||02-23-2012 11:35 AM
If you are planning to get a permit, you need to discuss the foundation question with the building inspector. They may allow you to place directly on the slab, they may require testing of the slab to determine if there are footings under the posts, or they may require a new footing under each post.
If you are not planning to get a permit, then the question is what is your basis of design? If you are planning to follow International Residential Code then you need a footing under each post. The required size and depth of footing is discussed extensively in the Prescriptive Residential Wood Deck Construction Guide, available over the internet or perhaps from your building inspector. Footing size and depth is a function of the frost depth, beam and joist span, and soil type.