TL;DR: A damp proof membrane (DPM) is a heavy-duty polythene sheet laid beneath concrete slabs or against walls to stop ground moisture rising into a building. Choosing the right gauge—1000g, 1200g, or 2000g—depends on the ground conditions and the level of protection required. Get it wrong, and your floor will know about it.
Damp is the uninvited guest that never leaves. It seeps in quietly, says nothing, and then causes thousands of pounds worth of damage before anyone notices. Fortunately, there's a straightforward way to stop it at the door—or, more accurately, beneath the floor.
A good damp proof membrane is one of the most cost-effective investments in any construction project. It's not glamorous. Nobody's putting photos of their DPM on Instagram (well, we hope not). But skip it, and you'll be dealing with damp floors, failing screeds, and a very unhappy building inspector.
Whether you're a ground worker, a site manager, or a contractor who's been quietly Googling "what gauge DPM do I need" at 7am — this guide has you covered. We'll walk through what a damp proof membrane actually does, which gauge is right for which job, and why getting this part right saves serious headaches further down the line.
What Does a Damp Proof Membrane Actually Do?
At its core, a DPM creates a continuous physical barrier between the ground and whatever sits on top of it — usually a concrete slab. Groundwater contains moisture vapour, and without a membrane in place, that moisture travels upward through the concrete by capillary action. Left unchecked, it causes floor coverings to lift, adhesives to fail, screeds to delaminate, and structural materials to degrade over time.
Beyond moisture control, a well-installed polythene sheeting layer also helps protect against radon gas ingress in certain high-risk areas — something Building Regulations (Part C) take very seriously. So it's doing more than one job, quietly and without complaint. Honestly, an underappreciated hero of the construction world.
What Gauge DPM Do You Need?
Gauge — measured in microns (µm) or grams per square metre (g/m²) — tells you how thick the membrane is. Thicker means more durable, more puncture-resistant, and better suited to demanding ground conditions. Here's a practical breakdown:
Is 1000g DPM Enough for Your Project?
A 1000g damp proof membrane (roughly 250 microns) is the entry-level option and is commonly used for light-duty applications where ground conditions are relatively clean and well-prepared. It meets the minimum requirements for many standard domestic builds, though it offers less resistance to tears and punctures during installation.
Think of it as the economy option — perfectly adequate when conditions are ideal, but a little less forgiving if your sub-base is less than pristine.
When Should You Use a 1200g Damp Proof Membrane?
The 1200g damp proof membrane is the middle ground, and for most standard construction projects, it's the sweet spot. 1200g polythene sheeting offers improved puncture resistance during installation, better durability when laid over uneven or rougher substrates, and solid compliance with BS 8102 for below-ground waterproofing guidance.
If you're unsure which gauge to specify and the ground conditions are average, the 1200g DPM is the sensible default. It handles most residential and commercial groundwork applications without issue. 1200g polythene is stocked widely for exactly this reason.
What Are the Benefits of a 2000g DPM for Heavy-Duty Applications?
This is where things get serious. A 2000g damp proof membrane (500 microns) is designed for demanding ground conditions — think aggressive backfill, sharp aggregate, high hydrostatic pressure, or commercial and industrial projects where failure simply isn't an option.
2000g polythene sheeting is significantly more resistant to tearing, puncturing, and stress during compaction. It also offers superior long-term performance in wet or chemically aggressive ground. If you're building on difficult land or specifying for a commercial client who wants belt-and-braces protection, the 2000g DPM is the right call.
2000g polythene is also commonly used as a temporary cover sheeting on sites where heavy foot traffic or plant movement is expected — it takes a beating and keeps doing its job.
Does Colour Matter When Choosing Polythene Sheeting?
Short answer: not for performance. Long answer: sometimes, yes — depending on your application.
Black polythene is the most widely used option for DPM applications. The carbon black additive that gives it its colour also provides UV stabilisation and adds to its durability. For below-ground use, it's the industry standard.
Clear or natural polythene tends to be used more for temporary protection, wrapping, or applications where visibility matters. For a damp proof membrane, stick with black — it's the tried and tested choice and one that survives any scrutiny from a building inspector who's having a difficult morning.
What About Fire Retardant Polythene?
