Metal over old roofing shingles

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billyjoe63

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I am going to strip my roof with 1/4's and put metal over the entire roof. I have been told I need something in between for a vapor barrier and have been told I won't need a vapor barrier because of the space from the old roof to the new. I need to know for sure if I need the barrier and what to use. Would roofing felt be okay.
 
When warm, moist air comes into contact with the cool underside of the metal roof, condensation will form and drip onto the wood sheathing below. In the Northeastern USA / Eastern Canada, where I install, temperature fluctuations from extremely cold to warm are the norm and metal roofs can sweat to a significant degree. In Texas, it will be less but your money will be well spent in protecting the wood sheathing. I recommend either a 30 # felt (tar paper) or a synthetic underlayment such as Grace Tri-Flex 30 or IKO Roof Top Guard.
 
Welcome BillyJoe:
You are installing this metal roof over old shingles which probably have 15# roofing felt underlayment. That is enough vapor barrier. The space created under the metal along with the corrugations in the metal will allow air flow under the metal; just make sure you leave an exit for the air at the top (or ridge).
Glenn
 
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