DPM under concrete slab: role, thickness and installation

When you pour a ground-bearing slab, there is one invisible but absolutely critical detail for the health of your home over the next 50 years: the DPM (Damp-Proof Membrane) laid between the sub-base and the concrete slab. It weighs almost nothing, costs less than €1/m², and yet its presence determines the thermal comfort of your floor, the smell of your indoor air, the longevity of your ground floor insulation and the condition of your floor coverings. Forgotten, punctured or poorly installed, it turns your slab into a moisture pump. This article gives you the exact role of the DPM, the thickness to choose, the installation method and the mistakes that 80% of building sites make.

What is a DPM and what does it do

The DPM — short for Damp-Proof Membrane — is a low-density polyethylene (LDPE) sheet supplied in rolls 4 to 8 m wide and 25 m long. Its colour varies: clear, translucent blue, matt black. It is used in construction as a physical barrier to water and vapour wherever a dry volume must be protected from a damp contact.

Under a house slab, the DPM plays three complementary roles:

• Capillary barrier: it blocks the migration of moisture from the ground upward through the slab by capillary action (the micro-pores in concrete draw up water like a sponge)
• Vapour barrier: it prevents water vapour from the ground (especially in summer, when the earth is warm and the slab cool) from condensing in the insulation or in the concrete
• Mechanical separation: it prevents fresh concrete from losing its mixing water into the sub-base gravel, which would cause uneven setting and a weaker slab

What happens without a DPM

A well-built sub-base — 20 cm of clean, compacted 20/40mm gravel — is not a moisture barrier. It is a mechanical and drainage filter, but the air circulating between the stones is saturated with ground moisture. Without a DPM:

1. The fresh concrete loses its mixing water into the gravel within a few hours → incomplete set, a slab that crumbles at the surface
2. Once hardened, the slab draws up moisture from the ground by capillarity — 2 to 5 litres of water per m² per year migrate upward
3. This moisture rises into the insulation, which it degrades within 5 to 10 years (especially mineral wool and graphite EPS)
4. It eventually reaches the floor covering: warping timber, detaching tile adhesive, blistering vinyl
5. In parallel, water vapour laden with VOCs and radon from the ground contaminates the indoor air

The DPM costs €0.80 to €1.50/m² — fit it, always, without exception. It is the best value for money of your entire build.

Choosing the right thickness

DPM thickness is measured in microns (µm) — not millimetres. This is the key parameter: too thin, it punctures on the first sharp stone; too thick, it becomes rigid and hard to handle.

Reading the label

On a quality roll, you should find:

• Thickness in µm
• Standard NF T 54-021 (polyethylene sheet for construction) or equivalent EN 13984
• SD class (vapour diffusion resistance) — at minimum Sd > 150 m to act as a true vapour barrier
• CE marking if the product is intended for waterproofing use

Avoid cheap “site tarpaulins” sold by the trade counter without a data sheet. They may look similar but have a negligible Sd value and degrade in UV light within a few weeks.

Installation — step by step

Here is the method for laying the DPM on an already-compacted sub-base, before pouring the slab.

Step 1: Check the substrate

The sub-base must be clean, flat and compacted. Any protruding stone, any irregularity over 3 cm must be corrected before laying. A blinding sand layer of 2 to 3 cm over the 20/40mm gravel is ideal — it levels the surface and protects the DPM from sharp edges.

Step 2: Unroll the first sheet

Unroll the first sheet along the length of the room. Leave it overhanging 20 cm against each perimeter wall — you will fold it up afterwards.

• A standard roll is 4 or 6 m wide × 25 m long
• Work in pairs: one pulls the roll, the other smooths and removes wrinkles
• Do not drag the sheet over sharp stones: lift it or slide it on a piece of cardboard

Step 3: Lay subsequent sheets with overlaps

Each sheet must overlap the previous one by at least 20 cm. Between the two layers, a double-sided vapour-control tape (e.g. Ampacoll XT, Siga Sicrall, Isover Vario Multitape) ensures airtightness. Single-sided tape on top also works but is less durable.

