Subgrade Prep for Cement Floors: Sand or Gravel?

The Basics of Subgrades

• Regardless of what type of concrete slab you are pouring or where in the world you live, concrete slabs require a base layer of compacted soil, gravel and sand to help cushion the concrete. These materials help the concrete breathe as well as expand in the warmer months and shrink in the cooler months. They also provide a base so the concrete doesn’t crack or sink into depressions.

Gravel

• Since concrete needs room to breathe to evaporate moisture, a layer of gravel underneath provides the concrete with a form of circulation from underneath as well as the sides and the surface. Gravel also helps water drain from the soil rather than build up and freeze during the winter months. As water turns into ice, it expands and can push concrete upward and outward if sufficient drainage is not provided.

Sand

• Sand is the cushion layer for a concrete slab. It helps support the cement as the cement settles over the years. When compacted properly with a combination of soil and gravel, sand can support a concrete slab indefinitely. Without a base layer of compacted sand, the concrete slab will drain into the gravel and settle unevenly across the installation area, leaving behind depressions and cracked areas in the slab.

Combination

• The perfect combination of sand, soil and cement is compacted so that the cement rests on top of the sand, which is on top of the gravel layer, which is on top of the soil. With the combination of the cushion layer, the drainage layer and the support of the soil below, your slab will survive the years. Too little sand or cement will result in failure of the slab in months -- or even weeks in extreme cases.