Construction sites generate a large amount of material wastes and have become a common problem with associated risks in Ethiopia. However, the sources of such wastes are not well recognised. Therefore, the purpose of this research was to analyse the risk factors that contribute to material wastes in building construction projects. To achieve this goal, the factors that cause construction wastes were identified from literature and construction experts via focus group discussions and personal interviews. Following this, the factors were subjected to a questionnaire survey to identify the most critical factors of construction wastes. The questionnaire was distributed purposively to 85 construction experts representing contractors, consultants, and clients and 70 questionnaires were duly received for analysis. The data were analysed with a mean score and ranked to identify the most critical factors generating material wastes at construction sites. According to the results of the study frequent changes made to the design, poor strategies for waste minimisation, improper storage of material, poor site management, poor planning and supervision, and errors of contract document were the most critical factors causing construction wastes.