This is our procedure of prepping to install glued or nailed down floor systems on a concrete slab. First, we will thoroughly inspect the slab for levelness, make any correction if necessary, apply a heavy coat of tar, lay down a 6 mil plastic sheet (visqueen), then we install (1/2 inch for glued down), (3/4 inch for nailed down) plywood fastened to the slab by 2 1/2 inch pins, every 2 feet apart. Now we are ready to install either type of floor.   If it is a typical residential or commercial wood framed structure on a second floor, glued or nail down installation, the previous described procedure isn't necessary. Please take a look at the pics below.