A damp proof membrane, more commonly known as a DPM, is a material that prevents moisture from penetrating the internal walls and floors of a property. It offers protection against a variety of problems associated with moisture ingress ranging from visible damp patches on walls and floors to salt damage, rot and mould.
It is often used underneath concrete floors to prevent moisture from seeping in, and is one of the most reliable damp proofing solutions for helping to protect your property. If a damp proof membrane is not in place, properties can experience rising damp, penetrating damp and condensation due to cold surfaces. If not treated properly and within a sufficient amount of time, these can cause damage to the infrastructure of buildings.
This blog outlines why a damp proof membrane should be used, how it works, and the benefits it can provide when installed correctly. In the first instance, we would always recommend that external defects, where present, are repaired and a reasonable drying out period allowed before determining the requirement for additional work.
The purpose of damp proof membranes
A damp proof membrane can be used to resolve issues such as damp walls and slabs, salt affected walls and to 
provide a water resistant key for replastering damp walls. The installation of the damp proof membrane is very important, as it acts as a barrier to prevent damp rising up from the ground below or migrating through damp walls.
To ensure that you have full protection from rising and penetrating dampness, a damp proof membrane should be continuous with the Damp Proof Course (DPC). Normally, the damp proof membrane overlaps the DPC, which is placed within the brickwork to stop any damp from rising up from the ground through capillary action. A DPC will be installed at a minimum of 150mm above the external ground level, with the damp proof membrane then folded up the inside wall and lapping over onto the DPC. This helps to form a continuous and more efficient barrier against water.
How does a damp proof membrane work?
A damp proof membrane is made from plastic, enabling it to protect against moisture. The damp proof membrane used for floors are sheet membranes, whilst for walls they are ideally studded sheet membranes, usually with a mesh thermic welded to the surface to allow for a direct plaster to be applied. The studded profile creates an air gap that allows moisture to evaporate while leaving the wall undamaged – it is therefore important that these wall membranes are vented.
It is also important to note the difference between damp proofing and waterproofing. For structures below ground we would always recommend you ensure that any system is designed and installed by a competent professional and in accordance with the British Standards BS 8102:2022. Standard practice for below slab damp proof membranes – as per the Government’s building regulations recommendations – propose that the sheet of polyethylene should be 1200 gauge. If you use a sheet that does not meet the building regulations, it could cause future water ingress damage to the property, resulting in increased costs that could have been avoided.
Care must also be taken when installing a damp proof membrane to ensure that there are no holes or tears in the sheet that could allow damp penetration. Therefore, it is important to contact a damp proofing professional to ensure the installation is carried out to a high standard.
The installation process
A damp proofing specialist typically instals a damp proof membrane in the following order:
1. The source of the damp problem is identified by a competent surveyor and a plan of remedial works provided, this is then rectified by a technician. Any damaged plaster will be removed from the walls and a reasonable drying out period allowed. It is advised that a period of 25mm per month of wall thickness is allowed. Where these timescales are not acceptable to the homeowner, then a damp proof membrane can be utilised to speed up the reinstatement of the plaster and ultimately the re-use of the space affected.
2. A damp proof membrane will be carefully placed onto the wall by a technician and secured with specialist plugs.
3. The membrane should be vented to allow moisture vapour to dissipate. This is usually done by installing vents at the top and bottom, or ventilating out to fresh air.
4. Electrical services are then placed on the dry side of the membrane prior to ideally being lined with insulated plasterboard. This is followed by a plaster skim finish.
Features and benefits
A damp proof membrane is very strong and extremely flexible – it can cope with structural movement, making it a long lasting and reliable damp proofing solution. It also comes with a mesh on one side for direct application of render and plaster, or plasterboard. This versatility makes a damp proof membrane an excellent option for commercial properties or homes, in order to maintain an aesthetically pleasing finish.
Liquid damp proof membrane
For existing concrete floors, you can also apply a liquid damp proof membrane, which provides a retrospective damp proof membrane.
The installation of a liquid damp proof membrane can be a vital step in the building process, for both refurbishment work and for new builds where a structural damp proof membrane may be damaged or missing. You should ensure that the liquid damp proof membrane product will protect the floor from residual construction moisture and rising damp. Choosing a product that also has excellent adhesion to damp concrete, even at lower temperatures, is also important for UK site conditions where inclement weather may be more common.
Closing thoughts
Peter Cox has years of experience fitting damp proof membranes as both a preventative and treatment solution for damp problems. If you think your property could benefit from a damp proof membrane or you just want to talk to a professional about a damp problem in your property, then call our team on 0808 115 4400 and we will be pleased to help. You can also find more information about our services here.
