Before the use of conventional slab-on-grade or floating slabs, the pier and beam foundation was the typical foundation used to support a house. This foundation was the most economical and practical for homes. With advancements in concrete design, contractors and developers began constructing floating slabs rather than pier and beam foundations.
Performance of a Pier and Beam Foundation
To understand how a pier and beam foundation performs, it is important to discuss how the foundation is constructed and what level of performance to expect. A true pier and beam foundation consists of pedestals (piers) embedded in the ground, spaced at an average of 5 to 6 feet on-center with wood beams supporting the floor joists.
The performance of the pier and beam foundation depends on two major factors.
The first is the structural integrity of the wood pier. Since wood decays with moisture, it is common practice to use cedar posts for the piers. Cedar is a naturally preserved wood that is able to resist decay due to moisture for a much longer period of time than other wood species. We recently worked on a leveling project for a 100-year-old home that was supported on cedar posts that had just recently been replaced.
The second factor is drainage. Poorly draining properties are more likely to have foundation issues and movement, regardless of the age of the foundation or the home.
Using a Pier and Beam Foundation
Economics and architecture were the driving forces behind the decision to construct a slab-on-grade foundation rather than a pier and beam foundation. Prior to 1950, the majority of homes used wood for exterior finishes and flooring. Since the framing of wood is considered flexible, the exterior finish and the floors seldom cracked or broke. The performance of the pier and beam foundation was incidentally compatible with the finish.
Common Problems of a Pier and Beam Foundation
Most pier and beam foundations that have leveling issues are a result of the piers decaying and settling as a result of less wood mass. As the wood decays, the rotted area of the wood becomes soft and creates a void between the soil and solid internal wood. As a result, the pier settles and so does any wood framing that it supports.
Moisture (water) is the second-most significant cause of foundation issues for a pier and beam foundation. Besides moisture decaying the wood pedestals, the soils that support the wood pier tend to swell when the amount of water they hold increases. Imagine a sponge has dried out and become stiff. As you add water to the sponge, the sponge begins to swell up and become more flexible. Soils tend to act in the same fashion.
Causes of Problems Affecting Pier and Beam Foundations
This picture exhibits a common issue with pier and beam foundations built from timber or cedar. Yes, the building and the foundation are actually listing over. The main cause of the listing is lateral (side-to-side) instability and deterioration and decay of the wood pier. This particular building did not have significant cracks in the walls or ceiling because the foundation was uniformly failing all at once. A structural engineer is needed in this case to design a new pier that can provide adequate lateral stability to the foundation AND provide better durability. Also, the debris below the building helps harbor rodents and allows water to accumulate near and around the foundation.
Consider the sponge analogy from before: soils tend to act like a sponge in that they swell in the same fashion when in contact with water. When the soil begins to swell up, it pushes around everything it supports- including a pier and beam foundation. It is very common to find a pier and beam foundation that has one corner of the building much lower (drier soils) in elevation (or height) than the area near a water faucet, plumbing, or a broken water drain that is leaking water into the soils. Broken roof gutters also add a substantial amount of water to the soils in concentrated areas along the foundation. With water decaying cedar posts and soil swelling in concentrated areas, the floor of a pier and beam foundation is often left much more uneven than a conventional concrete slab-on-grade. For this reason, stucco and tile finishes that are very sensitive to foundation movement should be avoided if the owner cannot live with the cracks that will result from the performance of a pier and beam foundation.
Moisture problems, which cause soil to swell and shrink, are the primary reason the building code does not allow for a traditional pier and beam foundation.