In this guide
Cavity wall ties sit on a grid. Approved Document A of the Building Regulations sets them at 900mm apart horizontally and 450mm apart vertically, with extra ties clustered within 225mm of window and door reveals. That grid is exactly why cavity wall tie failure produces such a recognisable defect. It is not a random crack. It is horizontal cracking in the mortar beds that repeats up the wall at roughly the same interval, over and over.
If the cracking on a house you are buying looks oddly regular, wall ties are the first thing an experienced surveyor will think of.
In short: Cavity wall tie failure happens when the steel ties joining a home’s two masonry leaves corrode and expand, lifting the brickwork above them. The signature is horizontal cracking along mortar joints at regular vertical intervals, sometimes with a bulging outer wall. A Level 3 Building Survey identifies the pattern and tells you whether a specialist investigation is worth commissioning.
The crack pattern that points to wall ties
A standard brick course, brick plus mortar joint, is about 75mm. Ties at 450mm vertical centres therefore land roughly every sixth course. When they corrode and swell, the crack follows the mortar bed the tie sits in, so the damage appears as a series of near-horizontal cracks stacked up the elevation at a repeating height.
Three secondary signs usually travel with it:
- Cracking concentrated around openings. The Building Regulations require additional ties down the sides of windows and doors at closer vertical centres, so there is simply more metal in those zones to corrode.
- A bulging or bowing outer leaf. Enough expanding ties will push the outer skin of brickwork outwards, and the bulge is often clearest when you sight along the wall from one corner rather than looking at it head on.
- One elevation far worse than the others. Across our patch in Cambridgeshire, Lincolnshire and Rutland, the south-westerly elevations take the driving rain, and that is where the cavity stays wettest.
What it is not: diagonal, tapering, stepped cracking that runs through bricks as well as joints, widest at the top and narrowing downwards. That pattern points at ground movement beneath the foundations rather than corroding metalwork, and we cover that separately in our guide to ground movement and what causes it. Getting the two confused is common, and it matters because the remedies have nothing in common.
Why the ties fail in the first place
The mechanism is simple and slightly counterintuitive. Steel that rusts does not just get weaker, it gets bigger. The iron oxide that forms occupies considerably more volume than the metal it replaced, and a tie bedded in mortar has nowhere to put that extra volume. So it pushes upwards, against the brick sitting on top of it.
One tie doing that is nothing. Several hundred doing it across an elevation lifts the outer leaf and cracks it from the inside out.
Galvanised ties are protected by a zinc coating, and that coating does its job well for a long time. But it is thin, and a cavity is a damp place. Once moisture has worked through to the bare steel, corrosion starts and does not stop. Cavity construction became common in British housing from the 1920s onwards, and galvanised steel ties remained standard for decades before stainless steel became the default. Broadly, the older the cavity wall, the more worth a careful look it is.
What most guides on this get wrong: almost every article you will find about wall tie failure is published by a company that sells wall tie replacement. Read them, by all means. But get the diagnosis from someone with no financial interest in the answer. That distinction is the whole reason independent surveyors exist.
How serious is the crack? A width-first triage
Crack width is the first filter, and RICS is refreshingly blunt about it: there is rarely any cause for real concern unless cracks appear suddenly and are more than 3mm wide. The same guidance notes that cracks may need measuring and monitoring for as long as 12 months before anyone can say what they are doing.
| What you can see | What it usually means | Sensible next step |
|---|---|---|
| Hairline cracking, no pattern, no bulge | Almost always decorative or thermal movement | Note it, redecorate, move on |
| Fine horizontal cracks in mortar beds, repeating up the wall | Early tie corrosion is plausible | Flag to your surveyor and have the pattern recorded |
| Cracks over about 3mm, or clearly widening | Beyond cosmetic, whatever the cause | Investigation before exchange, not after |
| Cracking plus a visible bulge in the outer leaf | Advanced tie failure is the leading explanation | Specialist wall tie investigation, priced quotes |
Width alone is not the whole story, though. A 2mm crack that is growing month on month is a more interesting problem than a 4mm crack that has been the same width since 1987 and has old paint sitting undisturbed inside it. Pattern, distribution and movement over time all feed into the judgement.
