The flat earthen roofs of Kemaliye houses represent a rare and well-preserved example of traditional earthen roofing systems in Anatolia. However, their material composition and performance mechanisms remain insufficiently documented in scientific literature. The main objective of this study is to characterize the earthen and stone materials used in the original flat roofs of Kemaliye houses and to evaluate their physical, geotechnical, and water-related performance in relation to roof functionality. For this purpose, original roof materials were collected in situ and analyzed using geotechnical index tests, mineralogical and chemical analyses as well as physical and mechanical tests conducted on mortar specimens prepared with different water contents. The results indicate that the upper roof soil (gavcin) exhibits low plasticity and a mineralogical composition dominated by quartz and calcite, which provides adequate workability while limiting excessive swelling behavior. Mechanical testing reveals compressive strength values in the range of approximately 2.7–3.7 N/mm² and a non-linear relationship between water content, porosity, and strength. Capillary absorption tests demon-strate that the material-level sensitivity to water is effectively mitigated in practice by the layered roof configuration and traditional construction details. These findings indicate that the long-term durability of Kemaliye flat roofs depends not solely on the use of water-resistant materials, but rather on the integrated interaction between material properties, compaction, and roof detailing. The study provides a scientific basis for the conservation of this heritage roofing system and offers transferable insights for the development of durable and sustainable earthen roof solutions in contemporary architecture.