An experimental model test study is carried out to investigate damage pattern and progression on a rock armoured breakwater roundhead subjected to multidirectional waves. Concerning damage pattern, the most critical sector is observed to shift leeward with increasing wave period. Taking angles relative to mean wave direction, the critical sector is observed in the sector 10°–55° for short waves and in the sector 100°–145° for long waves. A probabilistic approach is developed to predict for one typical roundhead geometry the damage distribution depending on the incomingwaves and structural characteristics. The damage progression is observed dependent on significant wave height and peak wave period, but not on the directional spreading and the spectral width of the incident waves. Combining the results of both damage pattern and damage progression, a stability formula for the distribution of damage over the roundhead is developed. Thus the formula also considers the shifting of the critical sector due to increasing wave period which existing formulae do not include. Finally, analysing the damage produced by double peaked spectra, it is shown that the armour may be designed by the formula when using the total significant wave height and an equivalent peak period.