In French road structure design, the capping layer is by default an essential element of it as it has to fulfil many functions. From an economical point of view, it is important to optimise its thickness as it is typically either made of imported granular or site-won stabilised materials. In 1983, a relationship was established by the main author to determine the thickness of granular capping layers covering all situations in terms of stiffness characteristics (subgrade, formation level and granular material). To date, in the case of stabilised capping layers, French engineers have simply been relying on well-established empirical tables published in the ‘GTS’ national notes for guidance. The major drawback of these tables is that only a few scenarios are covered. This article describes these methods of determining the thickness capping layers and completes it with a new approach in the case of stabilised materials so that all situations can be considered.