Metal over wood

The International Building Code calls for a maximum of two layers of roofing. While some heavily textured metal roofs such as corrugated products and metal shingles technically could perhaps be installed over what you have, I would advise against it. Even if code is not enforced in your area, going outside of code could be an issue in the future for insurance companies, home inspectors, or mortgage holders. Thanks for considering a metal roof.

I strongly strongly strongly suggest solid decking on ALL residential metal roof installations. Exposing the back side of the metal panels to the attic is begging for major condensation issues. Here's an article I wrote recently on this subject: https://blog.mcelroymetal.com/metal-roofing-and-siding/condensation-ventilation-and-your-metal-roof Then, once decking is exposed or in place, yes, underlayment should be installed prior to the metal, to meet building code. Much of the industry has adopted the use of quality synthetic underlayments.