Your guide to Damp Proof Course (DPC)

General

When it comes to construction, there’s a lot of talk about foundations, insulation, and structural strength, and rightly so. But there’s another key player in building integrity: damp proofing. More specifically, the damp proof course, or DPC.

Whether you’re building from scratch, renovating a period property, or tackling an unexpected damp patch on-site, understanding DPCs is crucial for keeping moisture at bay and structures sound. In this guide, we’ll walk you through everything you need to know about damp proof courses, from what they are and where they go, to the different types, how much they cost, and how they actually work.

What is a damp proof course?

It’s always best to start right at the beginning. So, what is a damp proof course? It’s a barrier usually installed in the walls of a building, designed to prevent moisture from the ground from rising up through the walls through capillary action. Without it, moisture can creep its way into a building’s structure, leading to a whole host of issues like:

Crumbling plaster
Stained walls
Bubbling paint
Mould

DPCs have been a requirement in new builds in the UK for over a century, and for good reason. They're one of the most effective defences against rising damp.

What causes damp walls? Not all damp is rising damp, and that is where the proper diagnosis matters. Here are some of the common causes of damp walls:

Rising damp - caused by moisture moving up from the ground. This is what DPCs protect against.
Penetrating damp - water coming in sideways, often due to faulty guttering, cracked render, or porous walls.
Condensation - from poor ventilation, especially in bathrooms and kitchens.
Leaks - from plumbing or roofs that go unnoticed for long periods.
Bridging - where the DPC is bypassed by debris in the cavity wall or high ground levels outside.

How much is a damp proof course? The cost of installing a damp proof course can vary widely depending on several factors, including:

The type of DPC you choose
The size of the area needing treatment
The severity of the damp issue

Is a damp proof course required in all homes?

In short, yes, by today’s building standards. It became compulsory in 1875 when the UK Building Regulations said that all new constructions must have adequate damp proofing to prevent moisture ingress. Older buildings (pre-1875) may not have one at all.

Types of damp proof course

Depending on the building, budget, and existing conditions, there are several types of damp proof course—the most common fall into two broad categories: physical and chemical. Let’s look into these in a bit more depth.

Physical DPC

Physical DPCs are exactly what they sound like. A solid impermeable barrier that’s physically built into the wall. These can be made from various materials such as:

Plastic (polyethylene) membranes – common in new builds, these membranes are easy to work with because they are lightweight, flexible, and can be cut or shaped on-site to suit various wall configurations.
Slate – A traditional choice in older properties because it’s naturally dense, durable, and highly water-resistant. Slate was widely available historically and could be easily cut into thin, flat sections to form a robust barrier.
Bituminous felt – once popular but less commonly used today due to its tendency to degrade over time.
Metal sheets – materials like lead and copper are used in very specific or heritage applications because they offer exceptional durability, malleability, and resistance to moisture over very long periods.

These are installed as part of the construction process, usually one or two brick courses above ground level.

Chemical DPC

Chemical DPCs are ideal for retrofit situations, usually when a physical barrier wasn’t installed initially or has failed. This process involves injecting a chemical solution into pre-drilled holes in the mortar bed, which then forms a water-repellent barrier inside the masonry.

What is the chemical solution?
The chemical solutions used in chemical DPCs generally fall into three main categories:

Silicone-based solutions: These are common and widely used because of their excellent water-repellent (hydrophobic) properties. They penetrate deep into the masonry, forming a long-lasting barrier that stops water from rising by capillary action.
Silane and Siloxane blends: These are advanced formulations that combine silane (small molecules that penetrate easily) with siloxane (larger molecules that provide durability). This blend provides deep penetration and a tough, flexible, water-resistant layer.
Silicate-based solutions: Less common than silicone options, these work by chemically reacting with the masonry to form a water-resistant crystalline structure inside the pores.

When are the different types used?

Silicone-based and silane/siloxane blends are the most commonly used for general retrofit damp proofing in older properties because of their proven effectiveness and durability. They are suitable for most types of brick and stone masonry.
Silicate-based solutions may be chosen for specific types of porous stone or masonry where a chemical reaction with the material can enhance performance, though these are less widespread.

This method is less invasive than rebuilding walls and is particularly favoured for damp proofing historic or period buildings where maintaining the original fabric is essential.

Where should a damp proof course be installed?

Correct placement of a DPC is crucial to ensure complete protection against rising damp and moisture ingress. While the exact position may vary slightly based on the building type and structure, here are the key areas to consider when installing a damp proof course:

External walls:
Install the DPC at least 150mm above the external ground level. This helps prevent moisture ingress caused by rain splashback or standing water and reduces the risk of bridging.
Internal walls:
The DPC should align with the base of the wall—the point where moisture is most likely to rise from the ground.
Cavity walls:
The DPC must run continuously through both the inner and outer leaves of the wall to provide complete moisture protection.
Solid walls:
For solid masonry construction, the DPC is placed in a single, continuous horizontal layer within the wall structure.

However, it’s vital that you follow the manufacturer’s guidelines for the specific product you’re using for any installation.

