A damp proof membrane (DPM) is a physical barrier that prevents moisture rising from the ground through a floor slab. A damp proof course (DPC) is the equivalent barrier in a wall, preventing rising damp from reaching the habitable fabric of a building. Both are mandatory under Building Regulations Approved Document C and, when correctly installed and linked to each other, form a continuous moisture-resistant envelope at low level.
Failures in DPM or DPC — wrong gauge, insufficient laps, gaps at penetrations, or failure to connect DPM to DPC — are among the most common causes of damp in UK buildings. The fix, once the floor is laid, is expensive; getting it right at construction costs almost nothing extra.
DPM vs DPC: What Each Does
| Item | Location | Purpose | Key Material |
|---|---|---|---|
| DPM (damp proof membrane) | Under floor slab or screed | Prevents ground moisture rising through floor | Polythene sheet, liquid-applied membrane |
| DPC (damp proof course) | In masonry wall at ≥150 mm above external ground | Prevents rising damp in wall | Polythene strip, engineering brick, slate |
| Radon barrier | Under slab in radon-affected areas | Resists radon gas as well as moisture | Reinforced HDPE membrane |
These are separate elements that must be physically connected. If the DPM under the slab is not lapped up the inside face of the wall and linked to the DPC in the wall, ground moisture can track up through the wall cavity or inner leaf below the DPC and enter the building.
Polythene DPM: Gauges and Grades
Polythene DPM is specified by thickness in micrometres (µm) or, traditionally in the UK, in gauge (where 1 gauge = 0.254 µm, making 1200 gauge = 304.8 µm ≈ 300 µm). The Building Regs minimum is 1000 gauge (250 µm), but 1200 gauge is the industry standard and the minimum most specification clauses and NHBC require.
| Gauge | Thickness (approx µm) | BBA / BR requirement | Typical Cost (m²) | Use |
|---|---|---|---|---|
| 1000 ga | 250 µm | Minimum per Part C | £0.30–£0.55 | Not recommended for new build |
| 1200 ga | 300 µm | Standard | £0.40–£0.80 | Standard ground-floor DPM |
| 1200 ga reinforced | 300 µm + mesh | Radon class 1 | £0.90–£1.60 | Radon and moisture barrier combined |
| 2500 ga | 625 µm | Heavy-duty | £1.20–£2.00 | Basements, aggressive ground |
Reinforced membranes incorporate a woven polypropylene mesh laminated between two polythene layers. This resists puncture during concrete pouring and is required in radon-affected zones (see below).
Polythene DPM is typically supplied in rolls 4 m wide × 25 m or 50 m long. Rolls of 2 m width are available for narrow applications. Store rolls flat and away from sunlight; UV degrades polythene rapidly.
Position: Above or Below the Slab?
Both positions are used and both are acceptable under Part C, but they have different characteristics:
DPM above the slab (under screed):
- Protects the screed and floor finish from ground moisture
- The slab itself can absorb moisture, which may cause heave in highly expansive clay if the slab is cast before the site dries out
- Easier to inspect and repair if damaged before screed is laid
- Not radon-resistant (the slab below is not sealed)
DPM below the slab:
- Protects the entire slab from ground moisture and radon
- The slab acts as a structural protection for the membrane during construction
- Must be carefully placed and protected during concrete pour — punctures cannot be seen or repaired once the slab is cast
- This is the standard position for radon barriers and is now the more common domestic specification
Most structural engineers and the NHBC specify DPM below the slab for new construction. For a renovation where a new screed is being laid over an existing slab, a DPM above the existing slab (under the new screed) is acceptable.
Radon: When a Radon Barrier is Required
Radon is a naturally occurring radioactive gas that can accumulate to harmful concentrations in poorly ventilated buildings, particularly in parts of the UK with granite geology. Areas of significant radon potential are defined by UKHSA (formerly PHE) mapping and incorporated into BS 8485:2015+A1:2019.
Radon protection requirements (Part C and BS 8485):
- Basic protection (1–3% of homes above action level): 1200-gauge polythene DPM with properly taped laps — a standard DPM also provides basic radon protection if correctly installed
- Full protection (> 3% of homes above action level): Reinforced HDPE membrane (minimum 0.25 mm) with all laps mechanically joined or heat-welded, plus a passive sump system or under-floor ventilation void
Check radon potential at your site using the UKHSA Radon Maps service or the postcode checker before specifying membranes. Full radon protection areas include parts of Cornwall, Devon, Somerset, Derbyshire, Northamptonshire and Lincolnshire.
Laps and Continuity
Laps in DPM joins are a critical detail. Polythene has no adhesion to itself; laps must be:
- Minimum 300 mm wide (Part C minimum)
- 150 mm minimum for taped joins (where a specialist DPM jointing tape is used)
- Turned up at wall perimeters at least 150 mm above the finished floor level
- Linked to the wall DPC by either overlapping or being bedded in the DPC mortar joint
At penetrations (pipe sleeves, duct entries), the membrane must be cut and fitted tightly around the penetration and sealed with self-amalgamating tape or a proprietary pipe collar. Unsealed penetrations are one of the most common DPM failures.
Linking DPM to DPC in the Wall
The DPM under the slab and the DPC in the wall must form a continuous barrier. The typical detail:
- DPC is laid in the inner leaf of the cavity wall at ≥150 mm above external ground level (Part C minimum)
- DPM is turned up the inner face of the inner leaf to overlap the DPC by at least 150 mm
- The turned-up DPM is held in place by the screed, insulation or timber batten (floor construction dependent)
- At the cavity, the DPC extends across the full width of the cavity (or a separate cavity tray is linked to both leaves)
Where the floor insulation sits on top of the DPM (below-slab position), the insulation layer bridging up to the wall face provides additional protection for the turned-up edge.
DPC in Walls: Types and Cost
| Type | Material | Standard | Cost (approx) | Notes |
|---|---|---|---|---|
| Polythene strip | LDPE sheet | BS 6515:1984 | £0.20–£0.45/m run | Most common in modern construction |
| Bitumen polymer | Felt + bitumen | BS EN 14967 | £0.40–£0.90/m run | Older build; remains serviceable if intact |
| Engineering brick | Class B engineering brick | BS EN 772 | £8–£18/m run inc. labour | Durable, no failure risk; used in severe exposures |
| Slate (two courses) | Natural slate | Traditional | £15–£30/m run inc. labour | Pre-1920s buildings; survives when mortar fails |
| Chemical injection | Silicone/boron solution | BBA Certificate required | £40–£80/m run (remedial) | For existing walls with no original DPC |
In new masonry construction, polythene DPC strip is universal. The course is bedded in mortar and lapped 100 mm at joints. Where the DPC sits below a timber plate (e.g. at wall plate in a timber-frame wall), a heavier-duty DPC to resist abrasion is advisable.
Building Regulations and Standards
- Approved Document C (2004, amended 2013) — site preparation and resistance to moisture; Sections 4 (floors) and 5 (walls) cover DPM and DPC requirements
- BS 8102:2022 — code of practice for protection of structures against water from the ground (basements)
- BS 6515:1984 — specification for polyethylene damp proof courses (DPC in walls)
- BS 8485:2015+A1:2019 — code of practice for design of protection against radon gas; specifies membrane classes
- NHBC Standards Chapter 5.1 — floors: DPM thickness and position requirements
- BRE Report BR 211 — radon: guidance on protective measures
Always specify membranes with a BBA (British Board of Agrément) certificate or equivalent UKCA-marked product. This ensures the membrane meets the declared performance in the certificate and gives an audit trail for Building Control sign-off.
