Garage Foundation Requirements

Basic Garage Construction Facts

• All residential garage construction in the U.S. must adhere to national standards, such as the International Residential Code (IRC). The most recent set of these standards -- the 2009 IRC -- contains distinctive requirements pertaining to garage construction as well as foundation requirements that are generally similar to those applied to home building. One example of a distinctive requirement is found in Section R309.1, which states that the floor of a garage must be sloped toward the overhead garage door opening to allow liquids to drain out.

Garage Foundation Design

• A garage foundation, like any structural foundation, must be able to support the weight, or load, of the building that rests on it. Basic minimum load-bearing standards for a garage foundation require the width of the foundation footage to be in accord with the capacity of the soil to hold the weight, as found in IRC Table R403.1. A spread footing must be at least 6 inches thick, with stem wall and slab-on-grade foundations requiring the addition of rebar. One-story garages must have a foundation footing that is at least 12 inches wide across the top.

Garage Foundation Depth

• The consideration of load-bearing capacity for a garage foundation also includes the depth of the foundation. One aspect of this requirement, which points to the diversity in application that individual states and counties exercise when adopting national building codes, is the frost line of the soil. Although the minimum required depth of any type of garage foundation is 24 inches, areas that experience extremely cold winters often require a deeper foundation. Michigan, for example, requires that a garage foundation extend 42 inches below finished grade into undisturbed soil.

Detached Garage Foundations

• Although both attached and detached garages must adhere to load-bearing requirements, the square footage of detached garages is often smaller than attached garages. However, slab-on-grade garage floors are still required to be at least 3 1/2 inches thick, with a minimum of 6 inches of clearance between the grade and the bottom of the exterior wall sheathing. It is important to consider that interior wood-framed walls on concrete must be bolted down using 10-inch anchor bolts with a minimum of a 7-inch penetration into the slab when installed on a slab-on-grade garage foundation.