Installing metal over partial Shingle

Jack Heckaman
9/18/2014
I am installing a metal roof over our small log cabin in N. Michigan. We added a new addition changing the roof line on 1/2 so that is all new sheeting. The other half is existing shingle roof that is in good shape but 17 years old. The roof boards are true 1 inch and the rafters are true 2 x 8 as we had the wood sawn on location when we harvested these trees. The shingle half we would like to add 1 - 1.5" of foam which I would place over a synthetic roof paper - so my question is do I run strips and place the foam in between? How close should these be - 16 O.C.? and should I use same thickness - so 5/4 for 1" foam and 2x for the 1.5 foam. Any help here would be appreciated
Info @windowsonwashington.net
An informed customer is our best customer.
9/19/2014
Are you planning on stripping the shingles? You should. Strip the shingles, cover the decking boards with a peel and stick, apply the 1.5" of foam, apply the furring strips, and have the roof vented over the top of the foam. An overdeck vent application would be the best of both worlds.
Guest User
9/19/2014
We were not planning on stripping the shingles only covering them with the synthetic paper. Downside to not removing them? also if I lay the foam do I run the furring strips on top of it?and should this be a vertical or a horizontal strip? The roof is the 29 awg US metals exposed fastener metal roof. ( this is to match the pole barn/ garage we did 2 years ago). If I strip the shingles then I would use the peel and stick instead of the synthetic paper? Sorry if I am asking a lot of silly questions as roofing is not my thing. Thanks again
Info @windowsonwashington.net
An informed customer is our best customer.
9/20/2014
Downside is the lack of intimate contact with the roof substrate and the R-Value reduction that will translate to. You can run the purlins on top or on the deck and the foam between. In between, you have weak spots in the thermal system though. Whether you just do horizontal, vertical, or both depends on how you want to vent it and the roof you are using. You wouldn't need both if you strip the roof. Peel and stick would be fine.

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