Are Concrete Slabs Poured on Wood Foundations?

Slab-on-Grade Foundations

• A concrete slab foundation consists of a slab of concrete that is about 4 inches thick, often laid on top of crushed gravel. The concrete is thicker around the edges and reinforced with steel rods to provide footing. Sometimes wire mesh is incorporated into the concrete pour to eliminate crack formation. Concrete slab foundations are not suitable for areas that are prone to freezing, as temperature changes can shift the foundation. For this reason, you will normally not find slab-on-grade foundations in cold, northern regions of the country unless special methods are used. Slab-on-grade foundation is most appropriate for level areas in warm regions with a high water table.

Other Concrete Foundations

• In regions that are prone to freezing, different methods are required for concrete foundations than are seen in slab-on-grade installation. A T-shaped foundation includes concrete footing that reaches down into the soil past the frost line. Walls are added on top of the footing. This method prevents the shifting that can result when soil expands and contracts from freezing weather. A frost-protected concrete foundation offers another option in areas prone to freezing. Insulation is placed along the outside of the foundation wall and on the gravel at the wall's base. This serves to contain the heat in the ground and prevent it from shifting.

Wood Foundations

• Wood foundations are an alternative type of foundation to concrete slabs. In some cases, a concrete slab is used to form the floor of a wood foundation, but often a treated wood floor system is preferred by builders. Wood foundations are light and do not require concrete footing. Some homeowners may be averse to the idea of a wood foundation because they believe wood will decay. But the lumber used for these systems is treated with preservatives and is long-lasting if installed correctly. Wood foundations are commonly found in homes with basements or with crawl spaces under the building. Raised floor foundations consist of a set of wood piers supported with concrete blocks.

Concrete Slab Applications

• A concrete slab is a cheap and easy foundation option with a variety of applications. Slabs are used for sidewalks and driveways and form the foundation of patios, sheds, homes and other structures. To lay a concrete slab, create a level area of gravel or dirt as a base. Use lumber pieces to define the shape. Pour a ready-mix concrete and let set. Once it is dry, remove the wood pieces.