Damp Proofing: DPC vs DPM Explained

Understanding Damp Proofing: DPC and DPM Explained

In the realm of property maintenance and construction, the battle against moisture is a constant one. Dampness can wreak havoc on structures, leading to costly repairs, compromised structural integrity, and unhealthy living environments. Two key components in the defence against rising damp and penetrating moisture are the Damp Proof Course (DPC) and the Damp Proof Membrane (DPM). While often used interchangeably in casual conversation, these elements serve distinct, albeit related, purposes. This comprehensive guide will delve into the intricacies of DPCs and DPMs, exploring their functions, installation, and crucial considerations, particularly when dealing with concrete slabs and brickwork.

What is a Damp Proof Course (DPC)?

A Damp Proof Course (DPC) is a barrier within a masonry structure, typically laid horizontally, designed to prevent moisture from travelling up through the walls via capillary action. Think of it as a horizontal break in the porous material that allows water to rise. These are commonly found in the lower courses of brickwork in buildings. Traditional DPCs were often made from materials like bitumen felt, lead, or slate. Modern DPCs are typically made from flexible plastic materials, such as polypropylene or PVC, often reinforced with glass fibre for added strength and durability. The primary function of a DPC is to interrupt the path of rising damp, a phenomenon where groundwater is drawn upwards through the pores of building materials like bricks and mortar.

What is a Damp Proof Membrane (DPM)?

A Damp Proof Membrane (DPM), on the other hand, is a waterproof barrier used to prevent the passage of moisture through a solid floor or wall. Unlike a DPC, which is a relatively narrow band within a wall, a DPM is typically a continuous sheet or coating that covers a larger area. DPMs are frequently used in ground-level construction, such as laying concrete floors on the ground, to prevent moisture from the soil below from migrating upwards into the building. They are also used in basements and other below-ground structures. Common DPM materials include heavy-duty polythene sheeting, bitumen, or specialised liquid-applied membranes.

Can a DPC be installed on a concrete slab?

The question of installing a DPC directly onto a concrete slab requires careful consideration of its purpose. Generally, a DPC is designed to be integrated within masonry. However, if the intention is to create a barrier at the interface between a concrete slab and a brick wall built upon it, a DPC can be incorporated. Membranes should be laid either onto a concrete slab or onto a minimum 5mm sand blinding (if laid below a floor slab). This ensures a stable and level surface for the DPC. In situations where a DPC needs to bridge a gap or discontinuity on a concrete surface before brickwork commences, it's crucial that the DPC is correctly positioned and sealed to prevent moisture ingress. It's also worth noting that other DPMs, with appropriate third-party certification and installed according to manufacturer's instructions, may be considered for similar applications.

Can a DPC be used to bridge damp proof concrete?

Using a DPC to 'bridge' damp-proof concrete isn't the most conventional approach and can be problematic. Concrete, even when damp-proofed, can still retain and transmit moisture. If a DPC is simply laid over damp concrete, the soil beneath can draw moisture from the concrete, potentially leading to issues like cracking. Furthermore, dampness can still rise through the concrete itself if not adequately contained. To effectively prevent rising damp when working with concrete, it's essential to use a DPM as the primary barrier. This DPM should be continuous and properly sealed. If you have existing damp-proof concrete and are building up with bricks, you will need to ensure a proper damp course is established to protect the bricks above the concrete. This means the DPC or DPM should be continuous and overlap correctly to prevent moisture from bypassing the barrier and causing rising damp in the newly constructed brickwork.

Do I need to lap DPM & DPC together?

Yes, lapping is absolutely critical when installing DPCs and DPMs, especially where they meet or overlap. Proper lapping ensures a continuous, unbroken barrier against moisture. Imagine a DPC meeting a DPM at a wall junction. If they are not lapped correctly, moisture can find a way through the gap. The general principle is that the DPC should overlap the DPM (or vice versa, depending on the specific detailing) by a sufficient margin, typically at least 150mm, and be sealed or integrated to form a watertight connection. This prevents water from travelling laterally beneath the DPM and then upwards through the DPC, or vice versa.

A Practical Scenario: Shed Base Construction

Let's consider a common scenario, as described by a user building a shed base:

“Have built a shed base, 10m2, and am having it filled with concrete tomorrow. I have done a 3 coarse brick wall which will have the timber mounted on it. Because plans changed half way through, I haven't managed to do the DPC properly, and now all the courses of brick are built. So question is, what is best to do now? I was thinking the best thing to do was to put the dpc in, fill it up, and then just cut it off level with the top of the concrete, on the inside of the bricks. you might see it a bit, but that's ok... is this the best option? Or would the fact that it would separate the concrete from the brick wall entirely create a problem? Only other options as I see it are not bothering at all, or cutting the DPC off to a level square piece at ground level, and not running it up the sides. It's too late to have it coming through the brick. Believe you can get a DPC for on top of the brick wall anyway, in thin strips??”

