Rain water harvesting system became feasible and viable option in reducing potable water consumption during dry season and possibly reducing the adverse effect of floods during heavy rain fall. This study aims to identify the optimum size of rainwater storage tanks for different roof materials which results in high water and energy savings and carbon dioxide emissions reduction with a reasonable payback period in hospital and hotel buildings. Analysis conducted showed that metal roof with a 0.95 run-off coefficient grossed the highest water and energy savings as well as carbon dioxide reduction when paired with a 30m3 tank in all three commercial buildings. Gravel in contrast yielded the least amount of water savings resulting in a low amount of energy savings and carbon dioxide reduction due to a small run-off coefficient of 0.25. The metal roof portrayed significant difference at an average of 11.6 percent rise in water savings as the tank size is increased, whereas gravel could barely show noticeable difference. Consecutively, a 25 years payback period analysis of the rainwater tanks revealed that larger tanks are apparently more expensive hence giving a higher payback period. © 2016 Institution of Engineering and Technology. All rights reserved.