Wall tie failure is one of the few structural defects with a genuinely characteristic signature, which makes it satisfying to diagnose. It is also a defect where almost every page you will find online is published by a company that sells the remedy, which makes an independent opinion worth having.
What wall ties do, and why they fail
A cavity wall is two separate leaves of masonry with a gap between them. Neither leaf is stable on its own; the ties bridging the cavity are what make the two act together. Without them the outer leaf is a tall, thin, unrestrained wall.
The failure mechanism is corrosion, and the important part is what corrosion does to the tie's volume. Steel expands as it rusts, so a tie corroding inside a mortar bed joint does not simply weaken - it swells, and jacks the joint open. That is why the damage is so regular: the ties sit in a grid, and every tie in a course lifts its bed joint by a similar amount.
Age is the main driver. Ties made before the early 1980s generally carried thinner protective coatings than later ones, and the earliest were often little more than mild steel. Exposure matters too - the weather-facing elevation usually goes first.
What is normal
Not every horizontal crack is a wall tie. Cavity walls also crack horizontally at damp-proof course level, at cavity trays, at floor restraint positions and above openings, and none of those is tie corrosion. Isolated horizontal cracking in one place is much more likely to be something else.
It is also worth saying that a great many houses of the right age have some tie corrosion without it being a problem yet. The presence of corrosion is not the same as the failure of the wall.
The signature to look for
- Horizontal cracking in the mortar bed joints, repeating at regular vertical intervals up the wall - the spacing reflects the tie grid rather than anything about the building's shape
- Cracking that is worse on the most weather-exposed elevation, often a gable or a front elevation facing the prevailing wind
- Stepped or vertical cracking at the corners, where the expansion of a whole elevation has to go somewhere
- The outer leaf bulging outwards, most obviously when sighted along the wall from a corner
- A roofline or a course of brickwork that is no longer straight, because cumulative jacking has lifted the wall unevenly
- Rust staining or a visible tie end where mortar has spalled away
- Windows and doors that no longer sit square in an elevation that has grown taller than it was
What you can check yourself this week
- Sight along each elevation from a corner at a shallow angle. Bulging shows up dramatically this way and is nearly invisible face-on.
- Measure the vertical spacing between horizontal cracks. Regular, repeating spacing is the giveaway; irregular spacing points elsewhere.
- Look at all four elevations, not just the one with the crack. A single affected elevation facing the weather is consistent; damage on the sheltered side is unusual.
- Check whether the cracking follows mortar joints continuously across the elevation, or is local to one feature such as a window head.
- Photograph with a scale in shot and date it, then repeat in a few months.
How it is properly diagnosed
Visual signs raise the question; they do not answer it. Proper diagnosis means locating the ties with a metal detector, then inspecting them - usually with a borescope through a small drilled hole, and by exposing a sample of ties to see their actual condition and type.
That matters because the survey determines the scope, and the scope determines the price. There is a real difference between a wall needing a full replacement tie system and a wall needing localised work on one elevation, and it is not a difference you can see from the pavement.
The commercial problem, stated plainly
Wall tie replacement is sold by contractors who carry it out. That is not a criticism of the trade - remedial tie systems are well developed and the work is straightforward when it is genuinely needed. But the firm inspecting your wall and the firm quoting to re-tie it are, in most cases, the same firm.
We do not install ties, and we take no commission from anyone who does. That leaves us free to reach the conclusions that are commercially inconvenient elsewhere: that the cracking is thermal rather than tie-related, that the corrosion is present but not yet consequential, that one elevation needs work and three do not, or that the specification you have been quoted is heavier than the evidence supports.
When to call an engineer
A threshold, not a plea. Get it looked at if you have regular repeating horizontal cracking, if any elevation is bulging when sighted along, if a roofline or brick course has gone out of line, or if you have been given a re-tie quotation and want the diagnosis checked before you spend the money. That last one is a common and entirely sensible instruction.
If you have a single horizontal crack in one place and nothing else, it is much more likely to be a lintel, a cavity tray or a DPC than a tie problem.
How we would investigate it
Inspection of all elevations with the cracking classified against the BRE Digest 251 categories, sighting for bulging, tie location and borescope inspection where the signs warrant it, and a written opinion on cause, extent and what actually needs doing - elevation by elevation rather than as a single blanket recommendation.
A structural survey and report is a fixed £580 + VAT, with the report inside 10 working days of the inspection, guaranteed. Where remedial work is justified we specify it; we do not carry it out.
Frequently asked questions
How do I know if my horizontal cracks are wall ties?
The signature is regularity. Tie-related cracking repeats at consistent vertical intervals up the wall because the ties sit in a grid, and it is usually worst on the most weather-exposed elevation. A single horizontal crack in one location is far more likely to be a lintel, a cavity tray or the damp-proof course.
Which houses are affected?
Cavity-walled housing is the population at risk, and age is the main driver - ties made before the early 1980s generally carried thinner protective coatings, and the earliest were close to bare mild steel. Interwar and post-war housing is where we see it most.
Can wall ties be checked without damaging the wall?
Partly. A metal detector locates them without damage, and a borescope inspection needs only a small drilled hole. Seeing a tie's actual condition and type usually means exposing a sample, which is minor, localised work and is made good afterwards.
Is it dangerous?
Advanced tie failure is serious, because it leaves the outer leaf as a tall unrestrained wall, and that is a genuine stability concern. Most cases we see are nowhere near that point. The distinction between corrosion being present and the wall being at risk is exactly what an inspection establishes.
I have been quoted for a full re-tie. Should I get a second opinion?
If the quotation came from the firm that diagnosed it, yes - and it is a common reason people instruct us. The scope is what drives the cost, and there is a real difference between one exposed elevation and a whole house. An independent assessment either confirms the specification or saves considerably more than it costs.