Weak subbase soils pose a significant challenge in road construction, particularly in tropical regions where expansive and low-bearing-capacity soils are prevalent. Conventional stabilizers such as cement and lime, while effective, are associated with high cost and substantial carbon emissions. This study investigates the combined effect of rice husk ash (RHA) and waste putty dust (WPD) both low-cost industrial and agricultural byproducts on the geotechnical properties of SC-classified clayey sand collected from Naththandiya, Sri Lanka. Eight mix combinations (WPD: 5% and 7%; RHA: 5%, 7%, 9%, 12% by dry weight of soil) were prepared and tested following BS 1377:1990 and ASTM standards for sieve analysis, Atterberg limits, modified Proctor compaction, and California Bearing Ratio (CBR). Increasing RHA content progressively reduced plasticity index (PI) from 21.11% to approximately 14.00% and raised optimum moisture content (OMC) while decreasing maximum dry density (MDD). The optimal mix (Soil + 5% WPD + 7% RHA) achieved a CBR of 32.48%, representing a 343% improvement over the natural soil CBR of 7.33%. Further increasing WPD to 7% reduced CBR to 23.56%, indicating an optimal WPD content of 5%.The combination of 5% WPD and 7% RHA provides optimal subbase performance for SC soil, offering a sustainable, cost-effective alternative to conventional stabilizers. Field-scale validation and long-term durability studies are recommended for practical implementation.