125mu (500G) Black Polythene Damp Proof Membrane (DPM) 4m x 25m (100m2/Roll)

What Is 125mu Polythene DPM ? 

125mu Polythene Damp Proof Membrane (DPM) acts as an unbroken waterproof shield beneath a building, stopping naturally occurring ground moisture from rising and seeping into the property's structure - a common issue known as 'rising damp'.

This flexible sheet is manufactured from high-quality low-density polythene (LDPE) and has a precisely defined thickness of 125 microns (125µm), also commonly referred to as 500 gauge (G). 

This specific thickness is carefully engineered to be substantial enough to effectively block moisture vapour from the ground, thereby safeguarding a building's foundations, floors, and internal environment from damp-related problems such as mould, rot, and structural weakening.

Understanding Thickness: Microns (µm) and Gauge (G) Explained

• What is a Micron? A micron (symbol: µm) is a very small unit of measurement, equal to one-millionth of a metre. To give you an idea, a human hair can be around 50-70 microns thick. So, 125µm means this DPM sheet is 0.125 millimetres thick.
• What is Gauge? Gauge (often abbreviated as 'G') is another way to express the thickness of polythene sheeting. For DPMs, the conversion is typically 1 micron = 4 gauge. Therefore, a 125-micron DPM is equivalent to 500 Gauge (125 x 4 = 500).
• Why Thickness Matters: The thickness of a DPM is critical to its performance. A thicker membrane generally offers: 
  • Greater resistance to tearing or being punctured during installation (e.g., by sharp stones in the sub-base or by site traffic).
  • A more robust and durable long-term barrier against moisture.
• 125µm / 500 Gauge - What does it mean for your project? This thickness is considered a reliable medium-duty DPM. It's suitable for many standard domestic and some light commercial applications where the ground beneath is well-prepared and there's no risk of significant water pressure from below. For very rough ground or heavy-duty situations, a thicker DPM (e.g., 250µm/1000G) might be recommended.

Applications

• Under Concrete Floors: Installed as a ground-level moisture barrier directly beneath new solid concrete floor slabs in houses, extensions, garages, and similar buildings.
• General Purpose Sheeting (Secondary Use): While designed as a DPM, its robust nature means it can also be used as a heavy-duty plastic sheet for temporary protection of materials or areas on a building site.

Benefits

• Effective Moisture Shield: Provides a strong barrier against rising damp, helping to keep the building's interior dry and protecting floor finishes and structural elements.
• Durable Material: The 125-micron (500 gauge) polythene offers good durability for standard site conditions, resisting common ground contaminants and rot.
• Economical Large Roll: The 4m x 25m roll covers a substantial 100m², which means fewer joints are needed across large areas, saving installation time and reducing potential weak points where moisture could seep through.
• Waterproof & Rot-Proof: Polythene is inherently waterproof and will not rot or decay when buried in the ground.
• User-Friendly: Being relatively lightweight and flexible, it's easier to handle, position, and cut on site compared to more rigid materials.
• Meets Key Standards (Check Manufacturer's Claims): Reputable DPMs like this are manufactured to meet performance criteria outlined in UK Building Regulations, often complying with standards like BS EN 13967.

Installation Guidelines

Correct installation is vital for a DPM to work effectively. Always consult the current UK Building Regulations (Approved Document C – which deals with site preparation and moisture resistance) and relevant British Standards (like BS 8215 – a code of practice for damp-proof courses) and industry guidance (e.g., NHBC Standards) for full details.

• Prepare the Ground (Sub-Base): The ground layer beneath the DPM, often made of compacted hardcore (broken stones/bricks providing a solid base), must be level and smooth. Crucially, it should be covered with sand blinding (a layer of soft sand, at least 50mm thick, well compacted) to create a protective cushion and prevent sharp stones from puncturing the DPM.
• Laying the DPM: Carefully unroll the DPM over the prepared sand blinding, ensuring it covers the entire floor area. Avoid dragging it over rough surfaces. Being relatively lightweight, this membrane is straightforward to handle.
• Overlapping (Lapping) & Sealing Joints: All adjacent sheets must be overlapped by at least 300mm (in line with current best practice, e.g., NHBC Standards 2025, 5.1.20). These laps must be meticulously cleaned and securely sealed using high-quality, compatible double-sided DPM jointing tape between the sheets. The edge of the lap should then be over-sealed with a compatible single-sided DPM lap tape. Apply firm, uniform pressure to all taped joints to ensure a complete, durable, and impermeable seal.
• Connecting to Wall DPC: The DPM in the floor must connect properly and continuously with the Damp Proof Course (DPC) in the surrounding walls. (A DPC is a similar barrier built into the walls themselves). The floor DPM should be carefully detailed and turned up at all perimeter edges to meet or overlap the wall DPC according to approved details, forming an unbroken moisture barrier.
• Dealing with Pipes (Service Penetrations): Any pipes, cables, or other services passing through the DPM require careful and durable sealing. This is typically achieved using proprietary pre-formed top-hat units or by meticulously taping the DPM to the pipe with appropriate high-performance DPM tapes and/or compatible mastics, ensuring a permanent, watertight seal around each penetration.
• Protect the Membrane: After laying, the DPM must be rigorously protected from any form of damage (punctures, tears) by site traffic, tools, or during subsequent construction activities like placing reinforcement or pouring concrete. Any punctures identified must be repaired immediately using a patch of the same DPM material, generously lapped (e.g., at least 150mm beyond the damage in all directions) and fully sealed with DPM jointing tapes as per jointing instructions. The DPM should be covered by the subsequent construction layer as soon as practically possible.

Disclaimer: This information is for general guidance only. Always refer to the specific manufacturer's product data sheet, current UK Building Regulations, and relevant British Standards for detailed technical specifications and installation instructions before use. Site conditions and project-specific requirements should always be taken into account.