Water proofing slate deck.

Hello, I have a slate deck that was installed 3 years ago and was never
sealed. It see a fair bit of water as there are alot of plants on it. There is
a car port under the deckand on a recent inspection I found that the ply wood
base the slate is sitting on has quite a bit of water staining and is rotting
in one particular place. I have just moved to the US from Australia and I am
not sure what products to look for. I imagine I will need something to seal
both the grout and the slate. Any advice you can give me would be
apprieciated. Thanks alot , Colin.