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How to build a concrete foundation for a garage

Updated February 21, 2017

Building and pouring a foundation for a garage allows you build a structure for storage, parking cars or use as a workshop. The foundation must be built level, and reinforced to handle any shifting ground as well as the weight of the structure. A good foundation has a solid footer to support the walls, and is several inches thicker than slab itself. This is a project that is best completed with several assistants to help with the workload and handling the concrete.

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  1. Measure off and mark the four corners of the garage slab. Hammer a stake into the ground to represent each corner of the slab. Tie the construction string between each stake to serve as a guide for the outline of the slab. Tie the string about 12.5 cm (5 inches) above the ground level and use the plumb level on the string to level the string.

  2. Dig out 15 cm (6 inches) of the top soil inside the string marker for the slab. This forms the bed in which the concrete slab will be poured. Use the shovel to remove the soil.

  3. Measure and cut the 5 x 15 cm (2 x 6 inch) boards to outline and serve as form boards for the shape of the slab. Run the boards from corner stake to corner stake, and drive an additional stake in the ground every 90 cm (3 feet) to provide additional support for the boards. Attach the form boards to the stake, by screwing in a 10 cm (4 inch) screw through the form boards into the stakes.

  4. Dig a 30 cm (12 inch) wide and 45 cm (18 inch) deep trench along the inside of the form boards. This trench will form the footer for the garage. Use the shovel and wheelbarrow to dig and remove the dirt.

  5. Place a concrete block in the trench at each corner and every 2.4 m (8 feet) in between. Lay out and cut the rebar to fit as needed down in the trench. Lay the rebar on the concrete blocks to suspend it and allow the concrete to pour around it and cure. The rebar provides strength and helps hold the concrete together. Run two to three lengths of rebar down the trench about 7.5 to 10 cm (3 to 4 inches) apart.

  6. Level the main bed of the slab using the level to detect high and low spots. Use the shovel to fill in the low spots and remove soil from the high spots.

  7. Run the ground compactor over the main bed for the slab to pack in the soil as tight as possible for the concrete as it is poured.

  8. Roll out the concrete fence from one side of the slab to the other and cut it to fit. Repeat this step until the entire area inside the form boards, including laying it over the top of the footer trench, is covered with the fence. Use the wire cutters to cut and trim the fence as needed.

  9. Pour the concrete in the footer trench first and allow that to cure over a couple of days. Pour the concrete in the trench up to the level of the bed for the main slab, and use the float to smooth out the top of the concrete.

  10. Pour the concrete, starting in one corner inside the form boards, filling in the bed for the slab, while working your way to the opposite side of the slab. Use the mason's float to smooth out the top of the concrete and make it level with the top of the 5 x 15 cm (2 x 6 inch) form boards. Allow the concrete to cure several days before allowing any heavy foot traffic or building to continue.

  11. Tip

    Use a concrete with a psi (pounds per square inch) that matches the use of your slab. A concrete slab with 17,236 kPa (2500 psi) is average for a two-car garage. Concrete with the fibre mesh pre-mixed in will add additional strength to the concrete as it cures.


    Avoid prolonged contact of the concrete to any parts of the skin as it may cause irritation or burns. Be sure to check the local planning permission for your area for any special requirements for pouring concrete slabs, and pull any required permits before any building is started.

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Things You'll Need

  • Measuring tape
  • stakes
  • Sledge hammer
  • Construction string
  • Plumb level
  • 5 x 15 cm (2 x 6 inch ) boards
  • Hand saw
  • 1 box of 10 cm (4 inch) wood screws
  • Drill
  • Screwdriver drill bit
  • Shovel
  • Concrete blocks
  • Rebar
  • Hacksaw
  • Wheelbarrow
  • Level
  • Compactor (gas or electric)
  • Roll of concrete fencing
  • Wire cutters
  • Concrete, 17,236 kPa (2500 psi) with fibre mesh
  • Mason's float

About the Author

Laurie Brown has worked as a high school English teacher for the last several years and loves writing. She enjoys helping her students develop a love and appreciation for writing, reading, and literature. Laurie has a degree in education with a major in English. Currently she is a writer for eHow.

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