Lack of sufficient attention to the possible benefits of adopting lean concept has hindered the performance of small and medium enterprise (SME) contractors in Sri Lanka. Insufficient knowledge on minimising non-value adding activities (NVAA) is considered as the major barrier to implementing lean. Moreover, there is a lack of empirical research identifying NVAA, in order to trigger lean adoption in Sri Lankan SME contractors. Hence, the paper investigates NVAA, their significance and the causes, which hinder lean implementation in Sri Lankan SME contractors. A literature review, followed by five case studies were carried out, and the data were analysed using 5-why analysis. According to findings, lean construction is still a relatively unfamiliar approach among SME contractors in Sri Lanka. Some organisations follow lean techniques in an ad-hoc manner without an adequate understanding of the concept. The study further identified defects, inventory and waiting as major NVAA categories relevant to SME contractors. Lack of finance, insufficient training, cultural inertia, lack of individual capacities, lack of networking and collaboration, and lack of action learning were identified as the root causes for NVAA of SME contractors. Although respondents expressed their willingness to implement lean to enhance value, they identified lack of capacities as a major constraint against enabling lean adoption among SME contractors in Sri Lanka.