The influence of wall roughness on the slip behavior of a viscous fluid has been discussed in several recent papers. We study the asymptotic behavior of a viscous fluid near a periodic oscillating wall with period epsilon and amplitude depending on the period. We assume the fluid to satisfy the so-called Navier’s boundary conditions. When the period and the amplitude have the same order, it is known that in the limit this boundary condition provides the adherence (or no-slip) condition. This gives a mathematical justification of why adherence conditions are usually imposed for viscous fluids, they are due to the microscopic asperities. In our work we consider the case where the amplitude is much smaller than the period. We show the existence of a boundary layer and, depending on if its value is zero, a positive number or infinite, we get different boundary conditions in the limit. As particular case, we can recover the adherence condition. The proof of our results is based on an original adaptation of the unfolding method, which is closely related to the two-scale convergence method.