You need a load span table to determine the number and distance between each pier/post supporting the floor. I'm posting a few links so you can get a general idea of how it works: For ceiling/roof rafters: http://www.mcvicker.com/resguide/page013c.htm For floor Joists: http://www.mcvicker.com/resguide/page013b.htm For girders (floor): http://www.mcvicker.com/resguide/page013a.htm All of that (and more) can be found here: http://www.mcvicker.com/resguide/ GIRDERS OF JOISTED FLOORS: would be the beams beneath that the joists connect too/rest upon. http://www.mcvicker.com/resguide/page013a.htm You get two measurements to work with 4' and 6' spacing. For a 6 x 8 girder you can set a post every 13' 3" on center - then you set each girder apart depending on the size of the joist wood you are using. Examples: 2 x 10's set 16 inches on center can span a distance of 14'-0" Most common construction sticks with cheaper members, your girders would consist of 4 x 6 set every 6' having a minimal span of 6'-6" Your joists consisting of 2 x 8's set every 16 inches apart meaning you would need a supporting girder set 11'-6" apart. (When working with spans and your lumber ROUND DOWN - meaning in this case to span 12' in one span you need a larger chunk of lumber, one that could span say 13'6" not the other way around) If you use 4x6 girders (off the shelf lumber at most home stores) then you would need at least three set in your case 6' apart. Along the length of each one you would need at least 6 piers/posts one at either end and one in the middle. On top of that or connected to those you would use 2x8 joists set 16 inches on center. 10 joists for 12' x 12' one on each end, plus the ones set in between. For such a "small" project the consideration would be available lumber sizes. Lumber usually comes in 8' lengths, 10' lengths and 20 ' lengths. Although you can span the 12' length in one span, ideally you have to think about the manageability of the materials you are using. for you 6' lengths would work best. The left over 2' length can be cut down to 16" less the thickness of a joist and placed between the joists to add extra support as a cross piece. Different regions have different laws on how many cross segments you need - in some areas you need one for each span (in your case 6') in others they need a cross piece for every 48 inches. Girders (the support of the floor joists) should be one solid piece of lumber. Many lumber places do supply or can supply 12' long beams/girders since the size of the wood (4 x 6) is usually used for such. However you can laminate 3 2x6 boards together. Most areas require a minimal crawl space of 18 inches. Your piers/post most likely have to be concrete or set on concrete footers. Many places have developed strict laws when it comes to earth and wood contact, not only for rot but to prevent infestation with termites. Putting it all together. You will have 9 piers/posts set every 6' apart - 8 around the perimeter and one dead center.