Direct bonding is of particular interest for optical system manufacturing for spatial application. This process requires very precise physical and chemical preparations of surface, especially roughness controls. Thus, this paper proposes to understand and discuss the roughness influence on mechanical strength and bonding energy of an elementary mechanical structure after a room temperature bonding and an annealing at optimal temperature. First, surfaces are characterized and roughness is measured. At the same time, the performances of fused silica and Zerodur® glasses are compared. Then interface properties are measured using double shear and wedge tests. At the end of the paper, a discussion, based on literature models, is proposed to explain the existence of an optimum of roughness put in evidence in experimental results.