Garage foundations can be slab-on-grade, shallow foundations or deep foundations with footings. If the foundation has footings, they must extend below the frost line. If it is slab-on-grade, the foundation must be thicker at the edges to support the weight of the structure, and it must be frost-protected if the climate is cold. In all cases, foundations must satisfy local building regulations, which take into account local climate and soil conditions.
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Slab-on-grade, no frost
The easiest and least expensive foundation for garages is built in climates where the ground doesn't freeze. It is poured on a bed of gravel with a deeper trench around the edges for extra thickness. It consists of a single layer of concrete with reinforcing rods inserted around the thicker edges for extra strength. Pouring over a wire mesh increases resistance to cracking.
Slab-on-grade, with frost
Frost in soil below a slab makes the soil rise and fall unpredictably. The frost action makes the slab unstable and it may crack. It is possible to protect a slab-on-grade foundation from frost for a heated garage by insulating the foundation, and keeping the soil under the garage warm. The builder extends the gravel bed under the foundation several feet beyond the edges of the garage and places rigid polystyrene insulation sheets horizontally over the gravel and vertically against the base of the foundation. This prevents frost from penetrating the ground under the foundation.
Shallow foundations with footings
In areas with heavy frost, foundations must be supported by footings that extend below the frost line. The footings consist of 60 cm (2 foot) wide, 25 cm (10 inch) high concrete at a depth of 90 cm to 1.8 m (3 to 6 feet). 20 cm (8 inch) thick concrete walls reach from the footings to just above grade, and support the structure of the garage. Bolts sunk into the top of the concrete hold the base of the frame in place. Local planning requirements may specify extra thickness, strength or reinforcement, based on the particular local conditions.
In some areas, soil conditions, sloping land or periodic flooding will make it necessary to install a deep foundation. Deep foundations penetrate weak soil layers to support the garage on a deeper, firmer layer or on bedrock. Drilled shafts, piles or piers made of steel, wood or concrete reach down to the deep layer and hold the garage structure in place. The design of such foundations is complex and usually requires the services of a structural engineer.
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