A homeowner wrote and asked:
We are getting estimates on replacing the tin roof on our house. One contractor has given us an estimate with a plywood sheathing base under the metal and another without. He said that adding the sheathing would enhance the wind resistance and leak resistance of the roof. (We live in southeast Louisiana.)Another contractor has given us a price without sheathing as he doesn’t believe it’s necessary. What would you recommend??
On residential applications experience has shown me to always have solid decking (plywood sheathing) in place.
In your case with an existing house that does not have decking but has a metal roof, and things have been okay, someone might argue that if it didn’t need decking before, why should it now? The issue is that, as time goes on, we tend to remodel our houses and do things to them that make them more airtight. As this happens, more moisture that originates inside the house is typically found to be migrating into the attic. In the case of a metal roof without decking, excessive attic moisture will result in condensation. That is not good.
So, in the best long term interest of the structure, I always suggest decking on residential jobs.