Foundations

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The house's foundation

A house needs a foundation to shoulder its considerable weight, provide a flat and level base for construction and separate wood-based materials from contact with the ground-contact that would otherwise cause rot and allow termite infestation.

Depending upon when and where a house was built, the foundation may be made of stone, brick, preservative-treated lumber, concrete block or poured concrete. By far the most common material for foundations is concrete.

Most houses have a raised, perimeter foundation that supports floors and load-bearing walls. Some are built on a flat, concrete slab that provides both a base for the structure and the bottom floor of the house. Still others, notably vacation homes or small, older houses, rest on a series of concrete piers. Some houses utilize all of these methods for different portions of the house. Houses with perimeter foundations, for example, often have post-and-pier supports beneath a beam that runs under a load-bearing wall along the middle of the house.

The bottom part of a foundation is called a footing (or footer). The footing is generally wider than the foundation wall and is located about 12 inches below the frost line (the average depth at which soil freezes year after year). The footing distributes the house's weight to prevent future settling or movement.

There are three types of conventional concrete foundations: poured concrete, concrete block and post-and-pier. Size and acceptable types are regulated by building codes. A poured-concrete foundation may be either a raised, perimeter foundation, a flat slab or a combination of the two. Houses in warm climates may have a monolithic slab, where footings, foundation and slab are a single, integral unit.

A conventional perimeter foundation, shown above, has a poured concrete wall supported by a poured concrete footing. Both are strengthened by steel reinforcing rods (sometimes called rebar). This type of foundation is used in connection with both raised floors and slabs.

Foundation Footings
Stepped footing can support a concrete block wall. Blocks have nominal dimensions of 8 by 8 by 16 inches (the actual dimensions are actually 3/8 inch smaller than these to allow for mortar joints). They are hollow when laid-up; steel reinforcing bar called rebar is added and the hollows in the blocks are often filled with concrete. They lend themselves to construction where forming concrete is difficult or impractical.

Concrete blocks are also used for standard foundation wall construction. Here they are supported by a concrete footing; both are reinforced with steel rods and the concrete blocks are filled with grout.

A concrete pier, resting on a footing, may be used to help support beams at mid-span. Though some older homes rest entirely on piers, this method has been phased out in favor of stronger foundations.

The foundation is the first piece of a home to be constructed and creates a base for the rest of a home's components. There are three types of foundations that are commonly used in the U.S.: slab, crawlspace, and basement.

Slab Foundation
Slab is a type of foundation consisting of a structural concrete slab poured directly on the grade. No accessible space exists in slab construction. Slab foundations are popular in areas (i.e. the Southern United States) where there is a relatively high water table. (Water table refers to the depth in the soil at which you find water).

Crawlspace Foundation
A crawlspace is an accessible space with limited headroom, typically between the soil and the bottom of the first floor of a home. Crawlspace construction is predominant in areas where there is heavy clay content in the soil.

Basement Foundation
A basement is an accessible space between the soil and the bottom of the first floor of a home. It usually has more headroom than a crawlspace. Basement foundation construction is predominant in cold climates where the foundation needs to be situated below the frost level.

All three foundation types are usually constructed out of concrete, but can also use concrete masonry units or insulated concrete forms.

Concrete Masonry Units (CMUs) are hollow, concrete blocks. To create the foundation wall, mortar is used between blocks to hold them together, forming the wall.

Insulated Concrete Forms (ICFs) are made of rigid foam insulation forms (a system of support assemblies, including mold, hardware, and necessary bracing to hold concrete) into which concrete is poured. Once the concrete has gained its full strength, the outside forms, the inside forms, or both are left in place to insulate the wall. ICFs are common in regions in which the local building code requires the foundation to be insulated. Another benefit is that the homeowner or builder is able to finish basement immediately, without adding studs.