Several changes have been introduced in going from the 2 nd Edition to the 3 rd Edition of the PTI procedure for the design of slabs-on-grade on expansive soils. The changes in the design procedure, when applied to desert regions, tend to result in doubling of the edge moisture variation distance and a change from center lift to edge lift design governing conditions. The new design parameters along with the change of critical mode of slab deformation lead, in general, to thicker slabs. It is the objective of this paper to examine the changes in the PTI design method and compare the results obtained with both the 2 nd and the 3 rd Edition design procedures for typical Phoenix, Arizona soils and climatic conditions. Copyright ASCE 2006.