Here is what I was told...Keep in mind, I am checking stuff out over the internet until I can get back to Va.
START QUOTED MESSAGE I'm a licensed home builder & foundation contractor in Alabama and I've got an hour at kill, so here's your own D-I-Y manual. To answer the question directly at face value, assuming you just want someone to pour & finish a slab, I pay my slab finishers appx $1/sf to $1.50/sf for labor-only to dig any grade beams in the backfill (assuming sand is your backfill material), install poly, WWM, rebar, any keyway, then pouring and finishing the slab using a trowel machine & then cutting control joints in the slab. They provide all tools & equipment, including trowel machine, rebar cutter/bender, floats, walk-behind concrete saw, etc. I would make my dirt contractor tamp the backfill, but my particular slab guy does it all for me since I provide the backhoe loader, otherwise I do my own dirt work since I own the equipment and know what I'm doing. This price varies depending on the size of the slab, and with fuel costs now, going more than 20 miles, he would hit me with a fuel surcharge, maybe $1/mi one-way as he might be bringing 2 vehicles. Alabama is cheap comparatively --- slab finishers in other states may get double or triple that.
But let's assume you need the entire foundation done. Before you attempt to do your own foundation work, I would pay the money for a geotechnical engineer to do borings and prepare a soil report to recommend the proper foundation to use, and I wouldn't D-I-Y any foundation work if you don't have proper experience as the foundation IS the workshop and it can't be fixed if you do it wrong. A soil report in my area costs appx $1,800 and takes about 2 to 4 weeks to schedule & prepare for you from the day you call to the day it's ready. You have to have plans or at least the area staked out so they know where the building corners will be. Here's what you're getting into and what must be done to insure a proper foundation and what to watch for if you sub it out:
To do a 28x32 slab-on-grade or built-up with block, assuming excellent soil, no trees to remove and assuming the soil report doesn't say otherwise, I would set the building corners and mark them with stakes and set up batter boards to find them later after you remove the stakes, then remove topsoil/grass to 6", build a small 12" compacted pad at least 5' outside the footprint of the slab to replace the scratched-out topsoil/grass for the slab to bear upon, then excavating & pouring an 18"-square reinforced concrete exterior footing (you have to set the tops of your footings below the frost line by code) and reinforced with 4 rods of #5 rebar stacked 2+2 with #3 stirrups to box them together in a caged format and wet-setting #4 vertical rebar to extend through the stem wall & 2' into the eventual slab every 48" OC after pouring, then my choice is to lay on top of that a course of 8" concrete block + a course of 8" header block to level out and form the slab (as opposed to forming the slab & stem wall with lumber and pouring the footings monolithically as I don't personally like monolithic slabs), then backfilling with "zero" sand & tamping, assuming I have to dig out a couple of grade beams (interior footings) into the sand appx 12"W x 8"D for interior roof support walls and assuming no more than 40/lf, then termite spraying & covering with a
6-mil polyethelene moisture barrier, adding 2 rods of #5 rebar on 3" foundation chairs in the grade beams, adding any keyway needed (assuming no more than a couple of sticks but probably none needed in this case since the whole slab can all be sawed with control joints after pouring), adding 6"x6" 10-gauge WWM reinforcement, adding grade pins, turning down the vertical rebar to extend 2' into the slab area and in the center of it, then pouring 3,000psi concrete with fibers (we have a lot of expansive clay here and need all the help we can get, so I use both WWM & fibers) and filling the block stem wall & grade beams at the same time, finishing the slab with a trowel machine (assuming you'd want a smooth finish), then cutting control joints that afternoon after we get done + hauling off all excavated soil we don't use for backfilling and creating positive grade away from the slab.
I would typically charge about $17,500 for that job, which is about $19.50/sf, lock & key with all materials, equipment, labor, engineering, compaction testing & permit included. Note that in Alabama, everything is cheaper and you could spend twice that and more elsewhere, and in some areas around here, depending on soil conditions, you would need a 5' compacted pad (about $15,000 for that size area is what I would charge, installed) or a mud sill exterior footing, which is usually a trench about 18"W x 36"D filled with concrete and no reinforcement, then you add the 18"W x 18"D reinforced footing on top of that, and I would charge an additional $3,500 to add the mud sill. A post-tensioned slab would be expensive also. I buy concrete for $78/sy with fibers & 10% sales tax included. My block work usually costs me about $3.50 per block for all materials & labor. To do a compacted pad like that (38' x 42' - remember you have to go at least
5' outside the footprint) would cost me about $875 for 5 loads of "select fill" + $100 diesel fuel in my equipment + labor for 2 operators, and I'd place it in 3 "lifts" of 6" each and compact at each lift to insure compaction. I usually sub out minor dirt work to my slab sub-contractor only because he has a good backhoe operator and I own the backhoe, so I only pay him enough to cover the labor for the operator and a stick man using the transit, with me building the pad myself & hiring a block mason to do the block work & calling my pest control service for the termite spraying. My slab sub costs me anywhere from $1.70/sf to $3/sf for labor only depending on the size of the slab with him doing the excavation of exterior footings & backfill the slab after the block is laid. This doesn't include plumbing or electrical rough-in (floor plugs, conduit, etc) as you would typically contract that work separately with the plumber & electrician. My slab guy inserts the grounding electrode, which is nothing more than a full
20' piece of rebar that runs into the slab 18' and sticks out 2' at the place where the electrical panel (a/k/a breaker box) will be and paints it green as the inspector requires so he knows what/where it is.
There are a lot of variables to all this --- the virgin soil may be weak or expansive, ground water may be high, a heavy sloping terrain, a lot of trees which have to be removed root & all --- any trees whose canopy may eventually hang over the footprint have to be removed as that means their roots may eventually invade and upset the slab --- hardwoods are bad about this and can reach twice as far as their canopy sometimes, then there's the possibility the dirt company brings you soil that doesn't meet engineering guidelines, and unless you know how to spot it (proof-rolling by the dump truck after compacting a lift is a great, low-tech field-measuring device), weather is a strong factor, you have to allow for curing of the concrete & block mortar (3 to 5 days on mortar depending on weather and size of wall) and a lot more. Most subs are lazy and can't be trusted to do it right and many contractors are just brokers out to make a buck, and if you don't know what's right, you could be in for a problem. Make sure they work the edges and smooth them out, especially if you use a block stem wall as the framing will be easily and will be much more level & sound if the substrate (in this case, the edges of the slab) are as smooth & level as possible.
In other words, use this as primer and what to watch for, then hire a contractor to do it all, then watch and learn. Make sure you tell them what specs you want when they give you a bid and give them a copy of the soil report also. END MESSAGE