We propose a generalisation of the cellular automaton model introduced recently by Lai, Jia and Chan [Phys. Rev. Lett. 79, 4994 (1997)] for simulating friction induced segregation in a rotating drum. Our model implements the spherical geometry of the drum explicitly. Therefore it can be used to investigate geometrical properties of the segregation phenomenon in contrast to the original version of the model. Our model reproduces the spontaneous stratication observed recently in experiments for more than half lled drums, as well as the transition to non-stratied segregation for higher rotation speeds. It predicts that stratication can also occur, if the drum is less than half lled.