Redecking of Rental House roof

[Brian Oney]

So we had a roofing company replace the roof on our rental property in Kansas. Naturally they bid the job as one layer of shingles and no re-decking required (said they looked in the attic and didn't see any need for redecking). But then when they got into the job they found 3 additional layers of shingles and roughly 80% of the house had to be redecked.

So after swallowing that, I go to inspect the work this past weekend. I notice a concave portion on a small part of the house about 4 foot from the guttering. My initial thought is that they didn't replace the decking on that part (possibly didn't replace the other decking). The sales rep tells me that it could be the house framing is warped in that area, that they can only follow the existing contours of the wood framing, and that it won't effect the ability of the roof to keep the house dry.

Should I believe this? My initial reaction is that they should have at least placed spacers under the decking in that spot to bring it back to the roof pitch - to me they took a shortcut. The coating they put on our porch's metal roof also seemed thin (can easily see waves in the coating and see through it).

Thank you in advance!