A good understanding of the subsurface geological conditions at proposed construction sites is a fundamental requirement to design appropriate building foundations. In this study, the 2D electrical resistivity tomography (ERT) method was used to characterize the subsurface geology at three active construction sites located on or near exposed bedrock in northeast Thailand. The resistivity tomograms proved useful for determining the thickness of intact bedrock overlying a potentially weaker weathered rock of variable saturation. The wide-area information provided by the ERT method should be helpful to foundation design engineers assuming they have confidence in the geophysical results. Geophysics was also useful to guide suitable locations for ongoing geotechnical tests at a given construction site especially if difficult ground conditions exist.