The use of flat plate solar collectors (FPSCs) is the most common technology to employ solar energy. FPSCs enhance thermal efficiency and, as a result, can reduce both the size and the costs of a system. Surface modification techniques are extensively used to improve the thermal performance of systems. Among such techniques, helically corrugated tubes play an important role in industrial applications. Therefore, an experimental study was conducted in accordance with ASHRAE 93 (2010) to examine the effects of roughness height and pitch length of corrugated risers on the heat transfer in an FPSC in a turbulent flow. Nine corrugated risers with roughness height to diameter ratios (e/D-H) of 0.05, 0.1, and 0.15, and pitch ratios (P/D-H) of 0.3, 0.5, and 0.7 were manufactured and examined. The results demonstrated that the maximum efficiency of the FPSC with corrugated risers (e/D-H = 0.15 and P/D-H = 0.3) was about 61.59%. The friction factor increased by about 59.3-97.6%. The thermal performance factor had the maximum value of 2.3 at the Reynolds number of 5153. Based on the experimental results, the helically corrugated risers in the FPSC have a potential for being used in thermal applications.