1. Position the new sheet leaving a 20 cm lap over the previous one
2. Lift the edge of the new sheet
3. Stick the double-sided tape to the old sheet (or alternatively, apply a continuous wide adhesive tape over the overlap once both sheets are flat)
4. Fold back the new sheet and press by hand from the centre outward to push out any air

Step 4: Form upstands against the walls

The sheet must turn up against perimeter walls by at least 15 cm above the finished slab level. This upstand prevents moisture from the wall (even if the foundations are waterproofed) from bypassing the barrier at the sides.

1. Press the sheet flat against the wall
2. Tape the top of the upstand with acrylic building tape compatible with polyethylene (not standard packaging tape, which will peel off within 6 months)
3. Leave 5 cm protruding above the planned slab level — trim cleanly with a knife after the concrete has cured
4. At internal corners, form a fold (do not cut) to maintain continuity of the sheet

Step 5: Treat service penetrations

Every penetration through the slab (soil pipe, PVC sleeve, pre-cast electrical conduit, heating duct) is a weak point that must be carefully detailed.

1. Cut the DPM in a cross around the penetration, as tightly as possible
2. Turn up the 4 petals around the pipe by 5 cm
3. Secure with a jubilee clip or spiral tape
4. Add a pre-formed pipe collar (e.g. KS Klingspor or Siga Rissan) for sensitive penetrations (heating pipe, ventilation duct)

Step 6: Protect before pouring

Once the DPM is laid and before pouring the slab, install the reinforcing mesh and chair supports with care. The feet of steel chairs are the DPM’s number one enemy: they puncture it with every footstep.

• Use flat-footed chairs rather than pointed ones
• Or place plastic washers (spacer pads) under each chair
• Strictly forbid studded boots or loose nails on the floor
• Repair any visible tear immediately with a piece of DPM + double-sided tape

DPM and under-slab insulation: which goes underneath?

This is the most frequently asked question: should the DPM go under or over the insulation? The answer depends on the insulation type and the build-up.

The general rule: DPM under the insulation, directly on the sub-base. This way, the capillary barrier protects both the insulation AND the slab. Some site managers recommend a second thin DPM (100 µm) between the insulation and the slab as a slip layer — useful mainly under a floating screed, not essential under a standard slab.

Special cases

Very damp or clay ground

If your geotechnical survey (G2AVP) revealed swelling clay or a water table less than 2 m deep:

• Use 300 µm (≈ 1200 gauge) DPM or a reinforced DPM (e.g. Visqueen, Delta-PM)
• Double the overlap (30 cm instead of 20)
• Consider a bituminous membrane sump under the sub-base, in addition to the DPM under the slab

Slab over a crawl space

Over a crawl space, the slab is not in contact with the ground but with air. A DPM is not required in the capillary barrier sense, but it may be installed under the soffit insulation as a vapour control layer if the crawl space is very damp or unventilated.

Garage slab

Same principle as for a habitable house: 200 µm DPM between sub-base and slab. The temptation to skip it (“it’s only a garage”) is misguided: a damp garage slab means condensation on your car, corrosion of stored equipment and concrete that spalls in frost.

Floating screed on insulation

In this case, the DPM (100–150 µm) is laid on top of the insulation (between insulation and screed) to decouple the layers and prevent screed mixing water from migrating into the insulation. This is called a slip membrane or debonding layer, not a capillary barrier.

Mistakes to never make

Standards and regulations

• DTU 13.3 (French building standard — industrial-type floor slabs) — requires a watertight barrier under the slab
• DTU 52.10 (screed installation) — specifies a polyethylene slip membrane of minimum 150 µm
• DTU 26.2 (screeds and slabs on insulation) — defines conditions for vapour barrier installation
• NF T 54-021: standard for LDPE sheets in construction
• EN 13984: European standard for plastic sheeting used as vapour barriers

Cost for a 100 m² slab

That is €2.30/m² all-in, including self-build labour. It is the best investment relative to risk avoided of the entire structural phase.

Scheduling on site

The DPM fits between the completion of the sub-base and the laying of the reinforcing mesh:

1. Day 0: sub-base compacted, blinding sand layer placed
2. Day 1: DPM unrolled, overlaps formed, upstands completed, penetrations detailed
3. Day 1: reinforcing mesh installed with flat-footed chair supports
4. Day 1 or 2: concrete slab poured — ideally the same day or the next to minimise UV exposure