Runoff Coefficient Variability in Green and Bitumen Roof Systems

Emil Tsanov (1) University of Architecture, Civil Engineering and Geodesy, Sofia 1000, Bulgaria. 2) Center of Competence “Clean Technologies for Sustainable Environment–Water, Waste, Energy for a Circular Economy”)
Dobril Valchev (1) University of Architecture, Civil Engineering and Geodesy, Sofia 1000, Bulgaria. 2) Center of Competence “Clean Technologies for Sustainable Environment–Water, Waste, Energy for a Circular Economy”)
Irina Ribarova (1) University of Architecture, Civil Engineering and Geodesy, Sofia 1000, Bulgaria. 2) Center of Competence “Clean Technologies for Sustainable Environment–Water, Waste, Energy for a Circular Economy”)
Valentina Dimova (Sofiyska Voda Part of Veolia, 159 "Tsar Boris III" Blvd., Sofia 1618)

Civil Engineering Journal
Vol. 12 No. 8 (2026): August

This study aimed to generate field-based evidence supporting sustainable urban stormwater management under the continental climatic conditions. The runoff behavior of a retrofitted green roof was monitored and compared with that of an adjacent conventional bitumen roof of identical geometry. Monitoring was conducted from March 2022 to January 2024. A total of 34 rainfall events (3–74 mm) were analyzed to evaluate runoff coefficients (RC) and their relationships with precipitation depth, antecedent dry days (ADD), and rainfall duration. The average RC was 0.24 (0–0.84) for the green roof and 0.81 (0.43–1.00) for the bitumen roof. The green roof effectively retained rainfall events below approximately 10 mm and showed increasing runoff variability with increasing precipitation depth. A moderate relationship was observed between RC and ADD for the green roof (R² = 0.518), whereas no meaningful relationship was found for the bitumen roof. Rainfall duration had no significant effect on runoff generation. The findings confirm the considerable runoff reduction potential of extensive green roofs under continental climatic conditions and demonstrate that rainfall depth and antecedent moisture conditions are key controls of runoff response. The long-term field evidence generated in this study provides a sound basis for improving green roof design and sustainable stormwater management.