The diffraction effects of screens and other obstacles have been integrated in a room acoustic computer model using a theoretical model by Pierce, which is a further development of the original Geometrical Theory of Diffraction by Keller. This diffraction model is particularly interesting because it allows the modelling of a screen as a wedge with any opening angle, or as a wide barrier with two diffracting edges. Also the combination of two screens is possible. Another feature is that the model is valid even with the source or receiver located at the surface of the barrier. A major advantage of the method as implemented in the Odeon room acoustics software is the automatic detection and calculation of transmission paths around one or two edges. The basic assumptions, limitations, and examples of verification will be discussed.