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What Is a Damp Proof Injection? How Chemical DPCs Work
Updated 5 November 2026. Reviewed by Michael Young, Head Contractor at Advanced Damp (CSSW and FODCS).
Key Takeaways
- A damp proof injection is a chemical damp proof course (DPC) injected into the mortar joint near the base of a wall to stop moisture rising from the ground.
- It only fixes rising damp. If the damp is caused by rain getting in, condensation, a leak or high outside ground levels, an injection will not help.
- Most modern injections use a silane or siloxane cream that spreads through the mortar and lines the pores with a water repellent layer.
- Replastering is part of the job. Salts left behind in old plaster will keep pulling moisture from the air, so contaminated plaster is removed and replaced with a salt resistant specification.
- Walls dry slowly after treatment. A useful rule of thumb is about a month for every 25 mm of wall thickness.
What is a damp proof injection?
A damp proof course is a barrier near the bottom of a wall that stops ground moisture travelling up through the masonry. Older houses either never had one or have one that has broken down or been bridged. A damp proof injection creates a new barrier without having to cut out the wall. Holes are drilled into the mortar bed along the base of the wall and a damp proofing cream is injected into each one. The cream spreads through the mortar and cures into a water repellent zone that blocks rising damp.
The method and diagnosis are covered by BS 6576, the British Standard code of practice for diagnosing rising damp and installing chemical damp proof courses.
How the damp proofing cream works
Older injection systems used liquid fluids pumped under pressure. Most specialists now use creams based on silane and siloxane, which are easier to control and less likely to run out of the wall. Once injected, the cream migrates through the mortar along the line of holes. As it cures it lines the capillaries in the mortar and brick with a water repellent layer. The wall still breathes, but liquid water can no longer travel upwards by capillary action.
The mortar course is used because it is continuous around the building, which makes it the most reliable route for the barrier. Drilling directly into bricks is avoided where possible.
Is an injection the right fix for your damp?
This is the most important question, and it is where most failed treatments go wrong. Rising damp is genuinely less common than many people think, and plenty of walls that look like rising damp are actually suffering from something else.
- Rising damp usually shows as a tide mark up to around a metre above floor level, with salts and blown plaster below it. It is present all year, not only after rain.
- Penetrating damp comes from rain getting through the wall, from defective pointing, render, gutters or downpipes. It often worsens after wet weather and can appear at any height.
- Condensation forms on cold surfaces, often in corners, behind furniture and around windows, and is usually accompanied by black mould.
- Bridging happens when outside ground, a path, a raised patio or a render finish sits above the existing damp proof course and lets moisture go around it.
If the cause is bridging, lowering the ground or cutting back the render may be all that is needed. If the cause is rain or condensation, an injection will cost money and change nothing. A proper damp survey should establish which problem you have before any injection is recommended. Our guide to damp walls explains the differences in more detail.
What happens on the day
A typical injection job follows the same stages.
- Preparation. Furniture is moved, skirting boards are removed and floors are protected.
- Plaster removal. Contaminated plaster is cut back, usually to around a metre high or at least 300 mm beyond the highest sign of damp and salts.
- Drilling. Holes are drilled into the mortar bed at regular centres along the base of the wall, at a level that sits at least 150 mm above outside ground.
- Injection. Each hole is filled with damp proofing cream to the depth set by the manufacturer for the wall thickness. Thicker walls may be drilled from both sides.
- Making good. Holes are filled and the wall is left to start drying before replastering.
Replastering after a damp proof injection
Replastering is not an optional extra. When water rises through a wall it carries salts, mainly chlorides and nitrates, which build up in the plaster at the top of the damp zone. These salts absorb moisture from the air, so even after the source is stopped, old plaster will keep looking damp and blowing. The contaminated plaster is removed and replaced with a salt resistant system, typically a sand and cement render with a salt inhibiting additive followed by a finish coat. Ordinary gypsum plaster should not be used directly on a wall that has been treated for rising damp.
