300mu (1200G) Black Polythene Damp Proof Membrane (DPM) 4m x 25m (100m2/Roll)

What Is 300mu Polythene DPM?

300mu Polythene Damp Proof Membrane (DPM) provides the highest standards of ground moisture protection when installed beneath concrete floor slabs. Its core purpose is to halt 'rising damp' - the pervasive issue of naturally occurring ground moisture seeping upwards into a property's structure. 

This flexible sheet is manufactured from high-quality low-density polythene (LDPE) and has a precisely defined, substantial thickness of 300 microns (300µm), also commonly referred to as 1200 gauge (G).   

Its premium thickness firmly places this DPM in the extra heavy-duty category. Its use safeguards a building's foundations, floors, and internal environment from damp-related problems such as mould, rot, and structural weakening, making it ideal for the most demanding site conditions.   

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, 300µm means this DPM sheet is a significant 0.30 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 300-micron DPM is equivalent to an impressive 1200 Gauge (300 x 4 = 1200).
• Why Thickness Matters: The thickness of a DPM is critical to its performance. A thicker membrane, like this 300mu version, offers: 
  • Superior resistance to tearing, punctures, and damage during the installation process, even with challenging sub-bases or heavy site traffic.   
  • An extremely robust and highly durable long-term barrier against moisture, offering maximum peace of mind.   
• 300µm / 1200 Gauge - What does it mean for your project? This thickness designates an extra heavy-duty DPM. It is the preferred choice for a wide array of domestic, commercial, and industrial applications where maximum durability and protection are non-negotiable. This includes sites with poor or aggressive ground conditions, areas subject to higher loadings, or where the highest specification for damp proofing is required by architects or engineers. It provides an exceptional level of resilience.   

Applications

• Under Concrete Floors (High-Performance): Installed as a ground-level moisture barrier directly beneath new solid concrete floor slabs in all types of construction - houses, extensions, and critically, in commercial or industrial buildings where a superior, highly resilient membrane is specified. Ideal for contaminated land applications when used as part of an approved system.
• Extra Heavy-Duty General Purpose Sheeting (Secondary Use): Its exceptional toughness makes it suitable for the most demanding temporary protection tasks on building sites, such as heavy-duty screening or covering large areas and valuable materials.

Benefits

• Maximum Moisture Shield: Provides an extremely strong, reliable, and long-lasting barrier against rising damp, ensuring the building's interior remains dry and protected.
• Superior Durability & Resilience: The 300-micron (1200 gauge) polythene offers market-leading durability with maximum resistance to punctures, tears, and abrasion during and after installation, while also 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 – even with this thicker material.   
• Waterproof & Rot-Proof: Polythene is inherently waterproof and will not rot or decay when buried in the ground, ensuring its integrity for the lifetime of the floor.   
• Manageable Installation (for its class): While notably heavier and somewhat stiffer than thinner membranes due to its thickness, it remains a flexible LDPE product that can be effectively installed with due care and attention.   
• Meets Highest Standards (Check Manufacturer's Claims): DPMs of this superior thickness are manufactured to meet the most rigorous performance criteria outlined in UK Building Regulations and often hold BBA certification or comply with standards like BS EN 13967.

Installation Guidelines

Correct installation is absolutely critical for a high-performance DPM to function 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) 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. It is absolutely essential that it is 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. Given its substantial weight and increased stiffness, this 300mu membrane will typically require two or more operatives for safe and effective handling and positioning.
• 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 significant, uniform pressure to all taped joints to ensure a complete, durable, and impermeable seal, noting that the material's thickness may require extra attention to achieve full adhesion.
• 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, often project-specific, details, forming an unbroken moisture barrier.
• Dealing with Pipes (Service Penetrations): Any pipes, cables, or other services passing through the DPM require exceptionally well-sealed and durable solutions. This is typically achieved using proprietary pre-formed top-hat units designed for thicker membranes, or by employing specialised high-bond 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, however small, 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, especially when specifying a premium product like a 300mu DPM.