Minimum Timber Clearance and Drainage Holes

150 mm

The lowest timber components of a structure must be at least 150mm above finished ground level, or 75mm where conditions permit

The lowest timber components of a structure must be at least 150mm above finished ground level, or 75mm where conditions permit. Essential drainage holes are required at the base of the cavity to discharge any moisture.

Basis in regulation

Part C
Approved Document C
Read the source

How to check it on site

Verify the minimum clearance of timber from ground level and ensure that cavity drainage holes are present and clear.

Common failure

Insufficient clearance of timber from ground level can lead to moisture damage and rot in structural elements.

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