Wall Tie Failure and Bulging Walls
Regular horizontal cracks in the brickwork, and a wall that has started to bow outward, are the classic signs of corroding wall ties. Here is what is happening inside the wall, and what it takes to put it right.
In short: a cavity wall is two leaves of brickwork held together by metal wall ties across the gap. In older walls those ties are mild steel, and as they rust they swell to several times their original size, forcing the mortar joints apart. The result is regular horizontal cracking, spaced evenly up the wall at roughly every sixth course, and eventually a wall that bows outward as the ties lose their grip. A modest, stable case is usually repaired with remedial stainless steel ties; a wall that is badly bulged or more than about 25 mm out of plumb needs an engineer's assessment and may need rebuilding.
How a cavity wall holds together
Most homes built from the early twentieth century onward have cavity walls: an outer leaf of brickwork, a gap, and an inner leaf, built as two separate skins. On their own, two thin leaves would be unstable, so they are pinned together across the cavity by wall ties, small metal ties bedded into the mortar joints at regular spacings, typically around every 450 mm up the wall and closer together at window and door reveals. The ties make the two leaves act as one and stop the outer leaf from peeling away.
The ties are doing quiet structural work, and everything depends on them staying sound. The problem in older housing is what those ties are made of.
Why the ties fail
For much of the twentieth century, wall ties were made of mild steel with only light galvanising for protection. Sitting in a damp cavity for decades, that protection eventually gives out and the steel corrodes. The crucial detail is that rust takes up far more space than the steel it comes from, several times the volume. As the tie rusts where it is bedded into the outer leaf, it swells and acts like a slow jack, prising the mortar bed joint apart. Do that along a line of ties, at the same height across the wall, and you get a crack running horizontally through the brickwork. Because the ties are also losing their strength as they corrode, the outer leaf gradually loses its restraint and begins to bow outward.
The tell-tale signs
Wall tie failure has a signature that is hard to mistake once you know it:
- Regular horizontal cracks. Fine cracks running along the mortar bed joints, evenly spaced up the wall at roughly every sixth course, following the line of the ties. This even spacing is the giveaway that separates it from a one-off crack.
- A bulge or bow. The outer leaf leaning or bellying out, most often on the elevations that take the most driving rain, such as a gable or an exposed flank wall.
- Cracking at openings. Because ties are closer together beside windows and doors, cracking often shows up around the reveals.
- A gable that has moved, sometimes with a horizontal crack near where the roof meets the wall.
These are the same horizontal cracks covered in our guide to types of cracks in walls, where horizontal patterns are flagged as the ones to take most seriously, precisely because they point to a wall being pushed or losing its restraint rather than simply shrinking.
Not every bulge is a wall tie
A bowing wall is worth diagnosing rather than assuming, because a few different problems produce a similar look. In solid walls, which have no ties at all, a bulge can come from decayed timber bonding pieces built into the inner face of older brickwork, or from an external wall that was never properly bonded to the wall next door. Higher up, a wall pushed out at the top can be roof spread rather than ties. Each has a different repair, which is why the first job is to work out what is actually happening before anyone drills a single hole.
When it is serious
Most wall tie cases are not an emergency, but they do not fix themselves and they get worse. The point at which a bulge stops being cosmetic and becomes a stability question is a judgement for an engineer, but as a general guide, a wall that is more than about 25 mm out of plumb, or bulging more than about 10 mm within a single storey height, should be assessed without delay. Any bulge that is visibly growing, or paired with wide cracking, should be treated as urgent. These thresholds are a rule of thumb to prompt a look, not a licence to leave a wall that is below them; a photo and a description are enough for us to advise.
How failed wall ties are fixed
As with any movement, the repair starts with diagnosis, not with filling the cracks. Once an engineer has confirmed that the ties are the cause and assessed how far the wall has moved, the common repairs are:
- Remedial ties. New stainless steel ties are drilled and fixed through the outer leaf into the inner leaf on a designed grid, restoring the connection between the two skins. The old corroded ties are isolated or removed so they cannot keep jacking the joints.
- Localised rebuilding. Where the outer leaf has bulged badly or cracked right through, sections are taken down and rebuilt true, then re-tied.
- Making good. Cracked bed joints are raked out and repointed once the wall is stable again.
This is the same engineering-led repair process we apply to any defect: diagnose the cause, specify the fix, and inspect the finished work. At EMA Structures we carry out crack and masonry repairs across London as well as diagnosing them, so the same team takes the wall from assessment through to a stable, made-good finish.
Worried about cracks or a bulging wall?
A few photos usually tell us a lot: a straight-on shot of the cracking, one along the wall to show any bow, and a note of which elevation it is. Send them over and a structural engineer will tell you whether it looks like wall ties and what the sensible next step is. A structural inspection with a full written report costs £495 + VAT, see our fees guide, correct as of July 2026.
Wall tie and bulging wall questions
What are the signs of wall tie failure?
The classic sign is regular horizontal cracking in the mortar bed joints, spaced evenly up the wall at roughly every sixth course of bricks, which is the spacing of the ties. It is often clearest on the most weather-exposed elevations. Other signs are an outward bulge or bow in the wall, cracking around window and door reveals, and on gable walls a horizontal crack near the roofline. Because the cracks follow the tie positions, the pattern is quite distinctive once you know to look for it.
Why do wall ties fail?
Older cavity walls were built with mild steel ties that were only lightly protected against corrosion. Over decades, moisture in the cavity rusts them, and because rust occupies several times the volume of the original steel, the expanding tie forces the mortar bed joint apart. That is what lifts and cracks the brickwork and, as the ties lose strength, lets the outer leaf bow outward. Modern ties are stainless steel and are not prone to this.
Is a bulging wall dangerous?
It can be, because a bulging outer leaf is losing its restraint and its ability to stay stable. As a general guide, a wall that is more than about 25 mm out of plumb, or bulging more than about 10 mm within a single storey height, should be assessed by a structural engineer without delay. A modest, stable bulge may simply need remedial ties, while a large or growing one can need sections of wall taken down and rebuilt. Any bulge that is visibly increasing should be treated as urgent.
How are failed wall ties fixed?
Once an engineer has confirmed the cause, the usual repair is to install remedial stainless steel ties, which are drilled and fixed through the outer leaf into the inner leaf to restore the connection between them, often after isolating or removing the old corroded ties. Where the outer leaf has already bulged badly or cracked through, localised sections may need rebuilding. The repair is specified to suit the wall, and the cracked joints are made good afterwards.
Do all cavity walls have this problem?
No. It affects older cavity walls built with mild steel ties, which became common through the twentieth century. Walls built more recently with stainless steel ties are not affected in the same way. Solid walls with no cavity do not have ties at all, so a bulge in a solid wall has a different cause, such as decayed internal bonding timbers or a wall that was never properly bonded to the wall next to it. That is why diagnosis comes first.
Cracks lining up across your brickwork?
Send us a photo and we will give you an honest first view, and a full engineer's inspection and report if it needs one.