You want to build your dream barndominium on that plot of land you bought several years ago and you’ve got the perfect floor plans. You want a 40×60 foot metal building that will house your 3 bedroom, 2 bath apartment within it. But you’ve got to figure up costs. Starting with the foundation.
How much does a 40×60 concrete slab cost?
It would be about $8,500 to $12,000 for a 40×60 foot concrete slab, 6 inches thick. That’s based on national averages of around $5 per square foot for a fully engineered, finished concrete slab, including labor and materials.
It would be capable of bearing the weight of a 40×60 steel building and any vehicles you might want to store in the shop area, if you have one. It’s also considered an “engineered” foundation if your contractor has utilized someone with the right credentials to apply the science of soil mechanics and rock mechanics to the soil underlying your particular pour.
The objective of that exercise is to make sure your home site will be able to take the weight of the slab and anything you want to put on top of it.
What does that price typically include?
- 3000 PSI concrete using 3/4 inch crushed stone, poured to a depth of 6 inches.
- Fibermesh embedded in the concrete for reinforcement.
- At least one row of rebar around the perimeter for reinforcement, A double row is even better.
- A form surrounding the slab made up of 2×6 inch lumber.
- A workforce of skilled concrete laborers to pour and finish it.
And, in case you were wondering, the difference between concrete and cement foundations is simple.Even though cement and concrete are often used to describe the material making up your slab, cement is actually just an ingredient of concrete. It is the paste that congregates on the very top of the pour.
A true cement pour would be for a much smaller, much more shallow application — like a sidewalk. or a patio. Its texture is much more fine and can tend to crack over time. Not so, concrete.
Is a poured concrete foundation better than block?
A poured foundation is generally considered to be stronger than cinder blocks, which would be inappropriate underneath a large metal building like this in any case. Poured foundations typically don’t need to be waterproofed, either.
Cement can be generally considered to be the thick (or thin) paste that results when, as a powder in a sack, it is combined with water — to be used in grouting cinder blocks, for example.
It’s made from silica and calcium rich materials, like limestone or clay. It makes a powerful bonding agent.
But by itself it tends to crack and become a problem within a fairly short time frame. How many building projects — like sidewalk or patios — have you seen lasting unscathed for longer than 10 years?
Your engineered concrete slab, on the other hand, will likely still be here long after your barndo has fallen in after a century or so of use.
Advantages of a concrete slab
Specifically, concrete slabs:
- offer added strength
- offer a stable footing upon which you can build
- provide reassurance that heavy objects like tractors won’t cause buckling of the surface
- won’t generally be affected by extremely cold weather
Disadvantages of a concrete slab
Concrete slabs can be impacted negatively by:
- Cold weather that lasts a very long time, causing cracking of the surface
Concrete Slab Types
An excellent source of information on concrete slabs called Improvenet.com details the vast variety of slabs you can have put in:
- Regular Concrete
- Self-Consolidating Concrete
- Stamped Concrete
- Glass Concrete
- Pervious Concrete
- Polymer Concrete
- Geopolymer Concrete
- Asphalt Concrete
- High-Strength Concrete
- High-Performance Concrete
- Micro-Reinforced Ultra-Performance Concrete
- Rapid-Strength Concrete
How do I prep the site?
First of all, you must dig up any grass or weeds that would get in the way of a smooth pour. Be methodical by placing stakes all around the perimeter of your slab area. Then clear that area and then some — all the way out six inches or more beyond.
Finally, take away four inches or so of the top soil that remains and level the site up.
Dig the footings
Go about 12 inches down into the soil and make your footing trenches at least 16 inches wide. Just to be sure you’re not going to have a problem later on with weak soil underlying your barndominium, engage the services of someone knowledgeable about such things. They are typically called “geotechnical engineers,” and if you’re having trouble locating one, ask the concrete company. They may even have someone on staff.
Do I need a foundation drain?
Now is definitely the time to worry about — and prepare for — the possibility of water undermining your foundation.
An article written by a man who makes a living by putting in foundations says underground water and runoff from rain are the two biggest causes of structural integrity issues. He adds that good perimeter draining is key.
Brent Anderson, writing on HomeAdvisor Pro’s website, says this about surface runoff:
“Although some wind-driven rain strikes the siding and drains onto the ground, most surface runoff comes from the roof, and the amount of runoff varies according to the size and style of the roof. A gable roof deposits all runoff onto the ground under the eaves, with little runoff at the gable ends; a hip roof distributes the runoff more evenly on all sides.”
More on the foundation drain
Anderson goes on to provide an interesting chart on the necessity for a foundation drain.
— Chart courtesy of HomeAdvisorPro.com. Check out more facts on how to drain water away from your foundation by reading the article for yourself.
Putting the forms in place
Forms — the outer temporary boundary that will keep your concrete foundation in place while being poured and firming up — needs to be made up of 2×6 inch boards, fastened tightly together.
In this way, they will hold the foundation in place despite the intense pressure of the concrete pressing against them.
One pro tip here: when putting the forms in place, leave troughs to capture runoff.
Pouring the concrete
Above, the concrete must have a 3,000 psi rating, and you must use fiber mesh reinforcement.
“Important – plan to use your shop area to store very heavy equipment, you had best bump the psi to 4,000 and add either rebar or wire mesh to help reinforce it.
Then, the folks over at Whirlwind Steel Buildings recommend taking these steps:
- Pour evenly to a depth of 4 inches. Pour 6 inches if heavy vehicles or equipment will be stored on top.
- Fully cover all steel reinforcing bars.
- Eliminate voids and air bubbles.
- Remove any water seeping to the surface.
Putting a good, smooth finish on your concrete slab
To put the finishing touches on, the experts at Whirlwind Steel go on to recommend a process called “screeding” — the process of leveling the floor of the foundation by pushing concrete into areas that are under filled.
Finish the foundation once it has been leveled.
- Use metal or wood floats to compact the concrete and push large aggregates down into the cement.
- Smooth the surface with trowels.
- Compact the foundation for a harder finish.
- Cut expansion joints with a saw soon after the concrete is poured and leveled and trowel the joints in while finishing
Now that the foundation has been poured, you should let it “cure.” That is, you must let it harden sufficiently to walk on it and install the walls.
HINT: This would be the ideal time to let your kids put their handprints down — with the date — to be preserved for posterity.