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Waterproofing timber deck.

I wish to waterproof my timber deck. To my knowledge first the deck will have to be covered with H3 treated ply (17mm at least). Then Butynol waterproofing membrane will have to be welded onto the surface. Water drain down-pipes may have to be installed too. The size of the deck is approximately 28 sq m. This is my view - is this the way to go about it? Is there a better way? I am looking for a basic, simple, economical solution tht will last.