Cavity walls are two walls with a gap between them, held together by metal wall ties. Many homes built between the wars used mild steel ties that, decades later, are rusting. As they corrode they expand, which can crack the outer wall.
Why it happens
Early wall ties were often made from mild steel with limited protection. Moisture in the cavity slowly corrodes them. Rust takes up more space than the original metal, so the ties expand inside the mortar joints and push the brickwork apart.
Signs of wall tie failure
- Horizontal cracks in mortar joints, often repeating every few courses where the ties sit
- Bulging or bowing in the outer wall
- Lifting window sills or cracks around windows
- Cracking in render at regular intervals
- Roof edges or gables that appear to have lifted
Not all cracks are caused by wall ties. Subsidence, heave, thermal movement and lintel failure can cause different patterns of cracking, which is why a proper inspection matters.
How wall ties are checked
A surveyor inspects the cracking pattern and uses a borescope, a small camera inserted through a drilled hole, to look at the ties inside the cavity and assess their condition.
How it is repaired
- New stainless steel ties are fixed between the inner and outer walls
- Old ties are isolated or removed so they can’t cause further damage
- Cracks are repaired, and helical bars are used to stitch more serious cracking
- Holes are made good to match the existing brickwork
Our surveyors carry out wall tie surveys across London and the Home Counties. Read more about our wall tie replacement and crack repair service.
