Abstract We present results of an experimental study on the stick-slip phenomenon focused on detailed measurements of the relative normal displacement. This was done mainly to determine whether or not the sliding surfaces become separated during the slip phase as suggested in previous works. Experiments were performed on PMMA as it has mechanical properties comparable to those of brittle rocks and has proved to be a good analogue for rocks in rupture mechanics experiments. Results show clearly that normal displacement occurs during and after the slip phase. The characterisation of the sliding surface enables us to compare the maximum relative normal displacement during the slip phase with asperity height. In most cases, the maximum relative normal displacement was found to be higher than the average value of the peak-to-trough relief of the surface, showing that sliding surfaces were at least partially separated. The phenomenon generally consists of a monotonous slip opening phase followed by a monotonous slip closure phase. In some cases, the slip closure phase shows oscillations which have been interpreted as elastic bounces. This last phenomenon was observed irrespective of the experimental conditions with a probability ranging from 0.1 to 0.3 but no clear variations of this probability have been found as a function of normal stress and roughness. These observations may give insights for the explanations of earthquake-related problems such as the heat flow paradox, anomalous P-wave radiation or the inferred low average friction coefficients in subduction zones.