Minimum thickness of strip and trench fill foundations

150 mm

The thickness of a strip foundation must be equal to its projection beyond the wall face or a minimum of 150mm, whichever is greater, typically ranging from 150mm to 500mm

The thickness of a strip foundation must be equal to its projection beyond the wall face or a minimum of 150mm, whichever is greater, typically ranging from 150mm to 500mm. Trench fill foundations require a minimum thickness of 500mm to ensure adequate load distribution and structural integrity.

Basis in regulation

Part A
Approved Document A
Read the source

How to check it on site

Verify the depth and thickness of both strip and trench fill foundations meet the minimum specified requirements.

Common failure

Foundations are often poured to insufficient depths or thicknesses, compromising their ability to safely transfer loads and leading to potential structural failure.

Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.

Related dimensions at this stage

Minimum

Ready-mixed concrete temperature and curing

5 °C

Ready-mixed concrete delivered to site should have a minimum temperature of 5°C, as specified in BS EN 206

Minimum

Low-Level Protection for Timber Frame and SIPs

150 mm

For closed panel timber frame and structural insulated panels (SIPs), the lowest timber element must be positioned at a minimum of 150mm above the finished ground level

Minimum

Foundation depth during cold weather

450 mm

When construction occurs in cold weather, foundations should be at least 450mm below existing ground level, or adequate measures must be taken to prevent ground freezing

Maximum

Pipe support spacing and thermal movement

110 mm

Horizontal drainage pipes, typically 110mm diameter, require support at regular intervals, often around 900mm to 1000mm centres, or at every joint/socket

Minimum

Traditional manhole construction

100 mm

Traditional manholes feature a concrete base at least 100mm thick and walls constructed from brick, blockwork, or concrete appropriate to ground conditions

Minimum

DPC and Cavity Tray Locations

150 mm

Damp-proof courses (DPCs) must be installed at the base of walls, at least 150mm above the external ground level, and linked to the damp-proof membrane (DPM) in solid floors to prevent rising damp