What Are the Requirements for Damp-Proof Membranes in Floors and Walls?

Quick Answer: A damp-proof membrane (DPM) in a ground floor slab must be at least 1200 gauge (300 micron) polythene, continuous, lapped and taped at joints, with a minimum 150mm upstand at all edges linking to the wall DPC. Approved Document C requires a DPM in all new ground-bearing concrete floors. For walls, the damp-proof course (DPC) must be a minimum 150mm above external ground level and must be a continuous horizontal barrier of impermeable material to BS 6515 or BS 743.

Summary

Damp-proof membranes and damp-proof courses are foundational requirements in UK building construction. They prevent ground moisture from rising through capillary action into floors and walls, which would otherwise lead to structural damage, finishes failure, and health issues associated with damp living conditions. Despite being a well-established requirement, failures in DPC and DPM installation or specification are among the most common defects found in surveys of UK housing.

The terminology is sometimes confused. A damp-proof course (DPC) is a horizontal (or occasionally vertical) barrier in a wall. A damp-proof membrane (DPM) is a continuous sheet installed under or within a floor slab. Both serve the same fundamental purpose — breaking the capillary path between ground moisture and the building fabric — but they are different products with different installation requirements.

In new build, requirements are set out primarily in Approved Document C (Site preparation and resistance to contaminants and moisture) of the Building Regulations. Approved Document C also covers radon protection, which overlaps significantly with DPM specification and in some parts of the UK must be combined with the DPM.

Key Facts

• Minimum DPM thickness — 1200 gauge (300 micron) polythene to BS 6515 is the accepted minimum; higher specification membranes (e.g. 1500–2000 gauge) are used where ground conditions are more severe or where puncture risk is higher
• Approved Document C — the primary regulation covering DPMs and DPCs in new and materially altered buildings; requires protection against moisture from the ground in all habitable floors
• 150mm upstand — the DPM must be turned up at all edges a minimum 150mm and linked to the wall DPC to create a continuous moisture barrier at the floor/wall junction
• DPC height — wall DPC must be a minimum 150mm above finished external ground level; a DPC installed lower than this can be bridged by external paving or soil build-up
• BS 6515:1984 — specification for polyethylene damp-proof courses for masonry; defines minimum thickness and physical properties; the reference standard for polythene DPCs
• BS 743:1970 (withdrawn but still referenced) — specification for materials for damp-proof courses including bituminous and flexible materials; largely superseded by product-specific standards but still widely cited
• Radon membrane — in radon-affected areas (high or medium radon potential), the DPM must be upgraded to a radon membrane (minimum 300 micron polyethylene or proprietary radon-resistant membrane), all joints taped, and laps minimum 300mm; may also require a sub-slab void or mechanical ventilation
• DPM position — can be above or below the slab (known as "oversite DPM" below concrete, or "sandwich DPM" above concrete below screed); each position has different implications for thermal mass and cold bridging
• Linking DPC and DPM — at the floor/wall junction, the DPM and wall DPC must be linked; if on different levels, a vertical DPM strip connects them
• Slab thickness — typical ground floor slab is 100mm concrete (C25/30 mix) over compacted hardcore; minimum 150mm of clean hardcore or sand blinding
• Cavity wall tray — where the ground floor DPM is below the cavity, a cavity tray DPC must bridge the cavity above the insulation to prevent lateral moisture transfer
• Failed DPC — historic buildings with no DPC, or with a failed DPC bridged by rendering or floor screeds, can often be diagnosed by the presence of efflorescence (white salt deposits) at low level, a distinct tideline of dampness on internal plaster, or by a calibrated moisture meter survey

Quick Reference Table

Detailed Guidance

Ground Floor DPM Installation

The DPM is typically laid after the hardcore and blinding are compacted and levelled, before or after the slab pour depending on the specified position.

Below-slab DPM (oversite) This is the simplest and most common position. The DPM is laid on clean sharp sand blinding (minimum 25mm) over compacted hardcore. The polythene sheet is lapped at joints by a minimum 150mm and taped with a compatible tape. At all edges, the DPM is turned up the wall face and secured temporarily before the slab is poured. The slab covers the DPM completely.