This scenario highlights a common challenge when construction plans evolve. Here's a breakdown of the best approach:

• The Problem: A partially completed DPC installation after brickwork is already in place.
• The Risk: Without a proper DPC, rising damp can affect the brickwork and the subsequent timber structure.
• Assessing Options:
  • Option 1 (Proposed): Installing DPC, filling with concrete, and trimming. This is a compromise. While it provides a barrier, the lack of proper integration with the brickwork below could still allow some moisture ingress. It also creates a separation that might not be ideal structurally, though for a shed base, this might be less critical than for a load-bearing wall.
  • Option 2: Not bothering. This is the worst option, inviting damp problems.
  • Option 3: Cutting DPC at ground level and not running up sides. This is ineffective against rising damp.
• Best Practice (given the constraints): Given that the brick courses are already built, integrating the DPC fully is difficult. The most practical solution, albeit not ideal, would be to attempt to insert a flexible DPC into the existing brickwork at the lowest possible course that can be accessed. This would ideally involve cutting a small slot and inserting the DPC, then sealing it. However, this is a skilled job. If this isn't feasible, consider a 'damp-proofing cream' injection system as an alternative to create a DPC within the brickwork. If you are simply laying concrete within brick courses, a continuous DPM laid over the existing brickwork and extending up the internal face of the brickwork before the concrete pour would be the most robust solution to prevent rising damp from the ground and any residual moisture in the surrounding soil. The DPM should ideally be lapped with any existing DPC if present, or run up the internal face of the brickwork to a sufficient height (above the expected damp level) and be secured.
• DPC for on top of brick wall: Yes, there are DPC 'strips' or 'cavity trays' that can be retrofitted or integrated into existing walls, but these are typically for stopping lateral water penetration or bridging cavities, not primarily for stopping rising damp from ground level in this specific scenario.

Key Considerations for Installation

Regardless of whether you are dealing with a DPC or a DPM, proper installation is paramount. Here are some key points to remember:

• Continuity: Ensure the barrier is continuous. Any gaps or breaks will compromise its effectiveness.
• Lapping: Overlap DPCs and DPMs by at least 150mm and seal or weld them to create a continuous barrier.
• Upstands: Where a DPM meets a wall, it should turn up the wall face to a sufficient height (usually at least 150mm above the finished ground level) to prevent moisture from getting behind the membrane.
• Protection: DPMs, especially polythene sheeting, are susceptible to damage during construction. Protect them with a layer of sand, screed, or a protective board.
• Ventilation: While the goal is to stop damp, ensure adequate ventilation in spaces like crawl spaces to prevent condensation build-up.
• Manufacturer's Instructions: Always follow the specific installation guidelines provided by the manufacturer of the DPC or DPM product you are using.

DPC vs DPM: A Comparison

To summarise the differences and applications:

Frequently Asked Questions

Q1: Can I use DPC instead of DPM for a concrete floor?
No, a DPC is not suitable for providing a continuous waterproof barrier for a concrete floor. You must use a DPM designed for this purpose.

Q2: How high should a DPC be?
A DPC should be installed at least 150mm above the external ground level to prevent it from being bypassed by splashing or bridging.

Q3: What happens if my DPC is bridged?
If a DPC is bridged by external elements (like raised soil or render) or internal elements (like plaster dropped below the DPC level), it can allow moisture to bypass the barrier, leading to rising damp in the wall above.

Q4: Is it important to lap DPMs when joining sections?
Absolutely. Lapping DPM sections by at least 150mm and sealing them is crucial for maintaining a continuous waterproof barrier.

Q5: Can I install a DPC myself?
While some basic DPC installations can be done by DIY enthusiasts, retrofitting a DPC into existing brickwork or ensuring proper lapping with DPMs requires skill and attention to detail. If in doubt, consult a professional damp-proofing specialist.

Conclusion

Understanding the distinct roles of DPCs and DPMs is fundamental to effective damp proofing. While a DPC interrupts rising damp within a wall structure, a DPM provides a broader barrier against ground moisture. When working with concrete slabs and brickwork, the correct application and meticulous installation, including proper lapping and upstands, are essential to protect your property from the detrimental effects of damp. Always prioritise continuity and follow manufacturer guidelines for the best results, ensuring a dry and healthy building for years to come.

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