The single lap joints analysis has been developed from different perspectives, including overlap length, adhesive thickness, spew fillet formation, stress distribution, and manufacturing parameters. This paper describes recent results in the study of the roughness influence on the mechanical adhesion of single lap joints in composite test specimens made of glass fiber with a polymeric matrix. Analysis techniques were based on both finite element and experimental approaches. For the above-explained purpose, two cases were simulated: one with a surface roughness of the adherend in the joint overlap, and another one with a minimal roughness surface in the overlap zone. It was considered that the adhesive behaves like an isotropic material. As for the specimens used in the experimental tests, the surface roughness of the adherends was controlled by using an anti-adhering fabric, and the adherend specimen's surface without roughness was controlled using glass as a mould. The numerical results were validated experimentally by means of tensile tests. In the simulation where substrates with surface roughness were used, it was observed that the maximum shear stress at the joint ends decreases by 21% when compared with the model developed with adherends without roughness. Nevertheless, in the test specimen with roughness, a redistribution of the stresses exists that move from the adhesive to the substrates, which causes an increase of the single lap joint resistance. The same was observed in the experimental tests where an increase of four times in the load supported by the joint was obtained.