I have a deck which I need to build for my own new home construction project. The deck sits partially on top of a conditioned space so I need to make certain that it is waterproof. The plan is to lay a slightly sloped (1/4" per 1') plywood deck down, waterproof that plywood, then lay sleepers over that and then cover it with Ipe or a synthetic 1x6 decking. It is a coastal northern california home so it won't be subject to freezing temperatures but will receive a significant amount of moisture/rain. There are no overhanging trees so it won't fill up with pine needles/leaves. I am curious as to how I might best waterproof that plywood subdeck. It will not have direct foot traffic on it, but it will have the weight and possible minor abrasion of the sleepers and decking on top of it. Thus UV exposure won't be an issue, but it needs to be very long lasting since it won't be easy to get down to repair/recoat it should it begin to leak. Any suggestions/thoughts would be appreciated. thanks.