steel roof on bowed roof

Greg Fontana
Greetings, I have a 100 year old home in northern Wisconsin with a 12:12 roof, shingled. Due to age the roof is bowed from the ridge to eave fairly significantly. The shingles are about 50 years old and need to be replaced. I would like to put a steel roof on the building. The shingles are brittle and covered with moss in some areas. The attic is vented with a thick layer of blown in insulation on the floor of the attic. I do not believe there is a vapor barrier. The building is primarily used three seasons with little or no use in the winter. I would like to remove the shingles and attach new roof to clean deck. 1. How do I deal with the bowing? 2. Will tar paper over the deck act as a vapor barrier? 3. Any other issues I'm missing? Planning on using corrugated, screwed with washers. thanks
Todd Miller
Isaiah Industries, Inc.
removing the bow may be difficult without removing the decking. The key is figuring out why it's happened, and addressing the root cause. If you install a metal roof that can be installed on battens, that might allow you to address the bowing a bit. But generally speaking vertical seam roofs can be tough to install well on surfaces that are bowed. You should use a quality underlayment beneath the metal roof. I would suggest a quality synthetic. Make sure the roof is installed per manufacturer specifications.
Greg Fontana
The bowing is due to age and probably heavy snow cover at times. The attic has been braced internally. This is a common site on the older structures up there. There really is no way of correcting the bow without replacing the entire roof and structure. Also I am NOT going with vertical seam I'm considering corrugated with washered screws. I am told this type of metal can conform to the bow to an extent.
Eric Novotny
An informed customer is the Best Customer!
Any metal roof can slightly float on the deck, but under heavy load will show the underlying structural defects. If you want the roof to be true, you will need to either strap the roof via purlins/battens or sister up new rafters to square up the deck.

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