Brick cleaning guide
How to repair cracks in brick walls
Cracks in brickwork range from harmless cosmetic hairlines to indicators of serious structural movement. Getting the repair right depends entirely on understanding which type of crack you have, what has caused it and whether the underlying cause has been resolved before the repair is carried out.
Cracks in brick walls are one of the most common concerns for London homeowners and one of the most frequently misunderstood. Because London is built predominantly on London clay, which is a highly expansive soil that moves significantly in response to changes in moisture content, cracking in brickwork is common across the capital. The overwhelming majority of cracks are cosmetic and the result of normal thermal movement, minor settlement or mortar shrinkage. However, a smaller proportion of cracks are indicators of foundation movement or structural problems that require professional assessment before any repair is attempted. The key is learning to distinguish between the two, and understanding that repairing a crack before addressing its cause produces a repair that will fail again.
Under 2mm
hairline and narrow cracks are generally cosmetic and repairable with repointing or flexible masonry sealant
2mm to 25mm
moderate cracks in this range should be assessed by a structural engineer before any repair work begins
Over 25mm
wide cracks require urgent structural engineering assessment and must not be cosmetically repaired without professional investigation
The main types of crack in brick walls and what they indicate
Critical rule before any repairAddress the cause before carrying out the repair
The most important principle in brick wall crack repair is that cosmetically repairing a crack before its underlying cause has been identified and resolved will produce a repair that fails again, often within months. If a crack has been caused by ongoing foundation movement, that movement must be stabilised first. If the cause is wall tie corrosion, new ties must be installed before the crack is made good. If the cause is a failing lintel above a window, the lintel must be replaced. Monitoring a crack over time with marked pencil lines or crack monitors can help determine whether movement is still ongoing before committing to a repair.
Repair methodsHow cracks in brick walls are repaired
Repointing for mortar joint cracks
For cracks confined to mortar joints, repointing is the standard repair. The cracked or deteriorated mortar is raked out to a minimum depth of 20mm using a cold chisel or oscillating tool, and new mortar is packed in flush with the brick face and tooled to match the existing joint profile. For Victorian and period properties, the replacement mortar must be lime-based to match the original, not Portland cement. Cement mortar is harder than the surrounding brick and will cause the bricks to crack as the wall moves naturally with temperature and moisture changes.
Crack stitching for structural reinforcement
Where cracking is caused by lateral wall movement, subsidence or wall tie failure, crack stitching uses stainless steel helical bar anchors set into the mortar joints across the crack at regular intervals to reinforce the wall and redistribute loads. Slots are cut 500mm either side of the crack, the bar is bedded in resin or a compatible mortar and the joints are made good to match the surrounding pointing. This is invisible from the outside when complete and provides a strong structural connection without requiring brick replacement.
Brick replacement for spalled or cracked bricks
Where individual bricks have cracked through or have spalled (the face of the brick has broken away), the affected bricks should be cut out and replaced. Replacement bricks must match the original in size, colour and material. For Victorian London properties, reclaimed period bricks are often sourced to achieve the closest visual match. The old mortar is fully removed, the new brick is bedded in matching lime mortar and the joints are pointed to blend with the surrounding wall.
When a structural engineer is needed
Any crack wider than 2mm, any horizontal cracking, any cracking accompanied by doors or windows becoming difficult to open, any crack that has appeared suddenly or continues to widen, and any crack in a foundation wall should be assessed by a qualified structural engineer before repair work is commissioned. A structural engineer can identify the cause, determine whether movement is active or historic and specify the appropriate repair solution. Attempting to cosmetically repair a structural crack without this assessment is a false economy that risks significant further damage.
Brick cleaning London
Brickwork cleaning and preparation across London by Cloud Nine
Cloud Nine cleans brickwork professionally before any repointing or restoration work begins. Clean brickwork is essential for good mortar adhesion and long-lasting repairs. If you are planning a repoint or brickwork repair, contact us to arrange the brick cleaning stage first.
Part of our guide
Brick cleaning help and guidance
Everything you need to know about brick cleaning for London properties.
Back to the guide