This paper is a study on the results of strength tests on single-lap joints used primarily in the aerospace industry, based on a Hi-Lok fastener. Two popular structural materials used in the aerospace industry were considered in this work. The test specimens were fabricated from the EN-AW 2024 T3 aluminium alloy and a carbon composite. It was decided to use the Naftoseal MC780 type C adhesive as a sealant to form the required hermetic, air-tight lap joints. In the definition of the joint type used, the results are presented for the tests of selected 2D and 3D surface roughness parameters, with isometric imaging of the surfaces related to this analysis. Microscopic images acquired at a magnification of x500 illustrate the surface quality. This work concludes with a discussion of the test results and conclusions.