In this paper we consider the effect of surface roughness on the rate of coagulation, and on the particle size dependence of the stability ratio W in slow coagulation in particular. Deviations from a smooth surface ("surface roughness") will affect coagulation rates, if and when their characteristic amplitudes, the interparticle distance, and the range of the interaction forces are commensurate. Colloidal particles will nearly always show a degree of surface roughness on the scale of the Debye length, which under coagulation conditions is on the order of 1 nm in the surrounding electrolyte. Representing surface irregularities by a function which scales with the interparticle distance, we have calculated the interaction with the DLVO theory. It is found that surface roughness must strongly affect the relation of W with the curvature of the particle surfaces. Computed log W - log c diagrams for various combinations of particle size and surface roughness suggest that the lack of sensitivity of log W - log c diagrams to the particle size, which is generally found in the experiments, can be due to surface roughness.