Your garage floor can be the ground (without a concrete floor) or a concrete slab

We've been in business a long time. Over the years we've seen a lot of engineered plans come across our desks. This has given us the opportunity to see various concrete and building codes from across the United States.

Based on this information, we have provided some baseline specs for garage floor concrete requirements. However, keep in mind, your local building department may require different specs depending on your unique situation. Please use the concrete information below as a guide, not gospel.

Your garage kit can be anchored on the ground (without a concrete floor) or on a concrete slab. The first thing to note is that if you are in an area that has a frost line, you will need to be approximately 12" under that frost line for your anchoring system to function properly. The below described concrete applications do not account for frost line requirements.

Ground Installation:

When installing your kit directly to bare ground, you will be required to dig a post hole approximately 10” in diameter and 30” deep every four or five feet. This distance depends upon whether you have purchased a 5’ on-center garage or a 4’ on-center garage. This type of mounting system is called a “caisson” (Fig. 1).

30" ground anchors (Fig. 2) are supplied to you with your building kit if you specify "Ground Mount" upon ordering.

Assemble your base rails and lay them in place. Mark where your holes should be dug. (A 16 penny nail poked into the ground through the pre-drilled hole in the base rails works very accurately for this.) Next, use a post hole digger or auger and drill your holes 30” deep. Clean out the holes. You would now lay out your base rails according to the instructions and drop the ground anchors into the pre-drilled holes. Use a 2500psi concrete (pre-mixed bags or mix your own) and fill your holes with the concrete.

You should wait at least seven days for the concrete to properly set before moving on with the assembly of your building.

Concrete Installation:

1) Size the slab: You should make your slab 4” wider than the width of the building and 6” longer that it's length. For instance, on a 20' x 30' garage you would make your slab 20' 4" x 30' 6". This is so you don’t break or crack the edge of the slab when affixing the concrete anchor expansion bolts into your slab.

2) Order the Correct Concrete Mix and Amount: Your floor should be a minimum of 4” thick. Make it 6” thick if you are intending large vehicles such as trucks or larger RVs to be parked on it. The concrete should be a minimum of 2500 psi concrete with fibermesh reinforcement added at the batch plant OR #3 rebar on 24” centers. You should also consider going with a concrete mixture of 4000psi along with the extra two inches of concrete if you are intending more than a normal amount of weight.

Be sure to saw cut expansion joints within an appropriate time after the pour is finished or trowel in the expansion joints while the finishing is occurring.

As far as knowing how much concrete to order, you can generally call the concrete company and give them the specs of your slab. They can tell you how much to order. Also, it's a good idea to have a place to pour any spare concrete when your slab is poured. The driver will want to empty his truck on-site. The cost of a concrete slab to be formed and poured, including expansion joints, would run approximately $2.75 per square foot in our local area for a 4" slab on a level site.

3) Footing Requirements: (International Building Code 2000 or “IBC 2000” requirements)
At the same time you pour the floor, you are going to pour the perimeter footings. This is called a monolithic pour (or monolithic slab). Perimeter footings need to be 12” deep (you can include the 4” of floor as part of the 12”) and 16” wide. The footings will have to have two #4 rebars top and bottom, continuous run around the complete perimeter. In our local area, the footings would cost approximately $11.50 per lineal (running) foot.

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