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Why does my Pier and Beam foundation move?

Why does my  Pier and Beam Foundation move?

Before the use of the conventional slab on grade or floating slabs, the pier and beam foundation was the typical foundation constructed to support a house. Before 1950, the pier and beam foundation was the most economical and practical foundation for homes. With the development and advances in concrete design, contractors and developers started to construct floating slabs as opposed to pier and beam foundations.

The driving force to constructing a slab on grade foundation as opposed to a pier and beam foundation was economics and architecture. Prior 1950, the majority of homes used wood as an exterior finish and for 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 to the finish.

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 from this foundation. A true pier and beam foundation consists of pedestals (piers) embedded into the ground and spaced at an average of 5 to 6’-0” on center with a perimeter concrete wall that acts as a beam. The idea was that the perimeter wall, or beam, enclosed the crawl space (air space) created by the pedestals that supported the wood beams that make up the floor and wall framing. The pier and beam foundation is commonly misunderstood to signify a pier supporting wood beams, hence “pier and beam”. The beam of a true pier and beam foundation is the concrete or masonry wall that is along the perimeter of the house.

The performance of the pier and beam foundation is dependent 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. The cedar is a naturally preserved wood that is able to resist the decay due to moisture for a much longer period of time than other wood species. We recently worked on leveling project for a 100 year old home that was supported on cedar posts and just recently had them replaced.

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 decayed area of the wood become soft and creates a void between the soils and solid internal wood. As a result, the pier settles and so does any wood framing that it supports.

Moisture (water) is the second and most significant cause to foundation issues with a pier and beam foundation. Besides moisture decaying the wood pedestals, the soils that support the wood pier tend to swell up when the amount of water that they are holding 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.  As water is added to the soils the soils begin to swell up and push everything that it supports up as well; to include 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 water drain is broken and 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 swelling the soils in concentrated areas, a floor of a pier and beam foundation typically is 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.