The Most Common Misconceptions About Slab-On-Ground Foundations

The average person knows a lot of things about slab foundations. Unfortunately, much of what they know is not true. This section addresses the most important of these misconceptions. Misconceptions are important because they inevitably result in unrealistic expectations about how slab foundations function.

Slab foundations are constructed level

This is not even remotely true. The American Concrete Institute (ACI) publishes levelness tolerances for slab-on-ground foundations. Although this is a slight simplification, the ACI says that the elevation difference between any two points anywhere on the slab can be as much as 1.5 inches and be in compliance with the ACI construction tolerances. Two points 10 feet apart are allowed to be as much as 1.25 inches difference in elevation.

To make matters worse, the ACI says that their construction tolerances are about as good as you can reasonably expect.

Also, as a concrete slab cures, it changes shape. The edges typically curl upward. This is inherent in the material; it happens even with what are called super-flat slabs.

Houses that have drywall or brick veneer cracks need foundation repair

This is wrong on numerous levels. First of all, drywall and brick veneer can fracture for reasons that have nothing to do with foundation movement. Second, neither are structural in the sense that they hold the house up or transmit any significant loads. Third, they are easy to repair without underpinning the foundation. Fourth, there is no guarantee that the foundation will be more level than it was before the slab was “leveled.”

If a slab foundation is out of level by some amount, the foundation should be re-leveled

Here is a direct quote from an Army Corps of Engineers manual titled Foundations In Expansive Soils:

Most slab foundations that experience some distress are not damaged sufficiently to warrant repairs.

Notice that there is no mention of levelness or even elevation measurements of any kind. The Army Corps of Engineers key publication that addresses slab foundation repair uses distress to indicate whether there is sufficient damage due to a foundation to warrant repairing the foundation.

A properly repaired foundation will be level and stable

This is not even remotely true. Between the two of us, we have read thousands of repair company warranties, sales proposals, and contracts. With over 40 years experience not once have I seen repair company document of any kind promise, much less guarantee, to make a slab foundation level within any specific tolerance.

With regard to stability, it is not hard to understand how lifting a slab to make it more level could make it less, not more, stable. Consider what happens when you lift the slab at, say the front right corner. The grade beam would necessarily be lifted up resulting it to lift above the supporting soil. The resulting gap between the bottom of the grade beam the bottom of the trench is now a water trap. The clay soil is now more likely to absorb water causing the foundation to be less stable than it was before.


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