Garage Foundation Types

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 2-foot-wide, 10-inch-high concrete at a depth of 3 to 6 feet. Eight-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 building codes may specify extra thickness, strength or reinforcement, based on the particular local conditions.

Deep Foundations

• 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.