The vast majority of homes have one of three types of foundations. Today, the most common foundation is a concrete slab on grade foundation. Such foundations are variously referred to as floating slabs, slab on grade, conventional slabs, and post tension slabs. In each case, the foundation consists of a sheet of concrete that is poured on the ground. To shape the foundation, form boards are placed around the edges and at appropriate interior locations. Such slabs are reinforced with steel bars (conventional slabs) or with post tension cables (post tension slabs).
Over time, the design of slab foundations has changed substantially. Some early slabs had little or no reinforcing. Some homes built in the 1950’s are reinforced with welded wire mesh. Such reinforcement can not meaningfully strengthen a slab. During the 1950’s, 1960’s, and early 1970’s, most slabs were reinforced with steel bars. During that period, some foundations had few interior beams. (Beams, or grade beams, are ridges on the bottom of your slab, like the ridges on a waffle.) Starting in the mid-70’s, builders began switching from steel bars to post tension cables to reinforce slabs. Today, we estimate that in Texas, over 90% of new homes are built on slabs reinforced with post tension cables. Over the last 15 years, in response to increasing problems with foundation failures, the number of grade beams in typical foundation has steadily increased. In addition, the thickness or depth of the average beam has increased.
Prior to the 1950’s, the most common type of foundation was pier and beam. Pier and beam foundations have a masonry beam under the perimeter walls of a home. Typically, the beam extends 1 to 3 feet into the ground. The interior portions of homes with pier and beam foundations rest on a grid of beams and joists, which are in turn supported by piers that typically rest on the surface of the ground. Today, pier and beam homes are typically built when the designer is trying to minimize the amount of contact between the foundation and underlying soils.
In the 19th century, and in the early 20th century for less costly homes, the dominant form of foundation was block and base. Block and base homes rest on piers, often made of blocks of wood or stone that sit on a base pad (which explains the name block and base), which support a grid of wooden beams and joists that underlie the floors of a home. Few block and base foundations are built today. Mobile homes that rest on blocks are a type of block and base foundation.