One of the limitations in room acoustic computer modelling is the characterisation of the reflecting surfaces by the sound absorption coefficient. Especially at low frequencies, say below 500 Hz, the phase of the reflections may be important for accurate simulations of the room impulse response. In order to allow the inclusion of the phase of reflections an impedance model for reflecting surfaces is suggested. The common sound absorbing materials are divided into three groups (porous, membrane, or resonant) each having a characteristic frequency dependency of the absorption coefficient. For each group a method has been derived for the translation of absorption coefficients (in octave bands) to the surface impedance with the basic elements: surface density, cavity depth, and flow resistance. The impedance model makes it possible to calculate the complex pressure reflection factor and thus the phase of the reflections as a function of frequency and angle of incidence. It is found that the phase is particularly important for porous absorbing materials, leading to a significant time delay of the reflections at low frequencies, typically around 1-2 ms at 125 Hz. To what extend this new impedance model will lead to improved room acoustic prediction models remains to be seen.