This paper presents results of a study of the performance of solar air heaters with 60 degrees v-down discrete rectangular cross-section repeated rib roughness on the air flow side of the absorber plate. A detailed investigation has been carried out using a mathematical model to study the effects of various ambient, operating and design parameters on the thermal efficiency and effective efficiency (based on the net gain after taking account of the pumping power) of such air heaters. The study shows that, at air mass flow rates less than about 0.04 kg s(-1) per m(2) of the absorber plate, roughened duct solar air heaters provide significant performance advantage over the smooth duct air heater. The thermal and effective efficiencies differ only marginally at low flow rates. With the increase in the flow rate, the difference between the thermal and effective efficiencies increases because of the increase in the pumping power. At the mass flow rate of about 0.045 kg s(-1) m(-2), the effective efficiencies of the roughened and smooth duct solar air heaters are practically the same. The results of the study are presented in the form of design plots. (C) 2009 Elsevier Ltd. All rights reserved.