Horizontal or Verticle Supports?

John Jennings
I would like to put a vertical panel type metal roof over an existing roof on a 50 year old summer cabin. I want to have an insulation layer under the metal roof to aid with keeping the cabin cool in the summer. Inside the cabin there is no insulation and the rafters are exposed. I have two different proposals for how to mount the metal roof, and I'm looking for advise on which way to go. The first proposal is to lay 2x4's flat in the horizontal direction across the roof, spaced 2 feet apart. In between the 2x4's, fill the space with closed cell foam. The metal roof would then be screwed to the 2x4's. Then add foam to the bottom ends to close the spaces of the metal contour to keep out bugs (wasps and bees are an issue). The second proposal is to lay the 2x4's from bottom to top of the roof along the slope. In between the 2x4's, put one inch thick foam and leave an air gap to allow air flow from bottom to top. The first proposal seems more structurally sound(?). The second proposal seems good for the airflow, but could allow for wasps and bees to get in and make nests. One more question: How is a metal roof joined and sealed around a square chimney located on a roof slope?
Todd Miller
Isaiah Industries, Inc.
Either system could probably work but my preference is for the system that would provide ventilation. I assume they would then put down horizontal battens before the roof is installed. Chimney flashing methods will vary by product and should be available from the manufacturer but I would suggest flashing that is saw cut into the chimney.
Eric Novotny
An informed customer is the Best Customer!
Here is a link to a very succinct system that entails both the ventilation and insulation you are wanting. http://danperkinsroof.com/1108_JCL_Perkins_A.pdf I agree with Todd than the ventilation will be a plus but you can have your cake and eat it too. You just need to screen those vented areas. It is also a good idea to put the foam down and screw the wood down to that to eliminate any thermal bridging. 2x4's are probably overkill in this case too because you are not looking for them to provide any additional structure. You can likely get away with battens/purlins for your venting. Don't tape the seams on the foam as you want there to be some air movement and the ability of the moisture to diffuse through. It is completely acceptable to go over top of asphalt in this application but I prefer to remove it when you can. The size of the roof deck is not so large that it would add considerable expense to the project and it will make the roof cooler in the summer months but getting that large thermal mass off the roof.

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