Why levelness is a legitimate foundation performance indicator
The reason may surprise you. The levelness of a slab-on-ground foundation has nothing to do with the structural performance of a foundation. Highways are ground-supported like slab-on-ground foundations. Highways are also not level. They have a crown in the middle and they slope downward toward the curb. No one thinks twice about this. The slope downward toward the curb is made so that rainwater will flow to the curb and then to a storm sewer system. The difference is that we are used to sloping roads. Most people are not used to their floors sloping.
From an engineering perspective, we prefer the floors in a house not slope so much that it is noticeable.
Why slab-on-ground foundations are never level
The surface levelness of a slab-on-foundation is determined by one factor and one factor alone: how the concrete is finished. The tools used are hand tools. The workers have a limited amount of time to level out the slab surface before it gets too hard to work the concrete. Studies made by the American Constitute Institute show that about the best you can ask for is a slab surface that is plus or minus 3/4-inch. In addition, there can be a difference in elevation over 10-feet of as much as 1.25 inches. There can also be 1.5 inches difference in elevation between two points on the slab surface.
Once the concrete begins to cure, the edges will curl potentially making the surface less level. So even if the concrete is dead level after finishing it will become out of level due to curling, something we have little control over.
Is it possible to make a slab dead level?
The short answer is: yes. There is a slab called a superflat slab. It is very expensive. I have never heard of anyone using it in a residential application. Here is a video that shows step-by-step how they are constructed. Warning: it is over 40-minutes long.
How important is levelness?
Some people are very sensitive to sloping floors. Others could care less. From the perspective of structural performance, levelness is a non-issue. It is essentially an aesthetic issue analogous to a cosmetic issue.
I always encourage buyers to walk the entire house and make sure they are comfortable with the levelness of the floors. Generally speaking, most people do not notice a floor slope unless it is more than 1%.
Can you sell a house that has a slab-on-ground foundation that is not level?
Of course. Slab foundations are not level. Not ever. They tend to get less level as they age. Houses with foundations that are not as level as we would like are bought and sold every day. Can it affect the price of the house? I assume in some situations that may be the case, but it an issue better addressed by an appraiser or real estate agent.
How levelness is misused
There are Professional Engineers and TREC inspectors who regularly recommend repair because a foundation is not level. Please read the following quote from a TBPE Policy Advisory. Now go back and read the sentence that is in bold. If a Texas PE recommends repair based solely on elevation measurements, in my opinion, he or she is in effect using a single box “checklist” report.
While the Board may use this policy statement as a tool to evaluate specific complaints, this statement is not intended to replace professional engineering judgment. This statement is intended to emphasize the professional judgment requirements of Board Rules 22 TAC 131.151 155, not to replace or modify them in any way. Under no circumstances should a professional engineer use this statement as a “checklist” of activities needed to adequately perform air engineering assignment related to residential foundations. In its evaluations of complaints, the Board has consistently been most concerned that the intent of the Board rules of conduct and ethics arc followed and that the public and client interests are well served. This statement is designed to underscore that concern.