Tropical home gardens in developing countries are a traditional land-use system, which contributes to the basic food requirements of the farming families, as well as providing a small but important income throughout the year. Most of these home gardens rely on rain for survival, i.e., rainfed systems. At present home gardens are being neglected due to social and environmental problems. Among these issues, the scarcity of water during cropping seasons is the most important issue that adversely affects crop production from home gardens. This is mainly due to the occurrence of unexpected dry periods during the cropping seasons, and the requirement to have a water source at the farmer's residence to irrigate the home garden during the dry spells (to capture and conserve roof runoff in the home gardens for subsequent use) is an appropriate green option to mitigate this situation. Microlevel rainwater tanks are promoted to be used to collect roof runoff water during wet periods enabling the farmer to irrigate his home garden crops during dry periods. Field experiments were carried out at Mapalana Research Farm in Sri Lanka to obtain the roof runoff coefficients from commonly used roof types; namely: asbestos, tiles, and thatched (either straw or woven coconut leaves). The objective of the case study is to develop a methodology to estimate the optimum size of a rainwater tank based on the roof size, roof cover, climatic conditions, and the irrigation requirement for home garden irrigation. This paper presents the experimental setup, the runoff coefficients obtained from different types of roof materials, and the methodology used to calculate the optimum storage capacity of the tank for home garden irrigation. In designing the rainwater tank sizes, microirrigation systems with both drip and pot irrigation systems were assessed. The water-use efficiency of drip systems was comparatively small compared to the subsurface pot irrigation. Based on the tank sizes obtained in the study, roof runoff water from the farmer's own roof is sufficient and recommended to be used as the irrigation source for home gardens. (C) 2014 American Society of Civil Engineers.