Render, composed of a binder and sands, is a protective skin for stone walls or masonry supports and plays an essential role in insulation and filtering between interior and exterior hygrometric exchanges.
The primary function is to ensure the waterproofing of facades. It allows condensation to evacuate through the walls while being impermeable to rainwater. This limits the amount of water that could be absorbed by the wall if it were “bare”. The second function is the aesthetics of the facade: making it flat, homogeneous across its entire surface, and giving it an aesthetic appearance through a particular finish (e.g., brushed, trowelled, scratched render…).
Using natural hydraulic limes makes renders flexible enough to absorb deformations and settlements of walls and foundations. The base of the wall, treated with lime, facilitates the evaporation of capillary rise from the ground. The cornice protects the facade from roof runoff. On an old rubble stone wall, which is laden with moisture, air lime or natural hydraulic lime should be used to avoid cracks, saltpeter, and mold.
Depending on the nature of the masonry, the construction period, and even the geographical location of the building (seafront, mountain…), it is crucial to understand the substrates and prepare them before applying render or joints. The adhesion of the render depends on the quality of this analysis and the preparation of the render.
