This article investigates the cause of the failure of some parts of a two-storey building under construction, in Oba-Ile, Akure, Ondo State, Nigeria. The article examines the effect of the quality of sand used for concrete production on the quality of concrete produced from them. The article also investigates whether the provided main and shear reinforcements in buildings complied with the provisions of the BS 8110. Site inspections were carried out, and concrete samples were taken. Many of the structural components were exposed, so as to determine the number of reinforcements in each structural element, the anchorage length of main reinforcement provided and the spacing of shear reinforcement. The field settling method was used to determine the percentage content of silt/clay impurity in the sand. The average silt/clay content of the sand was found to be 10.5 per cent, which is above the maximum 4 per cent value stipulated by the code. The results of both the destructive and non-destructive tests show that the average compressive strength of the concrete is 9.6 N/mm2, which is less than 20 N/mm2 specified minimum concrete strength for structural use. The anchorage length provided is less than the value stipulated by the code. The low quality of the concrete is due to the high percentage content of silt/clay in the sand used. From the above investigation, the possible cause of the collapse of the building is the provision of anchorage length less than required, spacing of shear reinforcement is less than required and, the poor quality of the concrete used for the construction.