Puncture during the pour is a risk. Avoid dragging reinforcement mesh across the membrane. Where thermal insulation is being incorporated at slab level (below the slab is unusual, but possible in some passive construction types), the insulation boards are typically placed on top of the DPM, with the slab poured over the insulation.

Above-slab DPM (sandwich) The slab is poured first, then the DPM is placed on the slab surface before the screed. This approach keeps the slab within the thermal envelope (useful for thermal mass), but requires the DPM to be protected from puncture by the screed aggregate — typically by placing a 50mm sand layer over the DPM before screeding.

The 150mm upstand must still be achieved; the DPM must be turned up at all edges and linked to the wall DPC. With an above-slab DPM, this linking junction can be awkward to detail, particularly at the inner face of a cavity wall.

Wall DPC Requirements

A horizontal DPC in a cavity wall must be positioned at two levels:

1. In the inner leaf — at or just above floor level, continuous across the full leaf width, minimum 150mm above external ground
2. In the outer leaf — at the same or slightly lower level, continuous across the full outer leaf width

Both must be continuous. The cavity tray bridges the cavity between the two leaves at window and door openings and at any level change between inner and outer leaves. At lintels, a combined lintel with integral DPC tray is standard (Catnic, IG, Teplo). At the base of the wall, the cavity tray sits above the cavity insulation with weep holes at 900mm centres on the outer leaf to allow any water collecting on the tray to discharge.

DPC materials include polythene (most common in new build, lightweight, easy to join), bituminous felt (traditional, still used in conservation and repair work), lead (used historically; still appropriate in conservation areas where lead flashings are maintained), and engineering brick (two courses as a physical DPC — used in traditional solid-wall construction).

Linking DPC and DPM

The connection between the wall DPC and the floor DPM is critical and is frequently poorly detailed on site. The intent is to create a continuous impermeable barrier from the base of the wall DPC down through the junction and across the floor. If the junction is not properly sealed, ground moisture can enter at the skirting/floor interface and rise up the internal wall plaster.

The standard approach for a cavity wall with a below-slab DPM:

• The DPM is turned up against the inner face of the inner leaf before the slab is poured
• The DPC in the inner leaf sits at or slightly above slab level
• The DPM upstand overlaps the DPC by at least 150mm; they are bonded with compatible mastic or pressure-sensitive tape
• The junction is embedded in the slab or screed and concealed by the skirting

Where the DPC and DPM are on very different levels (for example, a new DPM in an existing building with a historic DPC at a higher level), a vertical DPM strip bridges the gap — essentially a strip of DPM material fixed vertically on the wall face, bonded to the DPC above and the horizontal DPM below.

Radon DPM Requirements

In England and Wales, the radon potential map (BRE/PHE indicative atlas) identifies high, medium, and low radon risk areas. In areas of high radon potential (above 3% of homes expected to exceed the Action Level of 200 Bq/m³), full radon-protective measures are required by Building Regulations Part C. In medium-risk areas, basic radon protection (enhanced DPM plus provision for a future sump) is required.

A radon DPM is a 300 micron polyethylene sheet, typically Visqueen Gas Membrane or equivalent, with all joints lapped a minimum 300mm and taped with compatible tape. The key distinction from a standard DPM is that the continuity and sealing requirements are more stringent — every penetration (pipes, cables) must be sleeved and sealed.

In the highest-risk areas, a radon sump is provided below the slab, ventilated naturally or by a fan to depressurise the subfloor zone. The DPM forms the lid of this depressurised zone and must be perfectly sealed. Radon membranes must also be resistant to gas permeation — standard polythene is acceptable at 300 micron; thicker or proprietary cross-laminated membranes offer enhanced protection.

Diagnosing Failed or Missing DPC/DPM

In existing buildings, the absence or failure of a DPC is a common finding in surveys. Diagnostic signs include:

• Distinct tideline on internal plaster at a consistent height above floor level (typical of rising damp from a failed or absent DPC)
• Efflorescence (white salt deposits) on brickwork at low level
• High moisture meter readings on internal wall plaster that diminish at height above floor level
• Salt analysis showing ground salts (nitrates, chlorides) in the plaster — a chemical signature of rising damp

Note that these signs can be mimicked by penetrating damp and hygroscopic salts from historic dampness. A full moisture investigation by a CSRT surveyor, including salt analysis, is needed to confirm rising damp before DPC treatment is specified.

