Building a freestanding deck on concrete slab. Should I raise the joists from the concrete or just lay the joists on the concrete? My intuition says raise the joists to allow rain water to run out from under the deck and keep the joists dry. I don't want a high deck, just 7-8" high. If I want to keep the joists off the ground by about half an inch, what should I use to support the joists?