Foundation design plays a key role in minimizing the need for foundation repair. Sadly, many local building codes lag far behind what is feasible to prevent foundation damage. Because of the weak building codes, most houses built this year will not be immune to the various causes of foundation problems. If you begin to see signs of foundation problems, contact us for a free evaluation of your foundation.
To get a more comprehensive understanding of concrete slab design, we recommend the publication Design of slabs on ground by ACI Committee 360, the American Concrete Institute.
Here are a few pointers on design and how design affects the integrity of your foundation.
Whether you are living in an older home or have purchased a new home in Texas, during our free evaluation for homeowners we can walk you through the technical details of your foundation design and design the repair solution that’s best for your structure. If you want to learn more about existing foundation designs in the Dallas Fort Worth area, read on. If your home is under 10 years old, call your builder as the foundation may be warranted by the builder.
If your home was built in the late 50′s, it’s likely that you have a slab foundation design that utilized inadequate concrete reinforcement. Early concrete slab foundations consisted of a four-inch concrete pad with no reinforcing steel. In later construction in the ’50s and early ’60s, the reinforcing steel looks much like fence with four-inch squares. The slab and the whole house were designed to ‘float’ upon the Texas clay soils.
Here’s the flaw with a weak floating slab – as the house ages, localized moisture induces swelling, settling or soil consolidation, which leads to cracks and foundation failure. Without adequate steel reinforcement, concrete is seldom thick enough to support the enormous load of the house and resist the forces of expanding and contracting clay soil. Steel adds flexibility to concrete.
In 1968, the FHA created a number of standards for residential concrete slab foundations to address soil plasticity (sensitivity to moisture), climate changes, and house loading. Note: To date, the FHA has not established “approved foundation repair methods” – beware of advertising claims to that effect.
Slab Foundation Design At A Glance
Whether you are live in an older home or have purchased a new home in Texas, it’s often helpful to understand how your slab was designed – especially if you believe that you have foundation issues. We can walk you through the technical details and design the foundation repair solution that is best for your structure.
The following summaries offer a quick review of the different types of slabs used in current construction: Type I slabs are four inches thick with a perimeter beam 10 inches thick and six inches wide, with no requirement for wire or steel reinforcement.
Type II slabs require welded wire across the four-inch slab, with perimeter beams increased to a depth of 16 inches and a width of eight inches.
Type III slabs require rebar reinforcement in both the four-inch slab and beams, with perimeter beams plus internal beams every 15 feet, beams being 20 inches deep and eight inches wide. A variation to the Type III slab has been the addition of post tensioning cables, and a reduction in beam spacing. To offset the cost of post tension cables in a slab, much of the rebar reinforcement has been eliminated. Starting in the mid-90s, foundation beams are sometimes 36 inches deep and 12 inches wide. Most of the homes built in the DFW area since 1968 are Type III.
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 or at least to a depth of 20-30 feet.
While it is more expensive, Type IV concrete foundation construction is the most effective method for avoiding foundation problems in Texas. It is a high-tech approach to basic pier and beam construction, with the benefit that piers extend deeply to more stable load bearing strata.
Pier and Beam Foundations
Most pre-1960 homes were built using the pier and beam method. A raised wooden sub-floor containing wooden cross members known as beams was supported every six to 10 feet by either a wooden post or a concrete pier. Typically the perimeter of the house was supported by a two to four foot high continuous concrete beam. Often drilled piers were installed to support the beam.
The exterior concrete beam keeps rainwater out from under the pier and beam house. Over time, however, soil builds up around the concrete perimeter, and the under floor interior area may hold rainwater. Following a heavy rain, you may notice that doors and windows become stuck, and that walls develop minor cracks.
If water accumulates under a pier, it will heave. If clay soil becomes saturated, it loses the ability to support one or more piers and the effected piers will sink. Other issues with a pier and beam design include: wood rot in beams of posts, warped beams, broken beams or joists, and termite damage.
Overall, pier and beam houses have proven to be reliable structures, with occasional foundation repair requiring shimming, post replacement, extra beam support, replacing damaged beams, joists and sill plates, or adding additional support.