Quantities
Wall-tie allowance
- where
- field ties
- net wall area (m²)
- specified field tie density (ties/m²)
- edge ties
- number of separate edges
- height per edge (m)
- specified edge tie spacing (mm)
Explain this equation
This gives an allowance of wall ties for a cavity wall: ties across the main area of the wall, plus extra ties at edges such as openings and movement joints.
A is the net wall area and ρ is the specified field density in ties per m². e is the number of separate edges needing extra ties, h is the height of each edge in metres, and s is the vertical spacing of edge ties in mm. N_field, N_edges and N_total are the three tie counts.
Field ties are area times density, rounded up. Along each edge, the height in mm divided by the spacing, rounded up, gives the number of gaps; adding 1 gives the number of ties, because a line of ties has one more tie than it has gaps. The × 1000 converts the edge height into mm.
20 m² at 2.5 ties/m² is 50 field ties. Two 2 m edges at 300 mm spacing need 8 ties each, so 66 in total.
Watch out: this is a quantity allowance, not a designed layout. Edge and field ties may overlap.
Estimate field ties from a specified density, plus separate edge-tie positions.
Enter your values to see the result.
Good to know
Quantity allowance only. Use project tie type, density, spacing and edge offsets. Edge calculation includes both ends and may overlap field ties; do not count it as a designed layout. Defaults are examples, not a compliance check.
Sources
- NHBC wall tie guidance / project specification
Estimates are not design approval or British Standards compliance checks. Structural arch, retaining-wall, wind, foundation, lintel and masonry-capacity design are not performed. Use the project engineer’s specification.