Wall Ties Calculator — How Many Cavity Wall Ties Per m²?

Standard cavity walls need 2.5 ties per m², placed at 900mm horizontal and 450mm vertical centres in a staggered (diamond) pattern, where both leaves are at least 90mm thick (Approved Document A, PD 6697:2019). The density steps up to 5.0 ties per m² (450×450mm) where the thinner leaf is under 90mm, and a timber-frame inner leaf uses 4.4 ties per m² (rising to 7 per m² in severe wind). Building height and exposure govern the tie TYPE and may trigger a structural engineer’s assessment — they do not set a fixed per-m² density. Use our Blockwork Wall Calculator for the full wall material specification.

Brick-and-block cavity walls use the standard 2.5 ties per m² where both leaves are at least 90mm thick (Approved Document A Section 2C, PD 6697:2019). With UK standard 75mm brick coursing (65mm brick + 10mm bed joint), the 900mm horizontal × 450mm vertical maximum spacing works out to a tie roughly every fourth brick along the wall and on every sixth course up the height, set out in a staggered diamond pattern. The density does not change for an all-brick wall — 2.5/m² is the floor figure, and only a thin leaf (under 90mm) lifts it to 5.0/m².

2.5 ties per m² is achieved at 900mm maximum horizontal centres and 450mm maximum vertical centres, staggered so alternate rows are offset (a diamond grid), per Approved Document A and PD 6697:2019. These are maximum spacings, not targets — closer spacing is fine, wider is not. On standard 75mm UK brick coursing, 450mm vertical equals one tie every sixth course. The figure of 2.5 comes directly from 1 ÷ (0.9m × 0.45m) = 2.47, rounded up to 2.5/m².

Multiply your net wall area (length × height, less large openings if you wish) by the density: 2.5/m² for standard cavity walls, 5.0/m² for a thin leaf under 90mm, or 4.4/m² for a timber-frame inner leaf (Approved Document A, PD 6697:2019). Then add extra ties up both jambs of every opening, movement joint and free edge at one per 300mm of height within 225mm of the edge. Finally add about 10% for bent or dropped ties. As a rough check, a plain 10m² wall panel needs roughly 25 standard ties before openings and wastage.

The per-m² density stays at 2.5/m² for a standard wall regardless of cavity width — wider cavities change the tie LENGTH and, beyond about 150mm, the tie TYPE, not the count. Approved Document A Table 5 sets the length band (a 100mm cavity takes a 225mm tie, 125mm takes 250mm, 150mm takes 275mm), and cavities over roughly 150mm generally need a stiffer or two-part tie verified to BS EN 845-1, often with a structural engineer's check per PD 6697:2019. Always keep at least 50mm embedment in each leaf.

Approved Document A Table 5 bands each cavity width to a fixed stock tie length, giving at least 62.5mm embedment into each leaf: a 50–75mm cavity takes a 200mm tie, 76–100mm takes 225mm, 101–125mm takes 250mm, 126–150mm takes 275mm and 151–175mm takes 300mm. For a standard 100mm cavity you need 225mm ties. Above 175mm, take the cavity width plus 125mm and round up to the nearest stock length.

Yes. Approved Document A and PD 6697:2019 require additional ties up the two vertical edges (jambs) of every opening, movement joint and unbonded edge — placed at no more than 300mm vertical centres and within 225mm of the reveal. The head and cill of a window do not get this extra row. This calculator counts two jambs per opening based on the reveal height you enter, which is why it adds ties beyond the basic per-m² figure.

Wall ties should be inspected if the property was built before the mid-1980s. Before this, mild steel (black iron) ties were commonly used which can corrode, expand, and crack mortar joints. Signs include horizontal cracks in mortar beds (especially at every 6th course for butterfly ties), bulging walls, or rust staining. A surveyor can use a metal detector or borescope to assess condition.

For new build, austenitic stainless steel ties are standard, tested to BS EN 845-1. Ties are also graded Type 1 to Type 4 (DD 140-2 / PD 6697) by duty: Type 4 is light duty for housing up to about 10m, Type 2 general purpose up to 15m, and Type 1 heavy duty for tall or very exposed buildings. For remedial work, helical stainless steel ties are the most common — they’re drilled and mechanically fixed into the mortar. Resin-fix ties are used where the mortar is in poor condition. All wall ties should be stainless steel — galvanised mild steel is no longer recommended.