State of Practice of Residential Floor Slab Flatness

Forensic engineering investigations of residential structures often include measurements of elevation differences across the slab-on-grade floor as an indicator of movement (settlement or heave) of the underlying soil. Such assessments are predicated on an assumption that the slab was flat or very nearly so at the time of initial placement. Despite the relatively frequent use of such measurements in practice, confirmation of the underlying assumption is scarce in the archival literature. This paper describes a testing program to measure the deviation from level of a large number of residential concrete slab-on-grade floors in Phoenix, Arizona, within a few days of concrete placement. An average maximum elevation change of 13.5 mm was observed. Average and maximum slopes are also presented. The construction of the residential structure atop the slab was found to have no statistically significant effect on the levelness of the slab, nor did the season in which the concrete was placed. Residential slab-on-grade construction methods are reviewed, concluding that this degree of deviation from level is to be expected. Comparison of the data with recommendations in the literature suggests that caution must be taken in evaluation of cosmetic or structural impairment of residential structures based in large part on measurements of interior floor slab levelness.