Damp proof course injection is the installation of a new chemical barrier low in a wall to stop moisture from the ground soaking upwards. A line of holes is drilled into a mortar course, a silicone-based damp-proofing cream is injected, and it spreads through the mortar to form a water-repellent band. It is only worth doing where rising damp has been confirmed and the original damp proof course has failed or was never there, and it is normally combined with replastering the affected wall.
When is a new damp proof course needed?
A damp proof course, or DPC, is a horizontal barrier near the bottom of a wall. Many older homes were built with slate, bitumen or engineering brick courses, and some very early houses have none. A new injected DPC makes sense when:
- Rising damp has been diagnosed from the pattern, salts and, where needed, samples from the wall
- The original DPC is missing, broken or has failed along its length
- Bridging has been ruled out or already removed, and the damp is still coming up
- Leaks, splashback and condensation have been checked and are not the cause
If those boxes are not ticked, injection is likely to be money wasted. Our rising damp treatment page explains how we reach the diagnosis, and why so many low-level damp problems turn out to have simpler causes.
How does chemical DPC injection work?
Modern damp-proofing creams are usually based on silane and siloxane compounds. Once injected into the mortar, the cream migrates into the pores and lines them with a water-repellent layer. Water can no longer be drawn up through the capillaries in that band, so moisture from the ground stops rising past it. The wall above then needs time to dry out.
The British Standard that covers this work is BS 6576:2005+A1:2012, the code of practice for diagnosing rising damp and installing chemical damp-proof courses. It sets out the need for proper diagnosis first, choosing the right injection line and treating the wall as a whole rather than in short sections.
What happens on the day?
- Preparation. Furniture is moved back, floors are protected and skirting boards are taken off along the walls being treated. Any fitted units in the way are discussed beforehand.
- Removing old plaster. Salt-contaminated plaster is usually hacked off, typically to around a metre or to where the salts stop, so the injection line and the brickwork can be seen.
- Setting out the line. The injection line is chosen in a mortar course close to floor level, taking account of floor levels inside and ground levels outside.
- Drilling. Holes are drilled into the mortar at regular spacing and to the correct depth for the wall thickness. Solid nine-inch walls may need drilling from both sides or to a greater depth.
- Injecting. The cream is injected into each hole using a gun or pump, filling the hole so the right amount goes into the wall.
- Replastering and making good. The wall is replastered with a suitable system and the skirting is refitted or renewed once things are ready.
A typical room is usually a few days’ work including the replastering, though it depends on how many walls are affected and access. The new plaster then needs to dry before it is decorated. See our replastering after damp page for what happens to the wall after injection.
Chemical DPC injection involves drilling into structural walls and handling specialist products, and old plaster or textured coatings in pre-2000 homes may contain asbestos. This is not a DIY job. We do not disturb suspected asbestos; it must be tested by a licensed specialist first.
Why is the replastering part so important?
When ground water rises through a wall for years, it carries salts that build up in the plaster. Some of these salts are hygroscopic, which means they pull moisture straight out of the air. If the old plaster is left in place after injection, the wall can stay damp-looking and keep spoiling decoration even though the DPC is working. That is why injection without replastering usually disappoints, and why we quote for both together where they are needed.
What about Liverpool’s solid-wall terraces?
Across Liverpool, many terraced houses from the late Victorian and Edwardian period have solid brick walls, suspended timber floors and sometimes cellars. In areas such as Everton, Old Swan and Bootle, these homes can have original slate DPCs that have cracked with movement, or later extensions where the damp proofing does not line up with the main house. Party walls between terraces raise their own questions about where to inject and what can be done from one side.
Where the floor is a solid floor laid without a membrane, damp can also come up through the floor and around its edges, which a wall DPC alone will not stop. Our damp floor repair page covers that situation.
How much does DPC injection cost?
We price each job after a visit. The main cost factors are the length of wall being treated, the wall thickness, how much plaster must be removed and renewed, access around kitchens, stairs and built-in furniture, skirting and timber repairs, and redecoration. You can see published national guide figures on our damp and mould cost page; these are not our prices, and Liverpool jobs vary.
Injection is one part of our damp proofing in Liverpool work. Where it is not the right answer, we will say so and explain what is.
Been told you need a new damp proof course?
Call 07777 659786 or use the form. Whether it is your own assessment, a survey or a mortgage report, a local damp and mould specialist can look at the wall and tell you whether injection is really needed.
Frequently asked questions
How long does a chemical damp proof course last?
A properly installed injected DPC is intended to work for many years, but how long it performs depends on the product, how well it was installed and whether the wall is later bridged by raised ground, render or a new path. Keeping ground levels below the DPC line outside is the best way to protect it. We provide a written record of the work done.
Do I have to move out while the DPC is injected?
Usually not. The work is noisy and dusty in the rooms being treated, so most people stay in the house but keep out of those rooms while the plaster is off. If anyone in the home has breathing problems, let us know when we call so we can plan the work around them.
My mortgage survey says I need a DPC. Is that definitely true?
Not necessarily. Mortgage valuers often note high meter readings and recommend further investigation by a specialist. A meter reading alone does not confirm rising damp. A proper inspection looks at the pattern, salts, ground levels and other causes before recommending injection. Our mortgage damp report page explains what we provide.
Can you inject a DPC into a party wall?
Sometimes, but it needs care. A party wall is shared with a neighbour, and treatment usually has to be done from your side only. Depending on the wall, that may mean drilling deeper or treating the wall differently. We explain the options and limits after looking at the wall.
How soon can I decorate after DPC injection?
The new plaster needs to dry first, which usually takes several weeks and can take longer in winter or on thick walls. A breathable emulsion is often suitable for the first coat. Wallpaper, vinyl paints and heavy finishes are best left until the wall is fully dry.
Will a new DPC stop condensation and mould?
No. A damp proof course only stops moisture rising from the ground. Condensation comes from moisture in the air settling on cold surfaces, and that needs better ventilation and steadier heating. If you have mould in corners or around windows, our condensation control service is the place to start.