The radiative properties of a material can be modified by its surface microrelief. We show that if the typical length of the surface roughness is smaller than a fraction of wavelength, the roughness can be treated as a homogeneous equivalent layer. Using this model and standard optimization techniques, we have developed an algorithm for the design of surfaces of a given material with prescribed radiative properties. As examples, we propose a relief structure that decreases the reflectivity of glass between 8 and 12 mum and another one that increases the emissivity of platinum in the infrared.