What are wall ties, and how do I know if mine have failed?

A cavity wall's outer leaf is only half a brick thick, which on its own is nothing like stiff enough to stand against wind. The ties are what make the two leaves behave as a single wall. When they corrode they do two things at once: they stop tying, and they expand and push the brickwork apart. The second is what makes the problem visible. The cutaway shows where the ties sit and the crack rhythm their failure produces.

Safety

Step by step

1. Understand what a tie does.A metal component built into the bed joints of both leaves, bridging the cavity. It transfers wind load from the outer leaf to the inner one, and it stops the two leaves moving independently. Neither leaf on its own is stable against wind.
2. Know roughly where they should be.Ties are placed at regular horizontal and vertical spacings, commonly in the region of 900mm apart horizontally and 450mm vertically, with extra ties closely spaced up both sides of every opening and at movement joints. That regularity is what makes the failure pattern recognisable.
3. Note that ties slope outwards and have a drip.A tie is set with a slight fall towards the outer leaf and carries a twist or a drip in the middle, so any water crossing it falls into the cavity rather than tracking to the inner leaf. A tie laid the wrong way or fouled by mortar droppings becomes a damp bridge.
4. Look for the horizontal cracking rhythm.The give-away. Cracks along bed joints, repeating at a regular vertical interval up the wall, is not a pattern anything else produces. The regularity is more diagnostic than any single crack.
5. Look for bulging and for lifting at the verges.Expansion in the joints makes the outer leaf grow taller, which can lift a roof verge or open the joint under a sill. Sight up the wall from the corner — a bulge is far easier to see obliquely than face on.
6. Know which houses are at risk.Cavity construction with galvanised mild steel ties, broadly from the early twentieth century to the early 1980s, is where the problem sits. Exposure matters a great deal: a wall that stays wet corrodes ties far faster, so seaside and severely exposed elevations fail first.
7. Get a wall tie survey rather than guessing.A specialist uses a metal detector to find the ties and a borescope through a small hole into the cavity to look at their condition. It is a modest cost and it converts a suspicion into a fact. The same survey also finds mortar droppings and debris in the cavity.
8. Distinguish it from the alternatives.Sulphate attack also causes horizontal cracking, but without the regular spacing. Foundation movement causes stepped diagonal cracking. A single horizontal crack in one place is more likely a lintel, a bearing or a junction than ties.
9. Understand the repair.Remedial ties are installed from outside — drilled through the outer leaf, into or through the cavity, and mechanically or resin anchored into the inner leaf, at a designed spacing. Corroded ties are isolated or removed where they can be reached. It is done by contractors who specialise in it, usually with an insurance-backed guarantee.
10. Deal with the wetness as part of the repair.Ties corrode because they get wet. Pointing, gutters, cavity debris, a bridged cavity and saturating render all contribute, and fixing the wall without fixing the water shortens the life of the new ties.

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