Best Foundation Type?

What type of foundation is the best for my new home?

There are three basic foundation types you’ll find in central Oklahoma, and the “best” type depends on many factors, such as soil type, home type, construction standards, and budget.

• Pier and Grade Beam: This type of foundation is fairly common, and it is referred to in different ways, such as “pier and grade” or “pier and beam”. Its defining characteristic is that it is visible when the home is finished, almost always on the sides, and sometimes on the front. It is an engineered system, meaning it must be drawn and approved by a licensed Professional Engineer (P.E.). It is made up of a steel-reinforced concrete “beam” that sits on top of the ground (anything on top of the ground is referred to as being “on grade” or “on finished grade”, hence the name “grade beam”). Every few feet, the beam is supported by a pier that consists of a vertical column of steel-reinforced concrete set at least 18” into the ground. The main advantage of this foundation type is cost: since the foundation is set up and poured in one step, it requires less labor. Since it sits on top of the ground, it requires less concrete and less brick to cover the exterior walls (the brick sits on top of the grade beam, which is why the foundation is visible after the home is complete). The big disadvantage is realized when this foundation type is used on top of expansive (clay) soil. As the Oklahoma red dirt absorbs water, it expands. The opposite happens when the dirt dries out. Since the top layers of soil are the most susceptible to changes in moisture content, the grade beam is subject to the heaving and settling motion of the soil at the surface. This soil movement can result in vertical and horizontal foundation movement, and in some cases, twisting. While such movement isn’t necessarily an indication of failure, it can be a nuisance when it causes drywall and/or brick cracks.
• Footing and Stem Wall: This is possibly the most common foundation type found in central Oklahoma. It requires no engineering because its design and construction specifications are detailed in the International Residential Code, the guidelines used by the vast majority of cities and counties in Oklahoma and elsewhere. Its big advantage is ease of construction; since it is so widely used, any foundation contractor is well-versed in its installation requirements. It consists of a “footing” (sometimes called a “footer”), which is a steel-reinforced beam set into the ground at the home’s perimeter. Once the concrete in the footing has cured sufficiently, a “stem wall” is formed from plain concrete (non-reinforced) directly on top of the footing. Sometimes the stem wall is tied to the footing with steel reinforcing bars (re-bar), sometimes not.  The home’s exterior walls will be framed on top of the stem wall. The brick will sit just outside the stem wall on top of the footing. The disadvantage of this type of foundation is cost; it consumes more labor, concrete, and brick. It is not as susceptible to soil expansion as the Pier and Grade Beam, but it is not immune.
• Post-Tensioned Slab: This is the least common foundation type found in central Oklahoma, but its use is growing due to the foundation’s robust design and construction. This type of foundation has been used for years in commercial construction, where larger loads and lower tolerance for cracks are common. The post-tensioned slab consists of a footing and floor slab poured as one unit. Inside the slab there are high-strength steel cables running front-to-back and side-to-side through the slab and foundation. Once the concrete has cured for a few days, workers return to the job and pull the ends of the cables using hydraulic rams, putting them under high tension; once tensioned, the ends are allowed to pull on the concrete, compressing the concrete. This makes the foundation and slab become a single, extremely strong unit. This type of foundation is extremely robust, and can tolerate very high amounts of soil movement without showing any signs of deflection. Owners of homes built this way report fewer effects of movement, such as cracks in drywall and brick. One disadvantage of this foundation type is initial cost, although over time the absence of significant movement, or “settling”, probably outweighs the initial cost.

Now, to answer the question I initially posed, “What is the best Foundation Type?”.  The answer is, it depends. You’ll find existing homes with all 3 types of foundations, with the Footing and Stem being the most common. You’ll also find movement and the related cosmetic problems on many homes built with the first 2 types, while you’ll also find plenty of homes with the first 2 types that show no evidence of movement. That depends a great deal on the soil conditions, which can vary widely.

The third type, the Post-Tensioned Slab, is unquestionably the “best” in terms of performance over the long haul. You just need to decide whether it’s worth the extra cost. Some builders might charge extra for this type, while others absorb the extra initial cost, knowing it will pay off in fewer warranty calls and many more happy customers. There is a reason why commercial builders have been using the post-tensioned slab for years; it carries the loads better over a longer period of time, and isn’t subject to the vagaries of soil composition the way the others are.