How to lay a concrete garage floor

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If you're building a garage and want to complete the job yourself, an experienced homeowner with DIY skills can lay a concrete garage floor. It is a big project, but if you buy ready-mixed concrete that is delivered and poured by the concrete company, the rest of the job is straightforward.

Determine the finished size of your proposed garage. Pound stakes in at every corner and every 1.2 metres (4 feet) along the edges of the proposed slab. Run string from stake to stake, marking the perimeter of the concrete slab.

Remove any rocks or debris from the site. Dig out the top 15 cm (6 inches) of soil. Remove any high spots and fill in any low spots. Rake the area smooth and put down a 5 cm (2 inch) layer of aggregate gravel. Remove the strings installed earlier but leave the stakes.

Establish the ground level grade of the site, which will be the top of the concrete slab. Tie strings at ground level and lay a spirit level on them to level. These levelling strings are not the same as the ones used to mark the perimeter. Run the levelling strings from edge to edge across the interior of the area of the slab in both directions to determine the grade level.

Build the edging forms for the pouring process. This is usually 2.5 by 15 cm (1 by 6 inch) timber laid on its side around the perimeter of the proposed concrete slab. The edging forms contain the wet concrete so it hardens to the desired shape and size. Use the levelling strings as a guide and install the edging forms along the perimeter you staked out earlier. To secure the edging forms before pouring the concrete, pound in stakes. Insert the stakes outside of the concrete slab area right next to the edging forms. Securely nail the edging forms to the stakes.

Lay down concrete reinforcing rebar wire mesh. Cut it to fit inside of the edging forms and cover the entire area of the concrete slab with rebar.

As the concrete is poured, tamp it down. Use a concrete roller to compact the concrete, creating a stronger garage floor. Roll the tamper across the wet concrete to compact it.

Level the concrete. Use as many lengths of 5 by 10 cm (2 by 4 inch) timber fastened together as you need to make a screed equal to the widest part of the garage floor. Lay the screed on top of the edging forms and drag it across the surface of the garage floor, removing any concrete that is above the grade level. Work the screed back and forth in a sawing motion and cover every part of the concrete garage floor.

Make the contraction joints. Contraction joints are necessary to allow set concrete to expand and contract with varying temperatures. Form the joints by inserting the 120 by 5 cm (4 foot by 2 inch) plywood in a grid so that the contraction joints are 1.2 metres (4 feet) apart in all directions. Pound a nail into the top edge of one of the plywood sections. Paint both sides of it with motor oil, and use it to push the boards, one by one, into the concrete until only the nails are visible. Use the nails to remove the top board, leaving the other one embedded in the concrete below the surface.

Allow the concrete to harden and cure for seven to 10 days. Cover with plastic to protect it from rain.