Because of the expansion and contraction of large concrete slabs, cracks in basement floors are inevitable. Concrete floors and similar surfaces are constructed to direct the cracks along predetermined lines called control joints. These are visible as troweled joints on driveways, sidewalks, and some floors. Basement floors often have hidden control joints instead of troweled joints. Before the concrete cures, the finishers slice through the thickness of the concrete along a grid pattern and trowel lightly over the top of these joints to make a smooth surface. When the cured slab moves, it cracks along these fault lines.
The NAHB guidelines allow a 3/16-in. separation and/or vertical displacement for cracks in basement floors. There are a number of ways to deal with floor cracks that fall within acceptable guidelines. One is to do nothing. If you’re stretching a carpet and pad over the floor, most of the differences and separations will disappear. If you’re laying a thinner covering, such as low-pile carpet or linoleum, cracks and heaves will telegraph through.You’ll need to smooth out the differences with a grinder or a floor leveling compound.
Serious cracks or large vertical displacements in floors are harder to ascribe to a cause and more work to fix. Fortunately, they’re also less common, especially in newer homes. Here in the Midwest, bad floor problems are most often caused by a substrate of clay. The clay expands as it absorbs water and can exert enough pressure to bust up a concrete floor. The only solutions to this problem involve eliminating the source of water or removing the clay. Both are difficult tasks. If you have badly cracked, separated, or heaved floors, consult a professional to see what’s going on and what can be done.