Within a 1+1-dimensional SOS type model with a periodic rough substrate, we show that the differential wall tension, which governs wetting, has a maximum as a function of a certain aspect ratio of the substrate. This result is based on a low-temperature expansion leading, in a first approximation, to Wenzel's law for the wall tension and allowing us to study the corrections to this law. It implies that the contact angle is minimum for a substrate with the corresponding aspect ratio. Our results are in agreement and explain recent numerical simulations.