Why Is a Concrete Slab Not a Good Foundation for a House?

Soil and Foundation

• Soil is the main factor to consider for foundation design. Soils are subject to movement from freeze and thaw activity and when they have high moisture content. The movement can cause concrete slabs to settle at different rates, resulting in cracks and other structural damage. Footings distribute the weight of the house and transfer it to the soil; ultimately the soil bears the load. A concrete slab is an adequate foundation where soils are strong and temperatures don't freeze.

Slab-on-Grade Foundation

• Where a slab foundation suits the conditions, a single layer of concrete at least 4 inches thick is constructed. The slab is thickened at the edges as an integral footing; the edge is reinforced with steel bars for added strength, and welded wire mesh reinforces the body of the slab. The concrete for the slab can be poured all at the same time, instead of two or more pours as normally required for footing and foundation structures. This simplifies construction scheduling and is cost effective.

Insulated Foundations

• The slab-on-grade structure is suitable where the ground doesn't freeze. In cold climates, slab-on-grade foundations can be improved with insulation, if the house is heated. One sheet of rigid polystyrene insulation is placed on the outside of the foundation wall, and another is placed flat on a bed of gravel outside the slab. The insulation prevents heat loss from the slab and keeps the ground around it from freezing. Another insulated method is constructing a "floating" slab with interior foam board beneath it.

Raft Slab-on-Grade Foundations

• Raft slab-on-grade structures strengthen the slab design with the addition of ribs, constructed by trenching the ground to allow thicker concrete at regular intervals both ways to support the slab, creating a grid or waffle appearance. The ribs are reinforced with steel. This method is more economical than thickening and reinforcing the whole slab, but achieves the design objective. It becomes more expensive if soil conditions or heavy loads call for especially deep ribs and heavy reinforcement, and controlling cracking is difficult.