Garages are usually square or rectangular in shape, so laying out the foundation is very straight forward. Nevertheless, careful and detailed planning is always required because once the slab is poured, it is permanent. Mistakes and/or bad choices can be costly. Also, there are local building codes and practices that must be followed. The garage will only be as strong as its foundation, so it is crucial that the foundation be built correctly.
Determine the area where the garage will be built and stake out the dimensions of the foundation. Make sure the area staked out is square. You can determine this by measuring diagonally from the stake on the bottom right to the top left and from the bottom left to the top right. When the measurements are the same, the area staked out is square, which means the sides are parallel.
Dig out the area that you have staked off for the foundation. Dig up and take away any sod or other ground cover and remove any debris in the area of the garage. Dig down 5 cm (2 inches) to allow for sand and a vapour barrier.
Dig the footings for the foundation around the perimeter. Local planning regulations will dictate the width and depth of the footings, but a typical footing will be at least 30 cm (1 foot) wide and 45 cm (18 inches) deep. The footing adds thickness along the edges of the slab to alleviate stress to what would otherwise be the weakest part of the slab.
Place form boards around the outside parameter of the trench for the footings. Make the form boards 15 cm (6 inches) higher than the height of your slab to form a curb around the edge of the slab on three sides and then 60 cm (2 feet) on each end of the fourth side. Stake down the form boards, placing stakes 60 cm (24 inches) off centre all the way around the parameter. Remember that you are only making the curb 60 cm (2 feet) on each end of the fourth wall. You will need to place boards at the ends of these 60 cm (2 foot) sections to cap off the cement at that point.
Place form boards 15 cm (6 inches) inside the first set of form boards. Stake down the form boards by placing stakes 24 inches off centre all the way around the interior parameter.
Pour or shovel a layer of sand and/or gravel into the footer trench in accordance with the local building regulations. Place the visquine vapour barrier over this layer in accordance with the local building regs. This vapour barrier will stop moisture from seeping into the slab. If it is not required by the local building regulations, it is a good idea to install it anyway.
Place rebar (steel rods) over the visquine vapour barrier, the size of rebar will be dictated by the local building regulations. The rebar adds rigidity to the cement and prevents cracking.
Pour the cement for the footings into the form you have built with the form boards. Once the footings are poured set the bolts or foundation anchors, again in accordance with the local building regulations. These are used to attach the still plates to the foundation after the cement work is completed. Allow the footings to set for a couple of hours, then knock the inside form boards loose and move them out of the way.
Pour or shovel a layer of sand and/or gravel into the slab area in accordance with the local building regulations. Place the visquine vapour barrier over this layer in accordance with the local building specifications.
Place wire mesh over the visquine vapour barrier. The local building regulations will dictate the size or gague of wire mesh to be used.
Pour the cement into the slab area to the required thickness. Smooth the cement and let it set.