This paper examines the consequences of unmanaged wastes on building procurement activities across southwest Nigeria. To achieve this, two hundred and six-one (461) questionnaires were randomly administered across the six (6) that makes of the southwest states Nigeria. The result of the findings established that 75.9% of unmanaged building material wastes have a direct impact on project cost, 69.3% of unmanaged wastes have a direct impact on the environment, 65.1% of the wastes have a direct impact on the socio-economic well-being of the general population and the quality of life respectively. The study further showed that 78.5% of unmanaged building material wastes have a direct impact on the loss of significant revenue, 73.6% of wastes have a direct impact on the lengthening of contract execution time, while, 72%, 62.5%, 61.7%, 60.9%, 58.5% and 51.3% of wastes have a direct impact on a cleaner environment, less productivity, others (such as land and air pollution, and public health), increased project cost, increased patronage and longer lifespan of materials respectively. The findings recommend the development of wastes management interventions should place dual emphasis on building procurement activities and professionals (e.g. architects, builders, engineers, project manager etc.) to effectively train on the ways of handling materials, and waste reduction.