A system interaction analysis of the relationship between expansive soil damage to slab-on-grade foundations and the surrounding environmental parameters (soil properties, topography, vegetation, climate, etc.) shows that the systems interact in a complex manner. Variation of any environmental parameter can completely change the interactions. The system interaction determination was based on a statistical analysis of the damage-environmental parameter relationships of over 400 brick veneer, single family homes located in 3 cities in Texas. Study sites were selected for variable environments in an established neighborhood. In Beaumont (humid) the system interaction model related damage to the antecedent rainfall ratio, average distance of trees from the home, effective plasticity index, quality of lot drainage, post-construction vegetation, and the age of the structure. In San Antonio (semi-arid) a complex system interaction model related damage to the depth of the active zone, yard maintenance, topography, and the antecedent rainfall ratio. The most significant environmental parameters were the depth of the active zone and yard maintenance. In Waco (dry subhumid) the system interaction model related topography, average plasticity index, age, number of trees, post-construction vegetation, and average distance of the trees from the house to the severity of damage. The Waco results showed that variations in vegetation play a significant role in the damage process. The topographic variation in Waco (up to 30% slope) suggest an optimum slope to minimize damage. Too flat a slope limits lot drainage while too steep a slope results in downhill creep deformation of the soil. The results of this study plus one carried out in College Station (moist subhumid) show that the system interactions are site specific and suggest that each site must be evaluated individually.