When you create a foundation for a concrete garage floor you need to take into account what will be stored in it as well as the structure on top of it. The garage will only be as sturdy as the foundation so the way that you construct it is very important.
Because cars are heavy they require that you reinforce the concrete with rebar to make it strong enough. It will also be necessary to add foundation bolts to the concrete. These are the main two differences in making a foundation for cars versus a patio. Here are the instructions to make sure that your foundation is built to be sturdy and solid.
Step 1 - Measure and Mark
Use a measuring tape to find the corners of your foundation. Mark the corners with wood stakes. Use a chalk line to mark where the outer area will be.
Step 2 - Dig Footings
Use a trench digger or a shovel to dig your footings around the perimeter of the slab layout to a depth of 18 inches. The inside portion of the garage will be dug out to 6 inches deep.
Step 3 - Create the Form
Use the 2x10 boards around the perimeter using the corner stakes to help keep the level of the lines. Use 2x6 boards to frame in the interior slab. Place theses 6 inches from the outside boards. You are going to be creating a curb around the outside three walls as well as two feet on either side of the garage door. Use the stakes hammered into the ground to secure the boards with nails.
Step 4 - Add Rebar
Place two lengths of rebar along the bottom of the footing. You need to keep these from touching the dirt so use a concrete brick to keep them above dirt level but below the level of the concrete. Run two more pieces of rebar along the top of the footing. Connect it so that it will be at least three inches from the top of the finished footing.
Step 5 - Add Foundation Bolts
The foundation must be bolted to the structure. The building plans will state how far apart they should be placed. Place the bolts per the design plans.
Step 6 - Add Sand
Add 2 to 3 inches of sand to the slab floor. Wet the sand and rake it out evenly. Place a plastic barrier over the sand to keep moisture out of the garage.
Step 7 - Create A Rebar Grid
Using ½ inch rebar and metal ties, create a rebar grid that is above the ground by at least two inches. The grid should have a spacing of no more than 12 inches apart in each direction.
Step 8 - Pour Concrete
Pour the footings first and allow them to dry. Then remove the forms and pour the remaining slab.
There are a lot of steps in forming a frame for a concrete garage floor. None of the steps can be omitted. Keep these plans in mind as the minimum requirement and check your building plans for any specifics from the architect.