Damp Proof Membrane: What Is It?

A damp proof membrane creates a barrier that helps stop moisture from passing through parts of a building, particularly floors and walls.

Not every damp problem will need a membrane and there are different types available, so consulting a specialist is always advised. 

In this blog, we’ll explain what a damp proof membrane does, where you might find one and why identifying the cause of damp should always come first.

Summary

• A damp proof membrane, or DPM, creates a barrier that helps stop moisture passing through floors or walls

• DPMs come in different forms, most commonly sheet or liquid systems

• Builders will often use damp proof membranes with concrete ground floors to protect against moisture from the ground

• While a damp proof membrane and a damp proof course perform similar jobs, they aren’t the same thing

• Professional diagnosis is vital, as the appropriate treatment depends on where the moisture comes from

What is a damp proof membrane?

A damp proof membrane is a moisture-resistant layer (normally a flexible sheet or in liquid form) that helps protect a building from damp. You’ll often hear people shorten the name to DPM.

It forms a physical barrier between moisture and the part of the property you want to keep dry. That could mean stopping moisture from the ground from passing through a concrete floor or protecting an internal wall as part of a damp treatment.

Unsurprisingly, a membrane only works properly if installed correctly so that it forms an effective barrier. If there are gaps, damage or poorly sealed edges, moisture may still seep through.

How does a damp proof membrane work?

Building materials such as brick, concrete and mortar can come into contact with moisture. Without suitable protection, that moisture can make its way into parts of a property where you really don’t want it.

A DPM blocks that route.

This matters particularly where floors come into contact with the ground. Government guidance in Approved Document C says floors next to the ground should resist ground moisture and describes damp proof membranes as one way to achieve this.

The membrane also needs to work with other damp protection in the building. For example, guidance for a ground-supported concrete floor calls for the DPM to connect continuously with damp proof courses in walls.

In other words, leaving an easy route around the barrier won’t solve much.

Where are damp proof membranes used?

You’ll most commonly hear about DPMs in relation to floors and walls, although the exact system will depend on the building and the moisture problem.

Damp proof membranes for floors

A damp proof membrane for floors helps prevent moisture in the ground from reaching the floor above.

In a ground-supported concrete floor, builders can position the membrane above or below the concrete slab, depending on the construction. Approved Document C includes both arrangements in its guidance.

While researching DPM solutions for a concrete floor, you may also come across heavy-duty plastic sheets. These create a continuous layer across the floor and will need suitable joints and connections around the edges.

Existing properties can present a different challenge. Adding a membrane to a floor that’s already in place may require a different system and careful assessment of the existing construction.

Damp proof membranes for walls

A damp proof membrane for walls works on very much the same basic principle. It creates a barrier between moisture and the internal wall surface.

A damp proof membrane for walls can create a barrier between a damp wall and the new internal surface. This helps stop moisture from reaching plaster and other finishes.

However, putting a membrane over a damp wall doesn’t automatically fix whatever caused the moisture in the first place. A leaking pipe, penetrating damp, condensation and rising damp can all call for different treatments.

That’s why the damp patch you can see only tells part of the story – and highlights the importance of proper assessment. 

What is a liquid damp proof membrane?

So, not every DPM arrives on a roll.

A liquid damp proof membrane starts as a liquid coating that a specialist applies to a prepared surface. Once cured, it forms a continuous moisture-resistant barrier.

Liquid systems can prove useful in situations where fitting and joining sheets would be difficult. They can also work well across certain existing surfaces, depending on the product and condition of the substrate.

Preparation matters, though. The surface needs to suit the system, and the membrane needs correct application if it’s going to do its job.

So while a tin of liquid membrane might look fairly straightforward, the bigger question remains the same: is it actually the right treatment for the damp problem?

Is a damp proof membrane the same as a damp proof course?

The short answer is no. However, it’s really easy to see why people confuse the two.

A damp proof course, or DPC, usually creates a horizontal barrier within a wall to help stop moisture travelling upwards from the ground. If that barrier fails or a property doesn’t have an effective DPC, rising damp can become a concern.

On the other hand, damp proof membrane generally covers a larger surface. You might find one across a concrete floor, for example, while membrane systems can also protect walls.

You’ll often find DPMs and DPCs being used in conjunction. For example, in modern floor construction, the floor membrane should connect with the damp proof course to maintain the moisture barrier.

Do you need a damp proof membrane?

Certain moisture problems can produce similar symptoms but have completely different causes.

A proper assessment can establish where the moisture comes from before a decision is made on the best route forward.

A membrane may very well form a part of that solution, but it’s by focusing on the cause of the issue rather than plastering over the symptoms that you’ll have the most success in resolving the problem.

FAQs

Can you put a damp proof membrane on an existing floor?

This is slightly more nuanced. Some damp proof membrane systems can work with existing floors, but the right approach depends on the floor construction and condition, as well as the severity of moisture problem.

Does a damp proof membrane stop rising damp?

A DPM can form part of a building’s overall protection against ground moisture, but rising damp specifically concerns moisture travelling upwards through walls. This is where a damp proof course is normally opted for.

How long does a damp proof membrane last?

The material, installation, surrounding construction and any subsequent damage can all impact a DPM’s longevity. A properly specified and installed membrane can provide long-term protection, but that’s not to say problems won’t occur if the barrier becomes damaged or loses its continuity.

Final thoughts

A damp proof membrane has a fairly simple job: create a barrier that helps keep unwanted moisture away from parts of your property.

Choosing the right treatment isn’t always quite so simple. Where the damp comes from, how the property was built and which areas it affects can all influence the best approach.

That’s why it’s worth finding the cause before reaching for a particular solution.#

If you’ve found this article useful and want to learn more about damp and how it impacts your property, you might enjoy the following blogs: 

• How to damp proof your house
• Prevent damp causing problems in your home
• A visual guide to damp, mould and indoor air pollution
• Rising damp: Infographic guide

The team at Crown Preservation have more than 30 years’ experience dealing with damp problems in domestic and commercial properties across London and the Home Counties. 

If you’ve spotted signs of damp or aren’t sure whether your property needs a membrane or another form of treatment, don’t hesitate to contact us for a free, no-obligation chat, and to discuss the best course of action for your situation.