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What is a Damp Proof Course?

In construction and home maintenance, the damp proof course (DPC) is a critical element, though it might not be a term that every homeowner is familiar with. However, understanding the DPC can save you from potential property nightmares and extensive repair bills in the future. Let’s dive into the details.

What is a Damp Proof Course?

At its core, the DPC is a protective measure that prevents moisture from travelling upward from the ground into a structure. Dampness, in a construction context, refers to the presence of unwanted moisture in a building, typically seeping in from the ground. This can be due to a variety of reasons:

  1. Capillary Action: Like the way paper draws up water, building materials, especially bricks and mortar, can absorb moisture from the ground. This phenomenon is due to capillarity, where water molecules travel up narrow spaces against gravity.
  2. Water Table Fluctuations: Changes in the water table, especially in regions prone to flooding or heavy rainfall, can push moisture into foundations and walls.
  3. Soil and Site Conditions: Buildings on clay-rich soils or poor drainage sites can face increased moisture challenges.

Capillary action

The DPC’s primary function is to act as an impermeable barrier, interrupting the path moisture would take if left unchecked. By doing so, the DPC:

  1. Protects Building Materials: Constant moisture can degrade many building materials, weakening the structure.
  2. Prevents Health Issues: Damp environments within homes can lead to mould and mildew. These exacerbate respiratory problems and other health issues.
  3. Maintains Aesthetic Appeal: Damp patches, peeling paint, or mould spots are unsightly and can decrease the value of a property.
Placement and bridging

The exact placement of a DPC varies depending on building design and regulations, but it’s typically positioned at least 150mm above the ground level. This ensures that surface water (like splashes from rain) won’t bridge the DPC. In multi-story buildings, DPCs can also be incorporated at each floor level to prevent moisture transmission from one floor to another.

One challenge with DPCs is that they can be ‘bridged’. This means something (like a new construction, soil buildup, or debris) allows moisture to bypass the DPC, rendering it ineffective. It’s vital to regularly inspect the area around the DPC for potential bridging sources and rectify them promptly.

Materials used for the DPC

Bituminous DPC

Crafted from fabric layers generously coated with bitumen. This imparts a waterproof nature to them. What stands out with this material is its adaptability. It can gracefully handle various structural movements, making it apt for diverse building necessities. Furthermore, the pliability of bituminous DPCs ensures they can navigate corners seamlessly without succumbing to cracks. Predominantly seen in brick and blockwork, they also find a place in cavity wall constructions. However, one should bear in mind that bitumen, though robust, can over time become brittle in specific conditions, potentially diminishing its efficacy.

High-density polyethene sheets

Beyond their undeniable resistance to moisture, they bring with them the ease of installation. Their resilience extends to combatting environmental factors as well; they remain undeterred by acids, alkalis, and a gamut of soil contaminants. Given their durability and imperviousness to decay, they’ve emerged as favourites for contemporary buildings. As with all materials, ensuring correct installation and monitoring for damage is key to maximising their protective lifespan.

Slate and lead

Boast natural resistance to moisture, making them effective barriers in damp-proof courses. Slate, with its dense and layered composition, is particularly adept at blocking moisture’s upward movement. Lead, meanwhile, is malleable and seamlessly integrated into various building configurations. Although traditional, slate and lead DPCs might still be encountered in older structures. Their longevity is a testament to their effectiveness, but modern construction often leans towards newer materials due to ease of installation and cost considerations.

Concrete

A special mix, usually incorporating water-resisting additives or impermeable aggregates, is used. Concrete DPCs are especially valuable in situations where structural strength is as crucial as moisture resistance. For instance, in foundational slabs or base structures, a concrete DPC not only prevents rising dampness but also adds rigidity to the overall construction. However, the key lies in ensuring the mix is correctly formulated and properly laid. Any cracks or flaws compromise its damp-proofing capabilities.

Damp Proof Course in Action

Moisture, in the absence of barriers, tends to move through capillary action. Think of it akin to a sponge drawing up water or how a paper towel absorbs a spill. Building materials, especially those like bricks and mortar, possess tiny pores. These pores can draw moisture upwards due to capillarity, especially if there’s a consistent source of moisture, such as wet ground or soil.

Aerated concrete block which has visible pores that can draw up moisture through capillary action
An aerated concrete block which has visible pores that can draw up moisture through capillary action

The DPC  barrier interrupts the upward march of moisture. By being impermeable, it ensures that even if moisture tries to ascend via capillary action, it’s halted at the DPC level. Typically, a DPC is positioned at a point slightly above ground level. This ensures that even with a ground splash or minor ground-level changes, the barrier remains effective. It’s not just about halting the moisture. It is also about ensuring that the barrier remains untouched by external elements that could compromise it.

In cavity walls, the DPC is often combined with a cavity tray to direct any moisture outwards, ensuring that it doesn’t find its way to the inner layers of the structure.

Why maintenance and repair is important

While DPCs are designed for longevity, over time, various factors compromise their integrity. Environmental changes, ground movement, building settlement, or even external factors like construction work or landscaping can potentially damage or alter the DPC’s positioning. Without regular checks, these issues might go unnoticed until they manifest as evident damp problems in the property.

Maintenance and regular inspections are cost-effective in the long run. Spotting a minor issue, like a slight bridge over the DPC due to soil build-up, is vastly cheaper and easier to rectify than treating widespread dampness, mould growth, or timber rot that can result from a long-compromised DPC.

Properties with damp issues often see a reduction in their market value. Regular maintenance ensures that the DPC remains effective, preserving both the structural and aesthetic aspects of a property and thus maintaining its market worth.

A failed DPC can lead to rising dampness, which is not only a structural concern but also a health one. Damp environments are breeding grounds for moulds, fungi, and mites. These can exacerbate respiratory issues, allergies, and other health conditions. Ensuring that the DPC is in prime condition is a proactive step towards maintaining a healthy living environment.

Continuous exposure to moisture can weaken the building materials, leading to issues like plaster degradation, timber rot, and even compromised structural strength in severe cases. Regular maintenance and prompt repairs ensure the building remains structurally sound.

Protecting the DPC

Insulation cannot just stop at the DPC. Thermal bridges already plague many external wall insulation projects. Thankfully, at EWI Store, we have a specialised solution. Through a combination of XPS insulation, Aquabase, and Mosaic Render, an impervious layer is formed below the DPC. The application of these materials is the same as our other basecoats and renders.

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