Conservation of render and plaster

Repair of fissures in a façade

Fissures in renders are common on a façade. We define fissures as openings up to 2 mm in width. These appear in the render due to small movements induced by sun (temperature) and moisture. But these fissures can also indicate issues with tensions in the render.

Fissures with different origins and properties

Fissures are small and thin, while cracks are bigger and often associated with cracks also in the construction. Fissures in the render develop due to the influence of temperature and moisture in combination with the render properties. Fissures due to shrinkage in the hardening process are also normal. Looking at earlier repairs, we often come across the habit of using strong, cement-based renders. The difference in compression and flexural strength between old and new renders often results in fissures and cracks. If the render is stronger than the substrate, fissures and small cracks will occur together with a lack of adhesion (giving hollow sounds). If the render is weaker or adapted to the substrate, fissures may occur without losing adhesion, but then often in areas with internal stress (openings in the façade) and in the transition zone between new and old render.

Conservation adapted to origin

Cracking wall

An examination of the origin of the fissures is highly important before starting repairs. Depending on the width of the fissure, different actions need to be taken into account. Smaller fissures can be filled with adapted material before repainting the whole surface. Most paints based on lime and silicate/water glass that are suitable for older masonry have a mineral‑based filler (crushed limestone/dolomite up to 0.025 mm). These fillers can be small enough to fill and close the fissure when worked thoroughly into the surface. Specialised primers are also available to be used before the final coat. Be aware that differences in suction can yield colour differences on the surface, especially in wet and humid weather. An interesting effect of using lime is its self-healing properties. When moisture and CO2 are present, new crystals grow in the fissure and will, with time, close it naturally. This regenerating of crystals is one of the keys to the long lifetime of lime-based materials.