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Building Regulations · Guide 5

Cavity wall and ties

Approved Document A · Walls · 2C8, Table 5 · Published 11 October 2026

The ground-floor wall of a two-storey house with a window, built a lift at a time. It shows the minimum leaf and cavity sizes, how the ties are chosen and spaced, and the extra ties needed beside an opening. Missing or badly spaced ties let the two skins act separately, which can lead to bulging or cracking, so tie spacing is checked on site.

The build

Cavity wall and ties: the finished build

1Inner leaf first, a lift at a time

Example: ground-floor wall of a two-storey house, with a window opening. These simple rules apply to small buildings that meet the size and other limits in AD A Section 2C.

Step by step

The source

What Approved Document A says

The pages of Approved Document A this guide relies on, in the order they come up in the build. Each clause is highlighted; the notes beside the page follow as you read.

Approved Document A, page 14, with AD A 2C8 highlighted — Approved Document A, page 14

Approved Document A, page 18, with AD A 2C19 highlighted — Approved Document A, page 18

Approved Document A, page 23, with AD A Table 5 (with note 2) highlighted — Approved Document A, page 23

Approved Document A, page 16, with AD A Table 3 highlighted — Approved Document A, page 16

Approved Document A, page 25, with AD A 2C24(a) highlighted — Approved Document A, page 25

AD A 2C8

Approved Document A, page 14

In plain words

Leaves at least 90 mm thick, cavity at least 50 mm wide. Ties at 900 mm horizontally and 450 mm vertically, or at least 2.5 ties per m². Extra ties, no more than 300 mm apart vertically, within 225 mm of the vertical edges of all openings, movement joints and roof verges. The two leaves plus 10 mm should be no less than the solid wall thickness from 2C6 and Table 3.

What it means here

Normal ties go in a 900 x 450 grid; any other pattern is fine if it gives at least 2.5 ties per m². At each side of the window an extra column of ties runs up at 300 mm centres, here 150 mm from the edge. 102.5 + 100 + 10 = 212.5 mm, more than 190 mm.

In this build

Each leaf at least 90 mm thickOuter leaf
The two leaves together plus 10 mm at least the Table 3 solid wall thickness (190 mm for a typical house wall; here 102.5 + 100 + 10 = 212.5 mm)Outer leaf
Cavity at least 50 mm wide (here 100 mm in all)Clear cavity
Each leaf at least 90 mm thickInner leaf
Spaced 900 mm across and 450 mm up, or any pattern giving at least 2.5 ties per m²Wall ties
No more than 300 mm apart vertically (here 300 mm)Extra ties at the opening
Within 225 mm of the edge of the opening (here 150 mm)Extra ties at the opening

AD A 2C19

Approved Document A, page 18

In plain words

Wall ties should comply with BS EN 845-1, be austenitic stainless steel (material reference 1 or 3 in its Table A1), and be chosen using Table 5.

What it means here

Use stainless steel ties marked to BS EN 845-1, in the length Table 5 gives for the cavity. Galvanised ties do not meet this.

In this build

Austenitic stainless steel to BS EN 845-1Wall ties

AD A Table 5 (with note 2)

Approved Document A, page 23

In plain words

Table 5 gives the tie length for each band of cavity width: 225 mm ties for a 76 to 100 mm cavity. Note 2: the tie should be embedded (bedded into the mortar joint) at least 50 mm in both leaves.

What it means here

Cavity width is the gap between the two leaves (AD A 2.3), so it includes any insulation board in it; read the tie length off for that width. A 225 mm tie across a 100 mm cavity leaves 62.5 mm in each leaf, which clears the 50 mm minimum.

In this build

Embedded at least 50 mm into each leaf (here 62.5 mm)Wall ties
225 mm long for a 76–100 mm cavityWall ties

AD A Table 3

Approved Document A, page 16

In plain words

For a wall over 3.5 m and up to 9 m high and up to 9 m long, the minimum solid wall thickness is 190 mm for its whole height.

What it means here

2C8 compares the cavity wall against this. Treating this as a typical house wall in that band is our assumption.

In this build

The two leaves together plus 10 mm at least the Table 3 solid wall thickness (190 mm for a typical house wall; here 102.5 + 100 + 10 = 212.5 mm)Outer leaf

Where it shows in the build

Insulation board

AD A 2C24(a)

Approved Document A, page 25

In plain words

Vertical load on walls should be spread out (distributed). This may be assumed for concrete floor slabs, precast concrete floors and timber floors designed to Section 2B, and where lintel bearing (how far the lintel sits on the wall each side) is 150 mm or more; where the lintel's clear span is 1200 mm or less the bearing may be reduced to 100 mm.

What it means here

The 900 mm opening here could use 100 mm bearings; the model uses 150 mm each side, which is more than enough. Lintel sizing itself is not covered.

Where it shows in the build

Lintels

Approved Document A: Structure (2004 edition incorporating 2010 and 2013 amendments). Crown copyright, reproduced under the Open Government Licence v3.0. Highlights and notes are ours.

Good to know

Also in this section, not shown here

AD A 2C4 and Diagram 1, 2C14 to 2C16: the building size, floor area, load and height limits that must be met before Section 2C applies.
AD A 2C5 and Diagram 3: wall thickness rules apply only when the building and wall conditions are met.
AD A 2C6: solid walls at least 1/16 of the storey height.
AD A 2C12: single leaf of small single-storey non-residential buildings and annexes may be 90 mm.
AD A 2C20 to 2C22: masonry units, compressive strength and mortar.
AD A 2C28 to 2C31: openings, recesses, overhangs and chases, including sizes of openings and piers.

Examples and other sources

Partial-fill insulation leaving a clear cavity of at least 50 mm: AD C 5.15(a).
BS EN 845-1 is named in AD A 2C19; its contents are not covered here.
Drip in the middle of the tie: product detail, explained for the reader, not an AD A rule.
Cavity closers round the opening: outside Part A, not covered.
Brick 102.5 mm, block 100 mm, 100 mm cavity, 50 mm board, 900 mm opening, ties 150 mm from the opening: example sizes.
Building the inner leaf first, a lift at a time: good site practice.

This guide explains Approved Document A (2013 edition, with 2010 and 2013 amendments) for England. It is general guidance, not professional advice: check your own project with Building Control or a structural engineer.

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