Replacing 5v tin roof

Patrick Brewer
12/29/2017
Hello, I need to replace a tin roof on a 100 yr old farm house. The rafters are stripped with 1x rough sawn lathing strips approx every 12 inches. I have had two contractors give me prices, one says the roof needs to be sheeted with OSB and an underlayment material installed. The other says that the existing lathing is fine. That it can be repaired where needed and new roof installed directly to it. Can you tell me the pros and cons to this. Proposed roofing is 29ga exposed fastener. Thanks
Todd Miller
Isaiah Industries, Inc.
12/29/2017
While there are metal roofs that can be installed over purlins / lathe boards, I never recommend it for residential applications. We continually do things to make our homes more airtight -- new windows, doors, house wrap, siding ... additionally, we seem to do more and more things that introduce humidity into our homes ... the end result is that the ambient moisture that used to go out through a home's walls and windows now ends up in the attic. A metal roof with the panels exposed to the attic (or even with a sheet of underlayment in between) poses a large risk of condensation in the attic -- something you do not want.
Guest User
12/29/2017
Thank you for your response. If I understand you correctly you are suggesting that a sheeting layer should be installed and then a moisture barrier be installed between the sheeting and metal roofing material too reduce the risk of moisture created by condensation?
Todd Miller
Isaiah Industries, Inc.
12/29/2017
Generally, yes. The moisture barrier is an underlayment that goes on top of the roof decking (sheathing) which is required by code. The metal roof would go on top of that. At the same time, re-roofing is always a good time to think about attic ventilation and to make sure it's up to code. Proper ventilation requires intake and exhaust vents. Your contractor should be able to tell you what is required by code in your area, and help you discuss options for achieving it.

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