Physical model tests were done on an armour of Tetrapods, placed in a single layer. The objective of the investigations was to study the stability of the secondary layer, and to see if the material of this secondary layer could be washed out through the single layer of Tetrapods. It was concluded that secondary armour is not washed out through an undamaged layer of Tetrapods, and that all damage to the secondary layer is related to damage in the primary layer. To prevent damage to the Tetrapod layer, a high placing density is needed. In case of low placing density, the area around the waterline will slide down, creating gaps in the main armour and exposing the secondary layer. Because of this process the placing density in lower sections of the breakwater increases, and consequently also the strength increases at those places.