How to install a damp proof course

Installing a damp proof course (DPC) correctly is essential to prevent moisture problems in buildings. While the methods differ depending on whether you’re fitting a physical or chemical DPC, both require careful attention to detail so that the manufacturer’s guidance is followed. Here’s a straightforward step-by-step outline for each type:

Installing a physical DPC

Physical DPCs are typically installed during the construction phase of a new building or when major repairs are being undertaken. As mentioned before, this involves placing a physical barrier within the wall to stop moisture rising. These are the steps to follow for a basic installation:

Prepare the site and materials: Make sure the wall base is clean and level before laying bricks or blocks. Choose the appropriate DPC material recommended for the project.
Position the DPC: Place the physical DPC material (such as a sheet membrane) horizontally within the mortar course at the base of the wall. Make sure it sits correctly on the mortar bed.
Maintain continuity: Make sure the DPC runs continuously across the entire length of the wall, including around corners and junctions. Overlap or join sections as per the manufacturer’s guidance to avoid gaps.
Integrate with other waterproofing elements: Connect the DPC with other damp-proofing layers such as damp proof membranes (DPMs) in floors or cavity trays, to make sure there’s a continuous moisture barrier throughout.
Avoid damage during construction: Take care not to tear, fold, or displace the DPC as bricklaying continues above it. Protect the material from sharp tools or heavy loads.
Complete the build-up: Continue laying bricks or blocks above the DPC so that it’s visible and unbroken.

Installing a chemical DPC

Chemical DPCs are typically installed into existing walls suffering from rising damp. This process uses a chemical injection to create a moisture barrier inside the masonry.

Diagnose the damp issue: Before starting, confirm that rising damp is the cause of the problem. This step often involves a professional inspection or moisture survey.
Prepare the wall: Clear any loose mortar and surface debris from the wall. Mark the injection points along the mortar joints at intervals specified by the manufacturer.
Drill injection holes: Drill some small holes into the mortar joint along a horizontal line, following manufacturer recommendations on hole size, depth, and spacing.
Inject the chemical: Use the appropriate injection equipment to inject the damp proofing chemical into each hole, filling the masonry evenly.
Allow curing time: Leave the chemical to disperse and cure within the wall. This process can take several days to weeks, during which the barrier forms and moisture is repelled.
Repair the wall surface: Once the chemical has cured, remove any damaged or salt-contaminated plaster from the affected area. Replaster using salt-resistant render or a breathable finishing material as recommended.
Post-installation care: Follow any additional manufacturer advice regarding drying times, environmental protection, or follow-up treatments.

How does a damp proof course work?

A DPC works by breaking the path of moisture. Rising damp comes about when water from the ground is drawn up through porous bricks and mortar. By introducing an impermeable layer, whether that’s physical or chemical, you’re stopping water from making that journey upwards. This keeps internal walls dry and prevents all the issues associated with excess moisture in the structure.

Examples of damp proof course products

When it comes to selecting a damp proof course, there are several reliable products on the market designed to suit different construction needs. Here are a few popular options commonly used in the industry:

Visqueen High Bond DPC

Visqueen High Bond DPC is a self-adhesive damp proof course membrane designed for easy installation in new builds and refurbishment projects. It features a strong adhesive backing that sticks firmly to masonry, helping to ensure a continuous moisture barrier. This product is particularly useful where quick and reliable sealing is needed, and it bonds well to a variety of surfaces, reducing the risk of water penetration.

Visqueen Ultimate Gas DPC

The Visqueen Ultimate Gas DPC is a high-performance, multi-layer membrane designed to provide protection against a wide range of ground contaminants, including methane, carbon dioxide, radon, and volatile organic compounds (VOCs). Manufactured using Visqueen’s Advanced Barrier Technology, this 0.5mm thick co-extruded film is flexible and easy to work with, making it ideal for both residential and commercial construction.

It can be used in all masonry wall applications, either as a built-in or surface-fixed cavity tray, and is suitable for sites that fall under NHBC Amber 1 or Amber 2 gas classification. The product also conforms to key industry standards including BS 8485:2015 + A1:2019 and CIRIA C748:2014.

Visqueen Zedex CPT DPC

Visqueen Zedex CPT DPC is a robust, flexible damp proof course and cavity tray system made from 0.8mm co-polymer thermoplastic. Designed for both residential and commercial applications, it offers excellent resistance to moisture, tear damage, and ground gases, including radon. It’s particularly well-suited for multi-storey buildings and complex detailing thanks to its flexibility and the availability of preformed units.

This product is BBA certified and meets the standards required for NHBC Amber 1 applications, making it suitable for areas where gas protection is needed. It can be used either built into the masonry or surface-fixed, and integrates seamlessly with wider damp and gas protection systems.

Browse damp proofing products today!

Need a DPC for your upcoming project? Take a look at our range of waterproofing & damp proofing products. We have a collection of products made for use in these situations, such as Visqueen High Bond DPC, and many more. If you have any questions about our products, or want to learn more about how they can support your project, then contact us today.

By Katie Taylor