The use of artificial roughness in a solar air heater duct has been proposed to be an excellent option to enhance the heat transfer from absorber plate to the air. This paper presents an experimental investigation on heat transfer and friction characteristics of solar air heater ducts with integral repeated discrete square ribs on the absorber plate. The effect of geometrical parameters, especially, the gap width and gap position has been investigated. The roughened duct has a width to height ratio (W/H) of 5.83. The relative gap position (d/W) and relative gap width (g/e) has been varied from 0.16 to 0.5 and 0.5-2.0, respectively. Experiments have been carried out for the range of Reynolds number from 3000 to 18,000 with the relative roughness pitch (P/e) range of 4-10; relative roughness height (e/D) range of 0.018-0.037; and angle of attack (a) range of 30-90 degrees. The optimum values of parameters for rib arrangement have been obtained and discussed. For Nusselt number, the maximum enhancement of the order of 2.83 times of the corresponding value of the smooth duct has been obtained, however, the friction factor has also been seen to increase by 3.60 times of that of the smooth duct. The maximum enhancement is observed at a relative gap position of 0.25 for relative gap width of 1.0, relative roughness pitch of 8.0, angle of attack of 60 degrees and relative roughness height of 0.037. Based on the experimental data, correlations for Nusselt number and friction factor have been developed as function of roughness parameters of inclined discrete square ribs and flow Reynolds number. (C) 2009 Published by Elsevier Ltd.