I am considering pouring a slab that will be about 850 sq ft. to build a shop on.

I will do all the dirt work myself.

Here are my questions:

1.) How thick does the slab need to be?
2.) How much/what type of rebar or reinforcement do I need?
3.) Ball park on a price for the concrete (not labor)?
4.) What PSI should the concrete be?

I am in Denton, if that matters.

What type of shop? woodworking, metal working, car repair

I will be doing a bit of all three. No heavy machinery other than vehicles.

Depending on where you live, the county permitting office may have minimum requirements for the thickness and a perimeter grade beam unless you have PE stamped foundation dwgs. My brother just went through this for his shop, and I put together a dwg for his shop and a coworker PE stamped it. The permitting office saw the PE stamp and gave him his permit. When he had looked into it, they wanted a specific pad thickness and a 1' deep grade beam around the perimeter. This was in Victoria county.

This fdn was 1500 sq ft, it was 6" deep with #4 reinforcing spaced at 1'-0" centers with 2" cover from the top of the concrete. Perimeter grade beam was 2'-0" deep by 1'-0" wide and was wider at the columns for the anchor bolts.

4" thick, #4 @ 12" centers each way for the main slab, centered in the slab (preferably with bottom of bottom bar at center of slab)

I would strongly recommend a continuous thickened edge and a couple of grade beams at a bare minimum, but it is highly dependent on the type of building you will put over it as well.

The building will just be a standard metal building. Not sure which company I am going to use yet, but nothing out of the ordinary.

Where should I put the grade beams? How deep?

I would also thicken areas where eventually you might want to install like a 2 post lift, etc.

Yeah I had already thought about doing a couple of footings in the spot where a lift might one day go.

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The building will just be a standard metal building. Not sure which company I am going to use yet, but nothing out of the ordinary.

Where should I put the grade beams? How deep?

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Your building supplier might be able to help you with that after you select one.

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I am considering pouring a slab that will be about 850 sq ft. to build a shop on.

I will do all the dirt work myself.

Here are my questions:

1.) How thick does the slab need to be?
2.) How much/what type of rebar or reinforcement do I need?
3.) Ball park on a price for the concrete (not labor)?
4.) What PSI should the concrete be?

I am in Denton, if that matters.

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1. 4" is fine, 6" if you're overly paranoid.
2. #4s on 12" centers
3. Typically, I figure $1000/truck load(10 cu. yards) so $1050 for a 4" slab. 6" slab would be $1575.
4. 3600 psi. It's not that much more than 3000.

Honestly, one of the best things you can do is to make sure that the guys who do the concrete put chairs on the rebar. I've seen too many cracked slabs torn out that have had rebar sunk to the bottom because the contractor tried to hold the rebar in place while it was being poured.

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I am considering pouring a slab that will be about 850 sq ft. to build a shop on.

I will do all the dirt work myself.

Here are my questions:

1.) How thick does the slab need to be?
2.) How much/what type of rebar or reinforcement do I need?
3.) Ball park on a price for the concrete (not labor)?
4.) What PSI should the concrete be?

I am in Denton, if that matters.

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1. 4" is fine, 6" if you're overly paranoid.
2. #4s on 12" centers
3. Typically, I figure $1000/truck load(10 cu. yards) so $1050 for a 4" slab. 6" slab would be $1575.
4. 3600 psi. It's not that much more than 3000.

Honestly, one of the best things you can do is to make sure that the guys who do the concrete put chairs on the rebar. I've seen too many cracked slabs torn out that have had rebar sunk to the bottom because the contractor tried to hold the rebar in place while it was being poured.

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You are way off on concrete cost

Yep I have to second the concrete cost comment.

Also, if you are going to have any lift or floor mounted jib, now is the time to pour that foundation. Cutting out concrete sucks, speaking from firsthand experience.

Enlighten me then.

Surely I'm not low?

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Enlighten me then.

