Residential Foundation Design

• How are residential foundations built now and in the past?
• What is the evolution that has lead to today's building practices in the Dallas-Ft Worth area?
• Is new house construction adequate to eliminate the need for foundation repairs and house leveling?

Pre 1960 homes were predominantly pier and beam. A raised wooden sub-floor containing wooden cross members known as beams was supported every 6-10 feet by either a wooden post or a concrete pier. Typically the posts and piers were buried 1-4 feet into the ground. Sometimes the wooden posts rested on a concrete slab that was placed on the surface of the ground. Typically the perimeter of the house was supported by a continuous concrete beam.

A common problem with pier and beam homes is that following a heavy rain doors and windows become stuck, and walls develop minor cracks.

Occasionally, a pier will settle to the extent that it needs re-shimming, a wooden post would rot, or a beam would warp and twist.

Sometimes, the spacing between piers was too far, leading to sagging beams.

To this day pier and beam houses have proven to be reliable structures, with occasional foundation repair requiring shimming, post replacement, extra beam support, replacing rotted beam and sill plates, or adding additional support.

The post WW II building boom lead to a movement to slash construction costs by replacing the wooden floor and beams with a cheaper concrete slab. This technique really gained momentum in the late '50s.

Early concrete slab foundations typically consisted of a four inch concrete pad reinforced with heavy wire and a single concrete reinforcing beam 10-20 inches thick traversing the middle of the house. The theory was that the slab and the whole house would float upon the Texas clay soils. The flaw was that as the house aged, localized moisture induced swelling, or settling and soil compaction lead to cracks and foundation failure. The concrete simply was not thick enough to support the enormous load of the house while resisting the forces of the soil.

Foundation repair and house leveling is sometimes hard to accomplish on slab homes built in the 50s and 60s due to the fragile nature of the poorly reinforced slab.

FHA Standards for residential concrete slab foundations:

Universal design standards began in 1968 as a result of the establishment of a Federal Housing Authority standard for the "Criteria for Selection and Design of Residential Slab on Grade" This standard attempted to encompass soil plasticity (sensitivity to moisture), climate changes, and house loading. Several types of slabs were established. All except Type IV foundations often require foundation repair.

As a word of caution and truth in advertising, the FHA has not established "approved foundation repair methods".

Type I slabs are 4 inches thick with a perimeter beam 10 inches thick and 6 inches wide, with no requirement for wire or steel reinforcement.

Type II slabs require welded wire across the 4 inch slab, with perimeter beams increased to a depth of 16 inches and a width of 8 inches.

Type III slabs require rebar reinforcement in both the 4 inch slab and beams, with perimeter beams plus internal beams every 15 feet, beams being 20 inches deep and 8 inches wide. A variation to the Type III slab has been the addition of post tensioning cables.

Dallas-Ft Worth houses since '68 have typically been built to the Type III standard. The need for foundation repair in Type III construction arises with homes as little as 18 months old.

Type IV slabs are the new standard of excellence for DFW soil conditions. The slab is elevated by concrete beams supported by piers extending to load bearing strata.

While the most expensive, and prohibitive for many buyers, Type IV concrete foundation construction is the most effective method for avoiding foundation problems. It is the high tech throw back to the old pier and beam construction, with the benefit that piers extend deeply to more stable load bearing strata. Today less than 1% of the homes being built utilize Type IV construction methods.