Abstract The present work studies the mechanical behaviour of single-lap joints of PP reinforced with glass fibres. Failure loads were obtained experimentally for different superposition lengths (15, 30, 45 and 60 mm). A 2D numerical analysis was developed using the finite element method, and assuming a plane strain state, an orthotropic behaviour for the laminates and an elastic–plastic behaviour for the adhesive. It was found that the positions where σyy and τxy stresses have their maximum values, which are near the extremities of the joint and close to the interface adhesive/adherends, move inside the joint with load increasing. An equivalent stress was defined from σyy and τxy and was obtained for the failure loads obtained experimentally. This quantity varies 9.7% with superposition length, which can be considered reasonable therefore can be used as a damage criterion for single-lap joints.