How far out of level is a slab foundation allowed to be?
Why there is no simple answer every Houston Foundation Engineer agrees with
There is no answer to this question that universally accepted. The Post-Tensioning Institute has published a peer-reviewed paper in which it is stated that a diagnosis of excessive expansive soil movement cannot be made unless the slab surface is out of level substantially in excess of published American Concrete Institute (ACI) standardized construction levelness tolerances for slab-on-ground foundation construction. The ACI publishes several different construction tolerances but recommends the use of what are called F-numbers. The F-number system allows the elevation of two points 10-feet apart to be different by as much as 1.25 inches. If a foundation were to deflect L/360 in both directions (which most engineers would consider acceptable), the resulting slope (adding an as-constructed slope to the slope caused by foundation distortion) could result in a foundation surface slope of 1.65 inches or more over 10-feet. A slope greater than 1% (1.2 inches over 10-feet) is noticeable by most people. Thus, a noticeable floor slope may or may not indicate excessive foundation movement. You should also understand that the as-constructed slope and the slope due to foundation movement may not add together; the foundation may distort in a way that makes the slab surface more level, not less level.
How foundation engineers use countertops to judge foundation levelness
Some engineers prefer to judge the levelness of the foundation due to distortion by looking, not at the levelness of the slab surface, but at the levelness of first floor counter-tops and window stools since these elements are normally constructed to much tighter levelness tolerances than slab-on-ground foundation surface tolerances. If the countertops and stools are reasonably level within normal construction tolerances, then it is reasonable that any floor out-of-levelness is probably due to original construction.
