafters every 2'). I guess that would put more holes in my foam board and vapor barrier then if I use purlins, but it would be one less step on my steep pitched roof. The corrugations would provide some ventilation under the metal. What do you think, purlins or not?
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11/14/2017
The screws are usually pretty tight when it comes to sealing the air gap and seeing as it is going into the rafters, it should pull the foam tight at that location. If its a cabin, I don't think you really going to need the extra air flow. Is it helpful, yes. Necessary, not really. You are going to be moving some air under that roof panel as it is already via the corrugations.
Guest User
11/17/2017
Great, thanks. That seems to be the consensus of people who I have talked to as well. The next question I have is whether to screw the high points (ridges, humps), or the valleys of the corrugated metal. My dad says the ridges are better for avoiding leaks in the future when the neoprene gasket breaks down, because water will build up above the screw if it's in the valley. The manufacturer recommend valleys, but said that the high points are OK as long as care is taken not to dent the hump. It's 26 gauge. Thanks for the expert advice.
David Stermer
11/17/2017
Hi Jesse, Thanks for your interest in metal roofing. I have had success predrilling the stack of panels from the back side - will keep screw from walking if not predrilled. Clamp the stack and use and under-sized bit. Finally, it does take some care when seating the fastener not to 'dent the hump'.

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