Surely I'm not low?

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You might be close if he just does flatwork with no beams dug

So three people think his concrete cost is off but no one is offering up their estimate.

That would be helpful to op and others.

$125 a yard

Pricing I got recently to do my deck, tie in foundation, was about $140 per yard. Sorry, had to grab the estimate off my laptop, didn't have it at my fingertips last night. This is for work being done in Houston, so maybe concrete work is cheaper in Denton.

As a weekend warrior with concrete, you are looking at the $125-$135 per yard delivered cost.

Napkin math for a 29'x29'(841sqft slab)

For a 4" slab only you'd be close to 10 yds of concrete.

Add a 12"x12" grade beam around the perimeter you get an additional 4.5yds.

Fwiw when we design foundations for homes around Victoria we spec 36" deep beans. Fairly nasty clay around there.

Great thread with good information. Timely for me as well. Just demolished a 20' X 24' building in the back yard and wish to replace it with a 24' X 32' garage/workshop more or less what the OP is describing. The current foundation is actually 5 different slab sections from at least 2 different additions to the original 8' X 12' storage shed. In other words, it's absolute crap to build on so I'm going to start over and get some more space in the process.

I'm posting to ask how best to educate myself on how to DIY a foundation/slab. I don't have a construction background, but I am an engineer (thus believe that I can do anything ). Any links or books would be greatly appreciated. I'm in Dallas so any thoughts on dealing with the permit trolls would be appreciated as well.


OT - I'm planning to recycle pieces from the foundation demo to make a broken concrete patio , so any advice on that would be appreciated.

Edit to add denoting sarcasm

We had ours poured professionally, but I'd strongly suggest putting a 6" slab in a workshop.

Given the labor/etc involved, no reason to not have the additional peace of mind.

This matches my most recent project, admittedly several months back.

also, I was late, but just cleaned that up on the other job site and invite you to do the same if you have any references there.

2 # 5s T&B w/ #3 stirrup @ 18"O.C in edge beam
#4s @ 12"O.C. Each way in slab, Plastic stirrup or concrete brick to hold slab bars off the bottom.
90 degree # 4 hooks from slab bars to #5 longitudinal bars in edge beam
Plastic vapor barrier on top of gravel course under slab.
3500 psi Concrete

I'd recommend finding out what code's requirements you need to meet for a permit (I would guess IBC, but who knows). Once you figure that out, familiarize yourself with the applicable portions of your code. Or you could talk to a structural engineer who works on something similar just to get an idea of what you should be looking for.

6" Slab? Holy overkill batman!

While I'm all about overkill, 6" is way overkill for a typical shop foundation slab. Don't get me wrong, when I save up enough for a shop, it's going to likely have a 6" (maybe 8") thick mat slab, but I know full well that is in excess of anything normal.

Consider this - your driveway is 4" thick, and it gets regular use of vehicles weighing anywhere from 3000 pounds up to 7000+ pounds. Unless you are planning on putting fairly heavy machinery in your shop (large metal press or breaks), 4" with #4 reinforcing is more than enough.

I'm also not particularly fond of grade beams and prefer mat slabs instead. Easier to put in, less apt to have negative or detrimental effects from subsurface soil shrinking and swelling (which, and let's face it, nobody actually does good subgrade prep on anything in the residential category) and overall stronger IMO (not an engineer here, so I may be off on that last one).

But, your most used designs implement continuous exterior grade beams with usually a few crossing beams for additional support with a thinner slab on top, and that's likely going to get you the best overall price to dig, form, reinforce and place versus something not usually found in residential construction.

OP - don't overlook subgrade prep. Pay an engineer a few hundred bucks to come out and give you an assessment of your existing conditions, how much you really should overexcavate and what type of engineered fill you should bring in to actually put your foundation on. A few grand up front to get the subsurface conditions right will pay for itself many times over in the long run. The overwhelming majority of foundation issues on houses and light structures stems from craptastic subgrade prep and general poor placement practices of concrete. Eliminate those and you'll thank yourself 15 years from now when you aren't putting piers in under half your shop to get it close to level again.

