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. ( firstname.lastname@example.org). 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.
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
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.
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
Marvelous Marv wrote:
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?
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
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)
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
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.
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
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
Bob Morrison, PE, SE
R L Morrison Engineering Co
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