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In many concrete and rock structures, depending on the surface morphology, friction arises inside rough cracks and contributes to the stability of the structure. In this work, the stress transmission mechanisms inside rock and concrete cracks are investigated. The surface topography, experimentally digitized at the meso-scale, shows peculiar self-affine properties. As a consequence, the contact domain, obtained numerically, is a lacunar fractal set whose characteristics considerably change during the sliding process. The asperities interlocking mechanism is more effective than the purely adhesive one, and thus the dilatancy behaviour plays a fundamental role, especially if filtration across the cracks has to be taken into account. Finally, the distribution of the micro-forces within the contact domain is shown to be multifractal. This permits to give an interpretation of the typical stick-slip behaviour of these interfaces, in principle extendible up to the scale of the tectonic faults, which are responsible for earthquakes.