Conservation of render and plaster

Repair of cracks

Fissures and cracks in renders are common on a façade. The fissures most commonly occur in the surface, but cracks of more than 2 mm in width may also penetrate the substrate and masonry. 

Cracks in the render might be a sign of severe damages

Cracks are defined as long openings with a width of 2 mm or more. They can occur only in the render, in combination with hollow sounds (lack of adhesion) or following a construction crack into the masonry itself, often following joints in the construction. These cracks can be dead or alive (moving due to movements in the construction). Cracks that only appear in the render are often a result of wrongly used repair mortars, but also age and internal tensions in the render. Cracks that follow joints are often a result of the sudden impact of regular traffic, construction works or other acute happenings. Living cracks are a sign of fatal construction issues and should be addressed by a building physicist consultant to identify the origin and movement pattern.

Opening, repair and reinforcement of cracks in masonry

Cracking wall

It is very important to examine the origin before starting repairs. Cracks in the surface, often in combination with lack of adhesion, should be opened. Material up to 5 cm outside the loose area should be removed all the way down to the masonry, taking care not to widen the affected area. This area should then be filled up with repair material of the same or less strength as the surrounding material. Use the same build-up as a normal render, including primer, base coat and a final coat (normally not more than 1 cm per layer per day). The edges where new and old render meet should be consolidated with water glass before starting the repair works. If the cracks follow joints in the masonry, these joints should be scraped and repaired. If there are big openings, consider “pinning” the masonry with adapted bricks or material (type, suction and size) to avoid thick layers of mortar. With some projects, consider also helping the masonry using dilatation joints. When repairs in the brickwork are finished, repair the render as described above. If you suspect a moving crack, a specialist should be contacted to evaluate actions. These actions are just a few of the techniques used. Please contact your local organisation to get more advice on your project.