A bearing deck would have it's joists laid directly on top of the slab (these are called sleepers). Normally, a slab is not level (for drainage purposes). Therefore, the bottom of each joist for the deck needs to be cut at the angle of the slab to make the deck level. Posts or other support are not normally needed in this case. The joists would be attached by a perimeter board called a band board. I would lay 15 lb. Roofing felt down on the slab.