A 15-month ageing test of an ATLAS module ''0'' RPC has been performed at the GIF X5 CERN irradiation facility. The aged RPC performances largely fulfilled the ATLAS requirements. However, the test showed a significant reduction of the RPC maximum rate capability, from almost 2kHzcm - 2 to a few hundred Hzcm - 2 , due to the total charge integrated during the test. The reasons for this ageing phenomenon are found to be related to an increase of the total electrode plates resistance as it was already pointed out in previous tests on smaller prototypes. This is found to be not only due to a possible increase of the plate resistivity but mainly to a degradation of the graphite coating.