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What is damp proofing? DPC vs DPM

Moisture that penetrates walls, floors, and roofs can cause dampness in homes and other buildings, which can lead to mould growth and unpleasant conditions. Even worse, if left untreated, damp can eventually damage the building’s infrastructure, leading to costly repairs.

Damp proofing protects new or existing buildings against water and moisture infiltration. And it comes in two forms: a damp proofing course (DPC) or a damp proofing membrane (DPM). Each works differently, so it’s important to know which damp proofing method is best suited for your project.

Damp staining and peeling paint along the lower interior wall above a skirting board, indicating rising damp.

Types of damp

Determining the type of damp proofing you need starts with understanding the source of damp you’re protecting against. There are three types of damp that most commonly affect residential buildings:

Rising damp typically affects the lower floor as water from the ground moves upwards through walls (typically up to one metre). Potential signs include peeling paint, damaged plaster, and rotting skirting boards in ground-floor rooms.
Penetrating damp is when moisture moves from the outside inwards through walls, floors, or roofs. It can occur at any height and is often caused by damaged gutters, rendering, or roof tiles, or heavy rain. Localised damp patches and mould growth inside signal penetrating damp.
Condensation is the most common damp type. It occurs when warm air reacts with cold walls and windows. Water droplets on walls, peeling paint, and black mould in the corner of a room can all indicate condensation.

What is damp proofing and how does it work?

Damp proofing stops water and moisture from entering the building’s interior through the walls, floors, and roof. It’s a technique that creates a barrier to protect brickwork, timber, and plaster from water ingress, which can cause structural damage.

Damp proofing is essential in the construction of new buildings, as well as existing properties where a damp proofing system may have deteriorated or never been installed.

Without proper damp proofing, a building may experience:

structural instability when moisture weakens brickwork and mortar
rotting timber floorboards and skirting boards
peeling wallpaper and crumbling plaster
musty smells and an unpleasant environment
mould growth causing health problems.

How damp proofing works depends on the specific method applied. Let’s take a closer look at the differences.

What is a damp proof course?

A damp proof course (DPC) is a protective layer that prevents moisture from rising and entering walls from the ground. Rising damp can happen through capillary action, as water is drawn up into walls through small holes in bricks and mortar.

A DPC is normally a thin layer of polyethylene, bitumen, plastic, or another impervious material that’s placed horizontally between the mortar line during construction. It’s placed just above ground level to stop water absorption and moisture from travelling up the walls.

For new builds, it can be built into mortar joints and inserted into cavity walls to prevent moisture travelling between the inner and outer walls.

When to use a damp proof course

A physical DPC is usually installed during the construction of a new build property. You can also replace a damaged or non-existent damp proof course in an existing building by injecting a chemical DPC. It provides a protection barrier if you notice rising damp appearing in your ground-floor rooms.

Types of DPC

There are two main types of damp proof courses:

Construction foundation covered with a plastic damp-proof membrane sheet.

Physical DPC – a solid sheet that forms an impermeable barrier installed horizontally between the mortar line during construction. This can be a thin layer of bitumen, a polymer sheet, or a metal sheet.

Person wearing protective gloves applying a black damp-proof coating along the base of a brick wall.

Chemical DPC – cream, liquid, and silicone DPC injections create a new barrier or repair damaged DPC. Holes are drilled into the mortar bed, and the chemical DPC is injected. This is cheaper than retrofitting physical DPCs.

How to install a damp proof course

To install a damp proof course in a new building, lay a physical sheet in the mortar bed to create a moisture-resistant barrier. This must be installed at least 150mm above ground level so moisture doesn’t rise and penetrate the walls.

When installing a DPC in an existing property, a silicone injection is recommended. Drill small holes in the mortar joint at the level of the original DPC or 150mm high and about 110mm apart, to a depth between 10-20mm. Use reliable damp proofing tools such as an injection gun to fill with your chosen product.

What is a damp proof membrane?

A damp proof membrane (DPM) is a moisture-resistant barrier placed beneath a floor or along the surface of a wall. It covers a wide surface area and prevents water from rising through concrete and damaging masonry, brick walls, and wooden flooring.

Most damp proofing membranes are made from polythene. They’re much wider than damp proof courses but are often combined with them to cover walls and form a continuous bowl that protects against moisture responsible for rising and penetrating damp.

When you need a damp proof membrane

Adding a damp proof membrane is important when installing concrete floors, renovating basements, or covering subfloors. It prevents ground moisture from penetrating the floor and reaching the walls, which can cause rising damp.

DPMs are often installed during the construction of new buildings to cover subfloors. They’re also essential before laying new wood, laminate, or carpet on ground floors or basements to control moisture.

Types of DPM

There are a few types of DPM:

Polyethylene sheets – commonly used plastic sheets placed under concrete floors. They’re flexible, affordable, and puncture-resistant.
Bitumen membranes – these can be self-adhesive sheets that incorporate bitumen rubber to seal against water ingress, making them flexible and durable.
Liquid membranes – examples include epoxy and polyurethane coatings for concrete that create a waterproof barrier, often used in basements.

How to install a damp proof membrane

Before you install a damp proof membrane, first clean the floor and roll out the chosen membrane across it. Overlap the edges by 100-150mm and seal to the damp proof course to form a continuous system. Then install the flooring on top.

When installing a damp proof membrane to a wall, clean the surface and cut the membrane to fit the area, including extra for overlap. Position and fix the membrane by drilling small holes and using sealed plaster plugs as well as waterproof tape to seal joints. Then apply your chosen finish to the wall.

Choosing your damp proofing method

Deciding which damp proofing method to use depends on whether you’re working on a new build, a renovation, or a repair job.

Under the Building Regulations Part C, all new buildings in England must have a damp proof course fitted correctly. If you’ve spotted rising damp on your walls, injecting a chemical DPC should help fix the issue.

Applying a damp proof membrane is suitable when laying a new floor or to prevent moisture ingress when penetrating damp causes rotting floorboards or skirting boards.

However, these two damp proofing methods aren’t mutually exclusive. Most buildings require both as they work together to form complete protection against moisture infiltration and prevent damp.

Find more expert advice from Jewson to ensure your project succeeds.