One of the buildings I take care of was built as "Low Cost" housing in the seventies. The roof is sheathed with either 3/8 or 7/16-inch plywood.
It is now warped and wavy.
I have it under contract to resheath and reroof. The contractor has asked if it is possible to leave the old sheathing in place, and install the new 5/8-inch sheathing over the top of it. He is arguing that the old stuff, is so flimsy that the new stuff will flatten it out, and it lets him keep the house fairly dired in during the process.
What do you folks think?