Garage Slab

I currently live overseas and will be returning to Va in January. I want to build a detached garage in my backyard to use as an automotive workshop. I want to build the whole garage myself, but have a contractor prepare and poor the slab. I am thinking about 28' * 32. Could you please tell me ballpark (within 1000 dollars) about how much I would be looking at to poor the concrete slab?
If you have recently built a workshop, I would really be interested in looking at some of your pics if you would be interested in sending them to me. ( snipped-for-privacy@google.com). Also have some additional questions for you. Approximately how much did your workshop cost you to make? What would you have done different? I am considering putting in plumbing and HVAC. Thanks for any advice.
Pat
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komobu wrote:

Hi Pat,
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.
MM
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Marv,
Thanks for taking the time to explain what needs to be done. I was hoping to get it done for 10k but it looks now like 3 or 4 times that because I have to widen the driveway and extend the driveway around the attached garage to the backyard where the work shop will be. I am getting the feeling that the cement work will be 2/3rds the cost of the workshop. Take Care Pat Marvelous Marv wrote:
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Great job on all the details Marv. But could we boil it all down to some rather simple numbers for us laymen? First, answer the question given without all of the major details.
Q? How much does a 28'x32' slab cost? A: 896sf with a 6" depth would cost $n,000. Where n is the actual number for what ever. Now you get into more depth and details as to what all is involved. Anyone have a site where cost of cement by state might be shown?
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Marv gave you a very technical answer so that you would see that there is no way to give a simple answer.
Cement probably stays pretty consistent for price from state to state subject only to transportation from source. Be aware that there are cement shortages going on across the country. Concrete, on the other hand will vary a great deal from city to city. It is most subject to stone and sand availability, the predominant materials in concrete. Where I am, the closest stone is over 100 miles - imagine the fuel surcharges and transportation costs.
Flat work can be priced by the square foot, though the number will change radically across the country based on labor rates, site condition, project volume, and cost/yard variables. My SF number would scare you to death for a 5'x10' walk in the back yard of a "putting green" quality residence. The mobilize and mechanize dollars are just as big or bigger for that little project as opposed to laying down a 50'x100' parking lot.
The original poster asked about a building pad. This is not flatwork. It will require a footing. Footing depth varies by locality and latitude. Here it is 18". I have been told that in the Dakotas it is 48". That is a huge spread in excavation costs. What is happening to the excavated dirt? What reinforcing? Termite treatment? Edge form system? Building and anchor bolt layout?
richard, I'm sorry, but the very question you asked illustrates how much you don't understand about concrete and building.
______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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Thanks for the insight. I was not aware that a mere slab as proposed would be so detailed and involved. On the old "Home Time" tv show they showed building garages several times. To my best recollection, they never discussed the details. As I recall, they just smoothed out an area, tamped it down, and poured the cement over it. I do understand that certain requirements would have to be met in certain areas. So perhaps in those areas, these requirements were minimal. When I lived in a mobile home in Northern Ky, the pads were nearly 24" deep. In Wisconsin, the pads were less than a foot deep. While at another mobile home park, the "pad" was a bed of granite pebble stones, not sure how deep they were.
At my parents house, my dad finally had the driveway rebuilt. Imagine a sloped surface 25 ft long, that rises 12 ft or so above the roadway. I know for a fact he paid the contractor, a friend of his, a mere $3500 for the entire job. While otherwise, it would have cost nearly double that easily. All theydid was pour the cement over the existing ground. Nothing else done before the pour. Other than to smooth it out.
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In a previous post richard wrote...

Richard:
Don't forget that in most jurisdictions a 28x32 garage will require a building permit. Many people forget this, then have to apply retroactively and have retroactive engineering. This can add substantially to the cost when you add penalties and the cost of remedies for structural shortcomings.
I have a couple of these going right now. The owners could have saved themselves several thousands of dollars if they had done the job in the proper order.
--
Bob Morrison, PE, SE
R L Morrison Engineering Co
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