What a Level 3 survey will and won’t tell you
A RICS Level 3 Building Survey is the right level of inspection for a property showing this kind of cracking. The surveyor records the crack pattern and widths, maps where the damage is concentrated, sights the elevations for bulging, and reads all of that against the property’s age, construction and exposure. The report then sets out the likely cause and what it means for you as a buyer.
Here is the honest limit. A home survey is non-invasive. Nobody is drilling into your seller’s brickwork to look at the state of the metal inside it, and confirming the condition of the ties themselves needs a borescope investigation carried out by a specialist contractor. Blue Blossom Surveys does not carry out borescope wall tie investigations, and we are not going to pretend otherwise.
What we do is more useful at this stage of a purchase: we tell you whether the evidence justifies commissioning that specialist, or whether you are looking at cracking that does not warrant the expense. That is a genuinely different service from being quoted for the remedy by the firm that would carry it out.
A Level 2 Homebuyer Survey will still flag visible cracking and give it a condition rating. Where the pattern is pronounced, or the house is older and the cracking is repeating, Level 3 gives you the reasoning behind the rating rather than just the rating.
If cavity wall tie failure is confirmed, what happens to the purchase
Remediation is well established and not exotic. A contractor removes or isolates the failed ties and installs replacement stainless steel ties retrospectively, drilled and fixed through the outer leaf, usually with an insurance-backed guarantee on completion.
Cost depends almost entirely on things you can see before anyone quotes: how much wall area is affected, how many ties are involved, whether scaffolding is needed, and how accessible the elevations are. Treat any figure quoted before a proper inspection as an opening position rather than a price. Get two or three quotes, and ideally not only from the firm that diagnosed it.
On the purchase itself, a confirmed structural defect gives you a legitimate basis to go back to the seller, which is the same ground covered in our guide to what to do when a survey turns up problems. Be aware that the lender’s own valuer may also spot the cracking, and a lender can hold back part of the advance until works are completed. Tell your broker as soon as the report lands. Early notice gives you options; late notice removes them.
Wall tie failure also belongs on the shortlist of findings worth taking seriously rather than shrugging off, alongside the other survey findings that should give a buyer pause. It is fixable. It is just not something to discover after completion.
Frequently asked questions
How do I know if my wall ties have failed?
The clearest indicator is horizontal cracking along mortar joints that repeats up the wall at a regular height, roughly every sixth brick course, often worse around window and door reveals and on the most weather-exposed elevation. A bulge in the outer brickwork alongside that pattern strengthens the case considerably. Confirmation requires a borescope investigation by a specialist contractor.
Are horizontal cracks always wall tie failure?
No. Horizontal cracking can also result from thermal movement, a failed lintel, cavity tray issues or wall plate movement at roof level. What distinguishes tie failure is the repetition at consistent vertical intervals across an elevation rather than a single crack in one location. That is the pattern a surveyor is reading, not the crack in isolation.
Can you get a mortgage on a house with wall tie failure?
Usually yes, but it depends on severity and on what the lender’s valuer reports. Minor early corrosion often passes without comment. Where cracking is significant or the outer leaf is bulging, a lender may hold back funds until remedial works are completed and signed off, or ask for a specialist report before proceeding. Speak to your broker as soon as the finding appears.
How much does it cost to replace cavity wall ties?
There is no meaningful single figure, and be sceptical of anyone who gives you one before inspecting the property. Price is driven by the affected wall area, the number of ties, access and whether scaffolding is required, so a single elevation on a semi-detached house and a full wrap of a detached property are entirely different jobs. Get at least two quotes and check the guarantee that comes with the work.
If a property you are buying is showing cracking you cannot explain, an independent opinion is worth having before you commit. Blue Blossom Surveys covers Peterborough, Stamford, Grantham, Oakham, Corby, Wisbech and the surrounding area, carrying out RICS Level 2 Homebuyer Surveys and RICS Level 3 Building Surveys. We diagnose; we do not sell the repair. Get in touch to talk through what you are seeing.
Reviewed and signed off by Blue Blossom Surveys. Published 13 October 2026. Last updated 13 October 2026.
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