Many historic earthen structures in the Middle East, Central Asia, and parts of Africa and Latin America are traditionally protected against the rain by straw-reinforced clay-plaster applied as a protective layer to the exterior of the building. The plaster is effective in keeping the building dry and can sustain the severe temperature changes of the regions; however, the plaster is susceptible to relatively fast water erosion. This article presents the results of a program of experiments, aimed at enhancing the durability of this traditional plaster against rain erosion. Hydrated lime and a very fine-grained aggregate (crusher dust) are used to improve the resistance of the straw-reinforced plaster. Samples made of the improved plaster mixes are tested for compressive and flexural strengths, water tightness, and resistance to water erosion; the results are used to obtain an optimum mix proportion for the improved clay soil plaster. The results indicate a substantial increase in the strength and durability of the plaster by using small quantities of these low-cost additives.