I am in the process of forming up my foundation right now for my shop. I will be going with a 30' x 50' Mueller building.

Here is what I have figured for my foundation:
Grade beams - 16" deep and 12" wide they go around the perimeter as well as one down the length and two through the width
Slab - I figured on a 5" thick slab knowing it will be in the 5 1/2" range after its all said and done
Rebar - Grade Beams 2ea #5's top and bottom with #3 hoops at 16" on center
Slab #3 at 12" on center each way

I do heavy civil/concrete work for a living (build treatment plants like schmbella99) and honestly I have had to restrain from going overboard. My cost to purchase concrete in the Brazos Valley is $91cy granted that's the friend price. I bid work all over the state and concrete averages around $100-130cy just to buy it. I am cheap and will be doing all of the work myself except for placing a finishing.

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A shop already? I dislike you.

Thanks for all of the advice.

I am going to figure out what I have to do permit wise and go from there. I really feel like 4" will be more than enough. Everything going on this slab will at some point in time be on the driveway for extended periods of time, and that slab has held up just fine.

Seems like anything else, the prep work is key. I will focus on learning more about that.

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Seems like anything else, the prep work is key. I will focus on learning more about that.

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This, this, this, this...and this.

Learn and ask about your subgrade prep. Don't go by code, because it won't do you any good. A few hundred spent with a good civil engineer and a few hundred extra getting the contractor (or yourself) to have a quality subsurface foundation will be a life saver.

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I'm also not particularly fond of grade beams and prefer mat slabs instead. Easier to put in, less apt to have negative or detrimental effects from subsurface soil shrinking and swelling (which, and let's face it, nobody actually does good subgrade prep on anything in the residential category) and overall stronger IMO (not an engineer here, so I may be off on that last one).

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Learning a lot from this thread, thanks to all posters. Never heard of mat slabs, but that comment about soil shrinking and swelling really got my attention. We have very mobile clay soil in this part of Dallas (bordering Mesquite) that has given us some real problems with the drought/rain/drought cycle we're currently experiencing. There's not a room in our house that doesn't have sheetrock cracks from the clay expanding & contracting.

Also have a very large sycamore tree that might not like it's roots disturbed. Most of the new build would be soil already covered by the existing foundation that will be removed but I don't want to trench through any roots that I don't have to.

Would the mat foundation be a better choice for our area? And given what looks to be some very strong advantages with the mat foundation, why would anybody pour anything else in Texas where there is no frost heave?

I am not an engineer, so all I can give you is an opinion based on my experience doing heavy civil/municipal construction (basically pilfering off of the multiple designs I have constructed).

A mat slab is uniform thickness and sits on top of a prepared subgrade pad versus the thinner slab with grade beams that is more traditonal. I do not know how true this is across the board, but my dad's PE buddy that designed the slab on our addition at the house I grew up in said that grade beams are there strictly for mass to counter act uplift on residential structures. I am sure they have structural properties as well, but the mass was the primary design consideration. And I am sure one of our resident CE's can clarify.

To me, mat slabs are better simply because they do not give the area for expansive soils to push against like grade beams do. They are also easier to build and form, but generally require more reinforcing and slightly more concrete than more traditional designs do, depending on thickness.

And, quite honestly, almost every single structure I have ever built outside of a handful are of mat slab construction. That tells me something.

Again, all of this is strictly opinion on my part, so the advice is worth what you paid for it.

That does bring up another question:

This slab will be close, within a couple
Of feet, to a large oak tree.

Will the Slab cause problems for the tree? Vice versa?

I believe that if your slab covers much of the area under the tree's canopy, it stands a good chance of killing the tree.

I'll have to look at it. I'd really hate to lose the tree.