© 2022The Imperial Roman Baths in Sagalassos, Turkey, were built in the 2nd century C.E. and have been excavated since the 1990s. The harsh environmental conditions, especially during the winter, resulted in the accelerated decay of the building materials, material losses, and serious structural problems in the Baths. This study explores the comprehensive characterization and multiscale assessment of bricks and mortars for effective preservation of the Baths. Experimental results showed that although the interface between building materials weakened after years of accumulated degradation, the individual materials still had higher strength than those from the same period. The material scale finite element model used to understand the damage mechanism showed that brick failure was caused by the tensile stresses accumulated at the brick–mortar interface when some of the mortar crumbled and fell with deterioration. A global sensitivity analysis was also conducted to investigate the impact of individual layer thicknesses on the stress distribution in the brick and mortar composite. Results indicated that thicker bricks resulted in better confinement at mortar layer and smaller tensile stresses at the bricks.