Adequate design of post-tensioned residential slab-on-ground foundations in expansive soils is dependent on understanding the geotechnical and climatic factors inherent to the procedure. A method for design of ground-supported slab foundations was developed by the Post-Tensioning Institute (PTI). The design stiffness factors (edge moisture variation distance, em, and differential movement, ym) are derived in part based on the average Thornthwaite Climatic Index, Im, for the design area. The purpose of this paper is to show, by case study, the importance of changes in water balance as measured by the Im on expansive soil behavior and in particular to the PTI design stiffness parameters, em and ym. Surveyed differential movements of post-tensioned foundations in two expansive clay formations in the North Texas region, where average Im, not including the influence of irrigation, is 0, are provided. Data is presented to illustrate that use of the average Im value results in slab foundations that are under-designed. Recalculation of the published average Im value for the design area is recommended to account for changes in water balance caused by development, for example the increase in Im from landscape watering or extreme dry periods. The case histories show that the PTI values for em and ym, using the recalculated Im are in closer agreement with observed differential movement. The studies also show that PTI ym values derived using average Thornthwaite values can be in error by 100 percent or more.