I use metal roofing under decks for a dry area underneath. Some applications are harder than others of course,

Really no way to "waterproof" the surface unless you go with a concrete or tile deck that slops away from the house. That of course would be demo'ing your existing deck & starting over, don't think that's what you had in mind tho.

Our deck was rotting and horrible. We had to replace most of it. The contractors replaced rotting beams, then put new plywood over that, tar paper over that, and then they put some kind of surface coating over that. It's waterproof. Dry as a bone under there!

We keep our firewood under there and it's never wet. Problem is we keep summer umbrellas suspended under the beams, and birds are now nesting there....... hard to win!

Suzi