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Concrete Slab Foundations

What is a Concrete Slab Foundation?

A concrete slab foundation (or slab-on-grade foundation) is constructed by pouring a single layer of concrete on a compacted base of crushed stone or gravel. All of the concrete should be poured at the same time in order to prevent leaks and cracks in the future.

As opposed to a slab-below-grade or crawlspace foundation, there is no space between the ground and the bottom of the house with a slab foundation. As a result, plumbing and other utility lines have to be run through the slab or out of a wall.

A slab foundation may also be called:

- Slab-on-grade foundation
- Slab-on-ground foundation
- Floating slab foundation

Aggregate Base

A layer of gravel or crushed rock should be laid below the concrete slab in most cases. The gravel should be compacted thoroughly to ensure that the slab will be fully supported. An aggregate base helps to prevent the concrete from cracking, shifting, or sinking over time.

The amount of aggregate base will depend on several factors, including the soil and drainage conditions. In most cases, four inches is the minimum required. Certain soils can be problematic for slab foundations. For example, if the soil expands and contracts with changing temperatures, the concrete can become vulnerable to cracking and settling. Additionally, if there is inadequate drainage, a thicker layer of gravel will typically need to be installed. With a thicker layer of gravel, the concrete slab will be able to support heavier loads.

Concrete Slab

A 3000 PSI concrete is the most popular choice for most residential construction (including home foundations). The average thickness of a concrete slab foundation is between four and six inches.

For a floating foundation, the exterior edge of the concrete pad is slightly thicker than the interior area of the slab. This thicker edge serves as a footing, providing additional stability to the structure.

If soil conditions are more challenging, the contractor will need to pour separate concrete footings. The footings (or spread footings) serve as a base for the foundation, providing additional stability to the structure and improving its load-bearing capacity.


There are two ways to reinforce a concrete slab foundation.

1. Concrete slab foundations can be reinforced with wire mesh or rebar. Rebar are reinforcing steel bars that improve the tensile strength of the concrete. Rebar bonds with the concrete during the curing proccess and it helps to control shrinkage.

2. A post-tensioned slab foundation is reinforced with flexible cables called tendons (rather than rigid steel bars). There is no tension on the cables when the concrete is poured. After some time passes, the cables are tensioned, improving the strength of the concrete.


Building codes vary from location to location. Concrete slab foundations are not ideal for all regions of the country. If you live in an area with a high water table, harsh winters, earthquakes, or clay soil, a slab-on-grade foundation may not be the best option.

Slab foundations are particularly popular in California, Texas, and other places without freeze-thaw cycles.

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About the Author

ProMatcher Staff, ProMatcher
Orlando, FL 32803

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