For projects where fire safety is a consideration — think occupied sites, refurbishments, or works near ignition sources — standard polythene may not be suitable. Fire retardant polythene and flame retardant polythene sheeting is manufactured with additives that slow the spread of flame, meeting the requirements of certain site safety regulations.
It's worth noting that fire retardant grades are available across multiple gauges and configurations, so you don't have to sacrifice performance for safety compliance. If your project specifies FR materials, make sure your sheeting matches the spec — don't assume all polythene rolls behave the same under heat.
How to Install a Damp Proof Membrane Correctly
Getting the right material is half the battle. Installation matters just as much. A membrane full of holes or poorly lapped joints is barely more effective than no membrane at all.
Key installation steps:
- Prepare the sub-base — Remove sharp stones, debris, or anything that could puncture the membrane before it's laid. A sand blinding layer is often used to create a smooth, protective surface.
- Lay the membrane flat — Avoid stretching or bunching the sheeting. Smooth it out across the entire floor area.
- Lap the joints correctly — Overlaps should be a minimum of 300mm, sealed with appropriate jointing tape to maintain continuity.
- Turn up the edges — The membrane should extend up the internal wall face to tie into the wall's DPC (damp proof course), creating a complete moisture barrier.
- Protect during concrete pour — Once laid, avoid unnecessary foot traffic over the membrane before the slab is poured. Even a small puncture can compromise performance.
How Many Polythene Rolls Do You Need?
This one's simple maths — but it's worth double-checking before you order. Calculate the total floor area, add extra for overlaps (a minimum of 300mm per joint), and factor in the edge turn-ups. Running out mid-pour is not a position anyone wants to be in.
Polythene rolls are available in a range of widths and lengths to suit different project scales. Ordering slightly more than you think you need is always the smarter call — offcuts have a habit of coming in useful.
Frequently Asked Questions About Damp Proof Membranes
What is the minimum thickness for a damp proof membrane under a concrete slab?
Building Regulations in the UK (Approved Document C) require a minimum thickness of 300 microns (approximately 1200g) for a DPM beneath a concrete floor slab. Some specifications call for heavier grades, particularly in areas with high ground moisture or aggressive subsoil conditions.
Can I use a damp proof membrane on an existing concrete floor?
Yes. In retrofit situations, a DPM can be laid on top of an existing slab and overlaid with a new screed. In this case, the membrane acts as a vapour control layer rather than a ground barrier. A 1200g or 2000g grade is typically recommended to handle the screed load without puncturing.
How long does a polythene damp proof membrane last?
When properly installed and undamaged, a quality polythene DPM has a service life of 50+ years. The material is chemically inert, resistant to ground moisture, and does not degrade in normal below-ground conditions.
What's the difference between a DPM and a vapour barrier?
A DPM is installed below a concrete slab to prevent ground moisture rising into the structure. A vapour control layer (VCL) is used within the building fabric — typically in walls or roofs — to manage condensation risk. They perform similar functions but in different locations and at different stages of the build.
Is fire retardant polythene required on all construction sites?
Not universally, but it may be specified by a principal contractor's site safety policy, particularly on occupied or sensitive sites. Always check the project specification and site rules before ordering.
Can polythene sheeting be used for purposes other than DPM?
Absolutely. Polythene sheeting is one of the most versatile materials on a construction site — used for temporary weatherproofing, concrete curing, surface protection, dust control, waste containment, and more. Having the right grade for the right job is the key.
Stop Damp in Its Tracks — Shop Our Full Range
A solid build starts with sound groundwork, and a quality damp proof membrane is a non-negotiable part of that foundation. Get the gauge right, install it correctly, and this is one problem you won't be solving again ten years from now.
At Indigo Supplies, we stock a comprehensive range of polythene sheeting for the construction industry — from 1000g DPM for standard applications through to heavy-duty 2000g damp proof membrane and flame retardant polythene for specialist projects. We've been supplying trade customers since 1998, we hold a 5-star rating, and we offer a price beater guarantee — so if you find the same product cheaper elsewhere, we'll do our utmost to beat it.
Click here to buy now and browse our full range of polythene rolls and DPM sheeting. Fast delivery, competitive pricing, and stock levels that won't leave your team standing around waiting.