How to Repair Cracks in Exterior Masonry
Cracks in exterior masonry let water in. Once moisture gets behind the surface, it leads to further cracking, frost damage, blown render, and damp penetration into the building. Whether the crack is in brickwork, blockwork, render, or mortar joints, repairing it properly means cleaning it out, filling it with the right product, and finishing to a sealed, weatherproof surface.
What causes cracks in exterior masonry?
Exterior masonry may crack for a number of reasons: thermal movement as the wall expands and contracts through seasonal temperature changes, settlement in the building structure, shrinkage in render or mortar as it cures, frost damage where water has got into the surface and expanded as it freezes, and physical impact or loading. The cause determines whether the crack is likely to come back. If there's ongoing movement, see our guide on reappearing cracks for a flexible solution. If the crack is stable, a standard repair will hold.
Step 1: Prep - clean out and stabilise
Start by opening the crack up slightly with a chisel or scraper to create a clean edge for the filler to bond against. Remove all loose material, dust, and debris from the crack and the surrounding area, then brush out thoroughly. On exterior walls, check for dirt, moss, or algae in and around the crack, and clean off any biological growth so the surface is clean and dry. Test the substrate before filling. Rub your hand or a dark cloth across the area around the crack: if it comes away dusty or powdery, or the surface crumbles under light pressure, the substrate is failing and needs treating. Apply Wall Hardener, working it into both sides of the crack and the surrounding surface, and leave it to dry for at least 24 hours before filling.
Step 2: Fill - how to fill an exterior crack
Using a filling knife, press your chosen filler firmly into the crack, working it right into the depth of the crack rather than just skimming over the surface, hard enough to bond it to both sides. Work along the crack in one direction for a clean, consistent fill, and avoid dragging the knife back and forth, which can pull air into the repair.
Masonry Repair Filler is designed for exterior bare masonry repairs and works well in cracks of varying width and depth. Iit can also be applied to damp surfaces. If you are filling a crack on a painted exterior substrate, use Tough Multi Filler is a versatile alternative that can be used for both filling and finishing. Build the filler up flush with the surrounding surface.
Step 3: Finish – Finish and blend the repair
At this point, the crack is repaired, and the surface is ready for decoration. For a finish that blends the repair into the surrounding wall, apply a finish coat over the repair and feather it out across the surrounding masonry.
Tough Multi Filler can be used as both the fill and finish product. Skimcoat Filler is a powder option for covering larger areas with control over the consistency. Or this purpose it has a long working time to allow smoothing. Skimcoat Filler Ready to Use gives the same result in a ready-mixed format. If the surrounding surface is textured, shape the finish coat to match while it's still setting. Let it dry fully before decorating, and the surface is sealed, weatherproof, and ready for an exterior paint system.