SUMMARYThe mechanical behaviour of discontinuities in rock, such as joints, is known to be size-dependent. It is also suspected that the behaviour of larger size features, such as faults, is also size-dependent. This size dependence has serious implications for performing numerical response simulations of geological media. In this paper, we develop a new mathematical theory for scaling of one particular discontinuity property, namely the interface normal sti!ness. To accomplish this, we idealize an interface to have fractal geometry, and we develop analytical relations which show that the interface normal sti!ness, which is commonly thought to be a size-independent property, is in fact a size-dependent property and has fractal characteristics that may be exploited to develop a fundamental theory for scaling.