Curbing water scarcity problems in semi-arid regions is a top priority for economic and social development. Alternative strategies for sustainable domestic water supply are therefore required to augment water supply with affordable cost and technology. Scant research findings in sub-Saharan Africa have, however, reported inefficiency in adapting to water scarcity problems in the region. Therefore, an investigation of domestic rainwater harvesting (RWH) reliability is important to identify factors affecting the system. We found that rainwater harvesting with the existing traditional RWH systems in Mekelle city was unreliable. Unreliability was attributed to inefficient design (the systems were installed without considering deterministically the stochastic nature of rainfall), family size, water demand, rooftop area and storage tank size.