Installing a metal roof over a sagging shingle roof

Casey Clement
We have an older home and have met w/several contractors for estimates on installing a metal roof. We have a few rotted spots, which will be cut out & new decking laid. There are a few sagging spots but no visual rot underneath. We have been told that a metal roof can be put over the sagging areas by putting 1x4's across the shingles, screwing on each side of the sag. We wonder if the sag is going to be noticeable once the metal roof has been installed this way.
Todd Miller
Isaiah Industries, Inc.
If the sags are bad enough it will probably be impossible to eliminate them entirely. However, they should be better. I have seen some situations where horizontal metal shingles did a better job of hiding sags than vertical metal panels.
Eric Novotny
An informed customer is the Best Customer!
Why is the roof sagging? Structure or plywood? If it is the latter, why not just replace the plywood? You could hide much of the sag by putting the roof on purlins but some with likely come through as Todd mentions.
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