Over the last few years, reverberation times and sound pressure levels have been measured in many sports halls. Most of these halls, for instance those made from stony materials, perform as predicted. However, sports halls constructed with profiled perforated steel roof panels have an unexpected very low reverberation time in the 125 and 250 Hz octave bands. The aim of this study was to provide an explanation for this low-frequency anomaly. A 1:20 scale model of a sports hall was constructed and placed in a small anechoic chamber. The roof could be equipped with differing ceiling types: a flat non-absorbing ceiling, a flat absorbing ceiling, two different profiled non-absorbing ceilings and a profiled absorbing ceiling. With a spark sound source and a small microphone, the impulse-response of the scale model could be registered and analysed. Moreover, a Matlab model was constructed to simulate the acoustic behaviour of the sports hall. This model included the ‘ground effect’ of the roof surface, an effect typically not included in commercial ray-tracing programs. The measurements and the simulations showed that the high sound absorption values of a perforated panel roof structure at 125 and 250 Hz can be (partly) explained by its shape. The diffusing properties of the corrugated roof have an effect similar to sound absorption. Because the roof is the largest absorbing surface in a sports hall, this effect can have a significant effect on the low-frequency reverberation time.