What Causes a Basement to Be Uneven?

Soil Rebound

• One cause for a shifting foundation is the elasticity of clay soil. Soil compresses tightly under pressure and expands dramatically when that pressure is withdrawn. When contractors excavate for new basement construction, they remove a large weight of dirt which had exerted significant pressure on the soil levels below. With that weight removed, the remaining clay soil experiences a significant reduction in pressure and begins to swell. This process is called "soil rebound." Because the weight of the new basement, even with the house above it, is less than that of the removed mass of soil, the clay beneath the foundation continues to expand even after construction is complete. Thus, soil rebound can shift foundations and warp basement floors for years to come.

Effects of Moisture

• Soil also expands as it takes on moisture. Correspondingly, it shrinks as it dries out. This means that changes in soil moisture are a common cause of the shifting foundations that cause uneven floors. Excavating for a basement exposes the soil to rapid evaporation that will continue until the floor slab is placed. It's common for several weeks to elapse between excavation and slab placement, resulting in a huge moisture loss. Placing the floor slab halts further evaporation and allows soil moisture to increase. The resulting swelling of the soil can heave the foundation and damage your house for a long time after construction.

Vegetation

• Trees and bushes in close proximity to the house can exert significant influence over soil moisture levels. Their roots extract a large quantity of moisture from the soil, resulting in soil shrinkage and shifting foundations. You can minimize this effect by only planting trees at a distance from your house equal to or greater than the expected height of the full-grown tree.

Poor Drainage

• Various construction factors can result in water being channeled right up against the house, soaking the soil beside and under the foundation and causing it to expand. To avoid this, ensure that soil slopes away from your house rather than toward it. Extend gutters and down spouts so that water is directed at least three feet away from the house.