Metal Roof Layover

I have a client who wants to go with metal on a barn. The problem is the roof currently has 1 layer of comp shingles over 1 layer of wood shingles on space decking. If it were up to me i would tear it all off and install new osb but he doesnt want to spend the money on it. What do you guys think is my best aproach for doing a layover with metal on this job? Is it possible to do even do this without running into any issues? I already told him if he was serious about going this route i probably wouldn't warranty it but that didnt seem to phase him????

Just price it for the full tear off and tell him that's the only way it'll get done. Hand him your card and say "Give us a call when you're ready." Problem solved.

First check building codes for your area. It's most likely not allowed to go with three layers. If it is ,then you would need to insure you have something to screw to