DPC and DPM Lapping in Timber Frame

100 mm

Every figure for this topic is on one page: Cavity trays and DPC: where are they needed and how high?

DPCs must be installed at openings to prevent rain penetration and over horizontal timber cavity barriers. At ground floor level, DPCs are fitted below sole plates and should lap with the DPM and air and vapour control layer (AVCL) by at least 100mm to ensure airtightness and moisture protection.

Basis in regulation

Part C
Approved Document C
Read the source

How to check it on site

Check that DPCs are correctly installed at openings, over cavity barriers, and adequately lapped with the DPM/AVCL at sole plate level.

Common failure

Inadequate lapping or incorrect installation of DPCs and DPMs compromises the building's airtightness and moisture resistance.

Last updated 2 September 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