Malcolm, heres my thoughts. There is a waterproof membrane available which waterproofs the deck exterior, in Canada it is called "Deckking", however it does cover the beauty of the wood. The other way would be to secure a wood sleeper to roof top running horizontally eg. (2x6) in 2-3 places covering this piece with rolled roofing then securing deck joists to it and tarring the screws. With enough tar a leak would probably not occur. Think about it, hope it helps. Doug PS besides with a 20 degree pitch water will run down slope as opposed to flat.