For failed ground floor DPM, signs include damp patches on a solid floor, damp rising at the floor/wall junction, and carpet or wood flooring moisture-damaged at the edges.

Remedial DPC Options

Where no DPC exists or the existing DPC has failed, the options are:

1. Chemical injection DPC — proprietary silane/silicone or siloxane-based fluid injected under pressure into a course of mortar joints; creates a water-repellent band in the masonry; the most common remedial approach; BS 6576 covers installation; must be carried out by a CSRT-qualified contractor
2. Physical DPC insertion — the wall is cut in short sections (600mm at a time) and new DPC material inserted; used for walls too narrow for injection or where structural continuity must be maintained; labour-intensive
3. Electro-osmotic DPC — titanium anode strips installed in the wall connected to earth; attempts to reverse the electrical potential driving capillary rise; low cost but limited evidence base; not widely recommended by mainstream surveyors
4. Drainage and ventilation — indirect approach: improving external drainage to reduce soil moisture, installing sub-floor ventilation to reduce humidity; reduces but does not eliminate rising damp

Frequently Asked Questions

Do I need a DPM in an extension with an existing damp-proof floor?

Yes. Building Regulations Part C requires a DPM in any new ground-bearing floor. Even if the existing house floor does not have a DPM (which may be the case in older properties), the extension must comply with current regulations. The extension DPM and DPC must be continuous and linked; if the existing house has no DPC at the same level, a detail connecting the extension DPM to the external wall DPC level is required.

Can I use a concrete slab without a DPM if it is thick enough?

No. Concrete alone does not act as a DPM — it is permeable to moisture vapour. A 100mm slab with no DPM will allow moisture vapour to rise and will condense under impermeable floor finishes (vinyl, ceramic tile), causing adhesive failure and floor damage. The DPM is required in addition to the slab.

What is the difference between a DPM and a vapour control layer?

A DPM (damp-proof membrane) is specifically designed to block liquid water and water vapour from the ground. A vapour control layer (VCL) is used in wall and roof construction to control vapour diffusion through the building fabric. The terminology can overlap — some DPMs can also serve as VCLs — but they are specified for different purposes and locations.

How do I fix a failed DPC in a Victorian house without disturbing the floors?

The most practical option is chemical injection through the mortar course at the appropriate level. A CSRT surveyor will specify the product and method. The injection mortar course is typically the 3rd or 4th course above floor level (minimum 150mm above finished floor). After injection, the associated plaster should be hacked off, the wall allowed to dry, and new renovating plaster applied to manage residual hygroscopic salts.

Regulations & Standards

• Building Regulations Approved Document C — Site preparation and resistance to contaminants and moisture; DPM and DPC requirements for new build and material alterations

• BS 6515:1984 — Specification for polyethylene damp-proof courses for masonry

• BS 743:1970 — Specification for materials for damp-proof courses (withdrawn but widely referenced)

• BS 6576:2005 — Code of practice for diagnosis of rising damp and installation of chemical DPCs

• BRE BR 211 — Radon: guidance on protective measures for new dwellings; radon DPM specification

• BRE — Radon Protection for New Buildings — BR 211 guidance on radon membranes and sump specification

• Property Care Association — Rising Damp and DPC Guidance — CSRT qualification and member guidance on DPC diagnosis and treatment

• Visqueen Building Products — DPM Technical Datasheets — product specifications for ground floor and radon membranes

• NHBC Standards Chapter 5.1 — oversite, drainage, and ground floor construction requirements

• UK Health Security Agency — Radon Indicative Atlas — radon risk area mapping for England and Wales

• penetrating damp — horizontal moisture ingress; distinguish from rising damp

• basement waterproofing — BS 8102 system types for below-ground structures

• cavity drainage membrane — Type C managed drainage for existing basements

• salt damp diagnosis — hygroscopic salt analysis and its role in damp diagnosis