Are Wood Posts in Concrete a Bad Idea?

Underlying Principles

• Soil holds moisture and usually has moisture in it to varying degrees depending on weather conditions. Concrete is inclined to absorb moisture but may not absorb moisture at the same rate as soil. Wood tends to expand when it comes into contact with moisture. When expanding wood is encased in concrete that has been exposed to water, the concrete is what is likely to break. Additionally, water, when it freezes, expands, and the pressure of ice between concrete and post in a freeze-and-thaw climate can break the concrete. To complicate matters, concrete is fundamentally alkaline in composition. Alkalinity combined with moisture may increase the wood post's rate of decay. When wood has been treated with a preservative, the treatment is likely to help slow decay but not necessarily stop it completely.

Common Design Flaw

• Problems can develop if a fence post is set with its base fully surrounded by concrete. In effect, the post is then lodged in a concrete bowl. Water penetrates the wood post and runs downward long the wood. The water, with nowhere else to go, collects inside the concrete, and the wood steeps in the water. Rot can set into the wood quickly. The wood develops weakness far sooner than it would have if it had not been steeped in water. The post, now weakened, is inclined to break and fall during heavy winds. A series of posts all set at the same time under these conditions are likely to break at or near the same time, a potentially costly turn of events for the homeowner.

Potential Remedy

• A potential remedy is created in two parts. First, the post hole can be dug deeper than is needed to seat it and filled partway with gravel to promote drainage. The concrete can be poured only around the sides of the post. The post will then be permitted to extend through and beyond the concrete rather than being encased by the concrete. When the post is tamped down into the hole, it will extend into the gravel. Water will have no opportunity to become trapped. It will simply drain away into the gravel nested in the deeper hole.

Another Consideration

• In some climates, after about a year has passed, a post may shrink away slightly from its concrete socket. One way to preserve the wood is to apply a water-based wood preservative where the wood adjoins the concrete. When the preservative trickles between the wood and the concrete, it finds its way underground and helps extend the viable life of the wood beneath the surface.