This is Tongue and groove redwood decking. Underneeth are a layer of sleepers followed by a waterproof membrane. What we did here was to lift up the old decking (some boards didn't survive and had to be replaced), lift up the sleepers and replace the existing underfloor membrane (Actually the old membrane was roll roofing) We used a form of Bituathane ( a very heavy rubber that seals any nail penetrations). All of this was necessary because under the deck is a portion of the basement area and items are stored in this location. Anyway- the idea is that the work that was done was to look well worn and tattered. Ta Da!