Achieving good impact sound insulation with a timber floor is challenging, given its low weight. Therefore, the acoustic performance for a timber floor should be considered at the planning stage, and a prediction tool is required to evaluate the impact sound insulation design. This work investigates the prediction of the impact sound radiated by a timber joist floor. The floor properties are determined by modal tests of the separate floor components. The impact sound radiated by the floor is subsequently computed in two steps. In the first step, the impact force of an ISO tapping machine is computed by considering both the global and the local mobilities of the floor. In the second step, the response of the point-excited floor is computed by the finite element method, and the sound pressure level in the underlying room is computed by a diffuse room model. Special attention is paid to the influence of boundary conditions on the computed eigenmodes of the floor; these are also compared against identified eigenmodes of the floors. Finally, the sound pressure levels and the single number rating results are computed, for excitation by a instrumented hammer and the ISO tapping machine. The computed results are experimentally validated.