I agree with Slumlord, and Kellory. Listen to these guys. Unless you tear off the shingles you have no idea what is below, or what the condition is. Once you get it done and inspected you are ready to go. Unless you already have shingles off, you really have no idea what the condition of the decking is. If the decking is OSB then use 1X3 or 1X4 strips to screw to. The manufactures of metal roofing I've seen throw your warranty out if it's not attached to plywood. IF it's 3/8 ply you have the same problem. If it's a concern to you, why haven't you talked to the company providing the metal? If you want their warranty, find out what THEY require.
