Wall tie failure happens when the metal ties holding the two skins of a cavity wall together corrode. The classic sign is horizontal cracking along the mortar joints of the outer brickwork, repeating every few courses up the wall. It mainly affects houses built with cavity walls before the early 1980s, it’s well understood, and it’s fixable — but it does need confirming properly before anyone sells you wall tie replacement.
What are cavity wall ties?
A cavity wall is two separate walls (or “leaves”) with a gap between them: an outer leaf, usually brick or stone, and an inner leaf of brick or block. The gap keeps rain from tracking through to the inside.
The two leaves are joined by wall ties — short metal strips or wires built into the mortar joints of both leaves as the wall goes up. They let the two leaves work together, stopping the outer leaf from bowing or being sucked off by wind.
Cavity walls became increasingly common in the UK from the 1920s and 1930s and were standard for post-war housing. Many Yorkshire houses built before then — stone terraces, back-to-backs, older farmhouses — have solid walls with no cavity and no ties, so this particular problem doesn’t apply to them.
Why do wall ties fail?
The early ties were usually mild steel protected by a coating of zinc (galvanising) or bitumen. Those coatings were thinner than they needed to be. BRE research found that steel ties made before 1981 could corrode prematurely, and that the life of the coatings was often shorter than the expected life of the wall. Standards for galvanised ties were tightened around that time, and stainless steel is now the norm.
Corrosion is faster where:
- the outer leaf stays wet for long periods — exposed elevations facing the prevailing wind and rain, which on the Pennine fringes can be most of the year
- the mortar is black ash mortar — mortar coloured with ash from coal or furnaces, common in industrial towns. Its sulfur content can make the mortar acidic when wet, attacking the zinc coating
- the ties were poorly bedded or there were too few of them to begin with
How does wall tie corrosion cause cracking?
Rust occupies much more space than the steel it came from. As the part of the tie embedded in the outer leaf corrodes, it swells and splits, lifting the brickwork above it. Because ties are laid at regular vertical spacings, the result is horizontal cracks in the bed joints at roughly regular intervals — often every four to six courses of brickwork.
Some thinner wire ties corrode without expanding much, so they can fail with few visible signs at all. That’s one reason absence of cracking doesn’t rule the problem out.
Signs of wall tie failure
- Horizontal cracks in the mortar bed joints of the outer leaf, repeating at intervals up the wall
- Cracks that are more pronounced on exposed elevations, and often more noticeable towards the top of the wall where there’s less weight holding the brickwork down
- Bulging or bowing of the outer leaf
- The outer leaf lifting relative to the inner leaf, sometimes seen as cracking or disturbance at the eaves, at window heads, or where the wall meets the roof
- Cracks in internal plaster mirroring the external cracking in more advanced cases
- Gaps opening around window and door frames in the outer leaf
None of these on its own proves wall tie failure. Horizontal cracking can also come from roof spread, bowing of solid walls, or sulfate attack in mortar. Our guide to horizontal, vertical and diagonal cracks in walls explains how to tell them apart.
How is wall tie failure confirmed?
A visual inspection can raise the suspicion; confirming it needs a closer look. The usual steps are:
- Confirm it’s a cavity wall and roughly when it was built.
- Locate the ties with a metal detector, which shows where ties are and whether there are enough of them.
- Inspect with a borescope (sometimes called an endoscope) — a small camera on a probe passed through a drilled hole in a mortar joint, which shows the condition of the ties inside the cavity.
- In some cases, remove a brick to expose a tie and examine both ends.
BRE guidance sets out how to sample ties and classify the degree of corrosion. This matters because the tie can look fine in the cavity and be badly corroded in the outer leaf, where it’s wetter.
What does wall tie replacement involve?
Remedial wall ties are installed from outside, through the outer leaf into the inner leaf, at a pattern and spacing set by the specifier. Modern remedial ties are stainless steel and are fixed with resin or mechanical expansion, depending on the type and the masonry.
The old ties also need dealing with. Where they’ve been lifting the brickwork, they’re usually isolated or removed at the outer leaf so they can’t carry on expanding. Simply adding new ties next to rusting old ones doesn’t stop the cracking.
In more advanced cases, where the outer leaf has bulged significantly, sections may need rebuilding before or as part of the work.
The Property Care Association publishes a Code of Practice for installing remedial wall ties, and BRE has published guidance on replacing them. Reputable installers will follow both, test the ties once fitted, and provide a guarantee.
How much does wall tie replacement cost?
We don’t publish figures we can’t stand behind, and costs vary widely. The main drivers are:
- the area of wall affected — one elevation or the whole house
- access — scaffolding is often the biggest single cost on two-storey and taller houses
- whether old ties need removing or isolating, not just new ties adding
- any rebuilding of bulged or distorted brickwork
- making good — repointing and redecoration
Get more than one quote, and ideally get the extent of the problem confirmed independently before inviting quotes. A contractor who both diagnoses the problem and sells the cure has an obvious incentive.
Wall tie failure and buying a house
If you’re buying a pre-1980s cavity-walled house, wall ties are one of the things a surveyor will look at from outside. A survey can flag the signs — horizontal cracking, bulging, disturbance at the eaves — and recommend further investigation. It can’t confirm tie condition without drilling, which you can’t do in someone else’s house without permission.
What to do:
- Ask the seller whether ties have ever been replaced, and for any guarantee and the installer’s details. Ask too whether any guarantee can be transferred to you.
- If the survey flags it, get the ties inspected before exchange. A borescope inspection is a modest, quick investigation compared to the cost of finding out later.
- Use the findings to negotiate. If replacement is needed, it’s a known, quantifiable job. See using your survey to negotiate.
Cavity wall insulation is a related consideration on exposed walls. Our guide on whether cavity wall insulation causes damp covers that side.
Is wall tie failure dangerous?
In most houses it progresses slowly over years, and the main consequence is cracking and gradual distortion of the outer leaf. In severe cases, a badly corroded outer leaf has reduced resistance to wind, and BRE notes that in the worst cases walls can collapse. That’s rare, but it’s why the problem should be confirmed and dealt with rather than left.
Getting an independent view
If you’ve spotted horizontal cracking on a cavity-walled house and want to know whether it’s wall ties, roof spread or something else, our building defects investigation is designed to establish the cause before you spend money on a fix. We don’t install ties or carry out repairs, so our only interest is getting the diagnosis right.
If you’re buying, a Level 3 survey gives more depth on older or altered houses. Get in touch with the address and photos of the cracking.
Sources and further reading
- Cavity wall tie repairs — Property Care Association
- Code of Practice for the Installation of Remedial Wall Ties — Property Care Association
- Corrosion of steel wall ties: recognition and inspection (IP13/90) — BRE
- Replacing wall ties — BRE
- Corrosion of metal components in walls — BRE