Light and porous cement-bound gravel layers such as foam cement are sensitive to weather conditions. When it is rained upon or a few days after the execution, the surface may pulverize or absorb a considerable amount of moisture. It is therefore important to provide the roof waterproofing as soon as possible. An alternative roof sealing in case of insufficient cohesion is a loose and ballasted or mechanically fastened sealing system up to the roof floor. Foam cement can be provided with an adhesive finishing layer to limit water absorption and guarantee cohesion.