Heavy Duty 1200 Gauge Damp Proof Membrane (4m x 25m)

In-Depth Look

New concrete ground floors in the UK need protection against ground moisture, and a damp proof membrane is the usual way to provide it. 1200 gauge (300 micron) DPM is a commonly specified thickness. Check what your building control body and any warranty provider require.

Why 1200 Gauge

Approved Document C requires ground floors to resist moisture from the ground, but it does not set a membrane gauge. 1200 gauge is a common specification because the thickness helps it resist puncture during construction while keeping an impermeable barrier. Thinner membranes risk being damaged by aggregate or foot traffic before the slab is poured.

Installation

The DPM is laid over the compacted hardcore or sand blinding, with joints lapped to the width in your specification (commonly 300mm) and sealed with DPM tape. It should be turned up at the edges to connect with the damp proof course in the walls, creating a continuous moisture barrier. Care must be taken to avoid puncturing the membrane during reinforcement placement and concrete pouring. Radon and gas barriers are different products, so check whether your site needs one.

Verdict

1200 gauge DPM is a common choice beneath ground floor slabs. Lay it with sealed laps, protect it during the pour, and confirm the specification with building control before you start.

How It Compares

A 1200 gauge damp proof membrane (DPM) is a polythene sheet, and 1200 gauge equals 300 micron. It is commonly sold in 4m by 25m rolls, which cover 100m2. It is used below a ground floor slab, or in a sandwich form within the floor build-up, to stop ground moisture reaching the slab and floor finishes.

The sheet is laid over a blinding layer, such as sand, which reduces the risk of sharp stones puncturing it. Laps in the sheet are sealed with DPM tape. Lap widths are quoted from 150mm to 300mm depending on the specification, so follow your own specification and the manufacturer’s instructions. At the edges of the slab, the DPM is turned up to link with the damp-proof course (DPC) in the walls, which forms a continuous barrier.

Ask building control or your warranty provider to confirm the specification for your project. They set what is acceptable, not the product page.

A DPM is only one layer in the floor build-up. A typical ground-bearing floor runs compacted hardcore, sand blinding, DPM, then insulation and the slab, although insulation can sit above or below the slab depending on the design, so follow your specification. Each layer has a job, and the order matters. Planning the sequence before the delivery arrives helps, because a membrane laid and then walked over by a gang with barrows is easy to damage. Store the roll flat and dry, and keep it out of strong sunlight until it is needed. Follow the manufacturer’s instructions and your specification.

Common Questions

What is 1200 gauge, and why is it used under slabs? Gauge is an older way of describing the thickness of polythene, and 1200 gauge equals 300 micron. A thicker sheet is more resistant to tearing during installation, which matters when concrete and reinforcement are placed on top. It is a common choice for floor slabs.

How wide should the laps be, and how should they be sealed? Lap widths are quoted from 150mm to 300mm depending on the specification, so follow yours. Seal the laps with DPM tape, as the manufacturer instructs, and keep the surfaces clean and dry so the tape sticks.

Why do I need sand or blinding under the DPM? Hardcore has sharp edges, and a sheet laid straight onto it can be punctured. A layer of sand blinding smooths the surface and reduces the risk of puncture.

How do I link the DPM to the DPC, and avoid punctures when placing mesh and concrete? Turn the DPM up at the edges of the slab so it meets the DPC in the walls and forms a continuous barrier. Use spacers under the mesh so it does not dig in, avoid dragging barrows over the sheet, and check for holes before pouring. Confirm the detail with your building control or warranty provider.