Abstract: In order to determine the relationship between goaf roof thickness and critical span, an iron ore goaf is taken as an engineering instance. The rock mass mechanical parameters are obtained from the mechanical experiments of rock specimens sampled in situ. The relationship between the safe roof thickness and the critical goaf span is determinined according to K. B. Lu Peinie equation, ratio of thickness and span equation, structure mechanics of girder and plate beam equation. Meanwhile, the damage and failure process of goaf roof is simulated by RFPA numerical simulation program. The relationship between the safe roof thickness and the critical goaf span is determined, respectively, when the initial damage and instability of goaf roof are adopted as the criteria. The values predicted with K. B. Lu Peinie equation are most close to the numerical ones when the damage of roof begians. The ratio between them can be considered as a safety factor obtained from the numerical simulations. As it shows, it is kept a large safety factor by the critical roof thickness which is calculated by K. B. Lu Peinie equation. The numerical simulation with RFPA could obtain this safety factor, and it may have more extensive applicability.

Key words: goaf, roof, safe roof thickness, critical goaf span, RFPA numerical simulation