Home / Guides / Building Regulations / Guide 7

Building Regulations · Guide 7

Strip foundation

Approved Document A · Foundations · Section 2E

A cavity wall for a two-storey house, built up from a concrete strip foundation in medium-shrinkage clay. It shows how deep and how wide the strip goes, how thick it must be, and how the two leaves, cavity and ties above it are set out. Getting the depth wrong on clay is one of the commonest causes of cracking in houses, so it is one of the first things building control checks on site.

The build

1Dig the trench

Example: two-storey house wall on medium-shrinkage clay.

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 37, with AD A Table 10, AD A 2E4 highlighted — Approved Document A, page 37

Approved Document A, page 36, with AD A 2E2(a) and (c), AD A Diagram 23 highlighted — Approved Document A, page 36

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

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

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

AD A Table 10

Approved Document A, page 37

In plain words

Table 10 gives the minimum strip width for each type of ground and the load on the wall in kN per metre run. For stiff clay (ground type III, can be dented slightly with a thumb) at up to 50 kN/m it is 500 mm. It applies only within the strict terms it sets out.

What it means here

Identify the ground by the field test, work out the wall load, then read off the width. The stiff clay and 50 kN/m load are our assumptions for this example, giving 500 mm; the strip shown is 600 mm.

In this build

Width at least the Table 10 minimum (assumed stiff clay, up to 50 kN/m: 500 mm)Strip foundation

AD A 2E4

Approved Document A, page 37

In plain words

Unless built on rock, a strip foundation should go at least 0.45 m down to its underside, to keep it below frost; this commonly needs to be deeper where frosts are long or to reach good ground. On shrinkable clay (clay that swells and shrinks as it wets and dries; AD A means a Modified Plasticity Index of 10% or more) the strip should go deep enough that ground movement, including the effect of trees and plants, won't harm the building, and its underside should be at least 0.75 m, 0.9 m or 1.0 m down for low, medium or high shrinkage clay. These depths may need to be increased to reach good ground or where there are trees nearby.

What it means here

These are minimums, not targets: trees, poor ground or long frosts push the depth down. The exhibit's clay is our example of medium shrinkage (0.9 m minimum); it is dug to 1 m.

In this build

Underside at least 450 mm below groundStrip foundation
On shrinkable clay at least 750, 900 or 1000 mm by shrinkage (example: medium, 900 mm)Strip foundation

Where it shows in the build

1 · Dig the trenchSubsoil

AD A 2E2(a) and (c)

Approved Document A, page 36

In plain words

(a) The foundation should sit centrally under the wall. (c) The concrete should be at least 150 mm thick, or P if that is more, where P is the projection (how far the strip sticks out past the wall face each side), worked out from Table 10 and Diagram 23.

What it means here

Centre the wall on the strip, then check the strip is at least 150 mm thick and no thinner than it sticks out each side. Here the strip is 600 wide under a wall about 300 wide, so it projects about 150 mm each side, and 225 mm thick is enough.

In this build

Thickness at least 150 mm, and not less than the projection each sideStrip foundation
Wall central on the stripStrip foundation

Where it shows in the build

2 · Pour the concrete strip

AD A Diagram 23

Approved Document A, page 36

In plain words

Diagram 23 shows the wall central on the foundation, the projection P each side, and the thickness T, which should be P or 150 mm, whichever is greater. The foundation width should be no less than the figure in Table 10.

What it means here

This is the picture behind 2E2(a) and (c): wall in the middle, equal projections, thickness at least P and 150 mm.

In this build

Wall central on the stripStrip foundation

AD A 2C8

Approved Document A, page 14

In plain words

Cavity walls should have leaves (the two skins) at least 90 mm thick and a cavity at least 50 mm wide. Ties should be at 900 mm horizontally and 450 mm vertically, or at least 2.5 ties per m². The two leaves plus 10 mm should be no less than the solid wall thickness from 2C6 and Table 3 for a wall of the same height and length.

What it means here

Here the brick leaf is 102.5 mm, the block leaf 100 mm and the cavity 100 mm. Ties at 900 across and 450 up meet the spacing. 102.5 + 100 + 10 = 212.5 mm, more than the 190 mm Table 3 asks for.

In this build

Each leaf at least 90 mm thickOuter leaf
Leaves plus 10 mm at least the Table 3 solid wall thickness (here 190 mm for a 3.5–9 m wall up to 9 m long)Outer leaf
Cavity at least 50 mm wideClear cavity
Each leaf at least 90 mm thickInner leaf
No more than 900 mm apart horizontallyWall ties
Rows no more than 450 mm apart verticallyWall ties

Where it shows in the build

3 · Inner leaf up to DPC level4 · Outer leaf up to DPC level9 · Wall 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 uses this figure to check a cavity wall. The wall height and length here are our assumption for a typical two-storey house wall.

In this build

Leaves plus 10 mm at least the Table 3 solid wall thickness (here 190 mm for a 3.5–9 m wall up to 9 m long)Outer leaf

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 leafWall ties
225 mm long for a 76–100 mm cavityWall ties

Where it shows in the build

9 · Wall ties

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 2E1: no non-engineered fill or wide variation in ground under the foundation, and no weaker ground below it.
AD A 2E2(b): concrete mix, and guidance for chemically aggressive soils.
AD A 2E2(d) and (e), Diagram 21: overlaps and step heights for stepped foundations.
AD A 2E2(f), Diagram 22: projections for piers, buttresses and chimneys.
AD A 2E2(c): trench fill foundations as an alternative to strip foundations.
AD A 2E3: the Table 10 widths may be used.

Examples and other sources

Cavity at least 50 mm wide: also AD C 5.13(b).
Partial-fill insulation leaving a clear cavity of at least 50 mm: AD C 5.15(a).
DPC at least 150 mm above ground: AD C 5.5(b).
Cavity fill top at least 225 mm below the lowest DPC: AD C 5.5(c).
Medium-shrinkage clay, stiff clay, a 50 kN/m wall load and a 3.5–9 m by up to 9 m wall: example assumptions, not from the site.
Brick 102.5 mm, block 100 mm, 50 mm insulation board, 600 x 225 mm strip, 1 m depth: example sizes.
Lean-concrete cavity fill, backfill and build order (inner leaf first, a lift at a time): good practice, not AD A rules.
Stainless steel ties: the material rule is AD A 2C19, not cited by this exhibit.

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.

Tell us about the building.

Send an address and a line about what you need. We aim to reply within one working day to discuss the next step and arrange a site visit where needed.

Start an enquiry 020 7096 9989