We have a back porch that is 13 feet off the ground. Under the porh is a concrete slab that we would like to convert to sitting area and maybe add a hot tub later.

We need the sitting area to be protected from the rain. I looked into buying an under deck drainage system and even using pieces of metal roofing under the deck as a ceiling that would divert the water to one side of the porch. All of these options seem very expensive.

I'm wondering what might happen if I used Marine grade or pressure treated plywood on top of the existing decking with some sort of asphaly/rubber membrane under it.

My wife works for a Ben Moore dealer and suggest sealing it and then primer/painting the plywood for durabilty.

Does it do any good to seal pressure treated plywood?

Has anyone done this?