The Engineered Design of Building Drainage Systems addresses the central problem of building drainage design, namely the unsteady nature of the flow within the drainage system follow the random operation of system appliances. The flow within the building drainage systems is identified as belonging to the general family of unsteady flow conditions and solution techniques based upon the mathematical method of characteristics are proposed. Computing models based on these mathematical solutions and techniques are formulated and fully validated for full scale building drainage networks. The applications of the computer model to the prediction of flow depths and flow rates at any point within a multi-storey network at any point during a simulation is presented within the text.