How long does the wall take to dry?
The barrier works as soon as it cures, but the masonry above it still holds the moisture it absorbed over the years. That moisture has to dry out, and in solid walls it takes time. A commonly used rule of thumb is about one month for every 25 mm of wall thickness, so a solid brick wall around 225 mm thick can take the best part of a year to dry fully. Use breathable paint for the first decoration, avoid vinyl wallpaper and keep the room heated and ventilated. If damp readings stay high long after that, the cause should be checked again.
Common mistakes with damp proof injections
- Injecting a wall that does not have rising damp, when the real problem is rain penetration, a leak or condensation
- Leaving outside ground or a path above the new DPC, so moisture simply goes around it
- Skipping the replastering, or using standard gypsum plaster on salt contaminated masonry
- Not removing enough plaster, so salts are left above the new finish
- Drilling too high, too low or at the wrong spacing for the wall thickness
- Decorating too soon with non breathable paints or wallpaper
Avoiding these is mostly a matter of diagnosis and good workmanship, which is why the survey matters as much as the product.
Will an injection work in a period property?
Chemical DPCs are widely used in Victorian and Edwardian solid brick houses and generally work well when rising damp is the real cause. They are less suited to rubble filled or very soft stone walls, where the cream has no continuous mortar course to spread through. In those cases other approaches, such as improving drainage, lowering ground levels and using breathable lime finishes, may be better. A surveyor with the CSRT qualification (Certificated Surveyor in Remedial Treatment) can advise on the right approach for the wall in front of them.
When to book a damp survey
Book a survey before agreeing to an injection, especially if the damp comes and goes with the weather, appears above a metre, or if a previous treatment has failed. A written diagnosis means you only pay for work that addresses the actual cause. You can book a damp survey online and we usually respond the same day during working hours.
Frequently asked questions
Does a damp proof injection actually work?
Yes, when the problem is genuinely rising damp and the work is done properly, including salt resistant replastering. It will not fix damp caused by rain penetration, leaks, condensation or high ground levels, which is why diagnosis comes first.
How long does a damp proof injection take?
Most jobs on a single wall or room take one to two days, including plaster removal and injection. Replastering follows, and full drying of the wall takes much longer, often several months.
Do I need to replaster after a damp proof injection?
In almost every case, yes. Salts in the old plaster keep drawing moisture from the air, so contaminated plaster is removed and replaced with a salt resistant specification.
How high should a damp proof course be?
A new DPC should sit at least 150 mm above outside ground level. If paths, patios or flower beds are higher than that, they need lowering or the DPC will be bridged.
Can I inject a damp proof course myself?
DIY injection kits are available, but most failures come from misdiagnosis, poor plaster removal and the wrong replastering, not from the cream itself. Getting the cause confirmed first is the most important step.
Is a damp proof injection guaranteed?
Specialist installers usually provide a long-term written guarantee covering the DPC. Check what it covers, whether it transfers if you sell, and that it includes the replastering.
Reviewed by Michael Young, Head Contractor at Advanced Damp (CSSW and FODCS). Last reviewed November 2026.
Related Case Studies
Our recent work includes lower ground flat, Kings Cross WC1X; first floor flat, Brixton SW9 and lower ground floor, Blackheath SE3. Each case study sets out what the survey found and how the problem was fixed.
Lower ground flat, Kings Cross WC1X
Rising damp returned around an arch and back door despite an earlier DPC. Treated with a new DPC, waterproof render and skim ahead of a sale.
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Lower ground floor, Blackheath SE3
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Sam Fitzgerald is the founder of Advanced Damp and a working damp and structural waterproofing surveyor diagnosing and treating moisture problems in residential and commercial properties across London and the South East. He is FODCS certified (a Federation of Damp Certified Surveyor) and a member of the Federation of Damp. His hands-on work covers rising damp, penetrating damp, condensation, basement waterproofing to BS 8102, and thermal imaging surveys, and he has personally completed over 400 damp surveys across properties of every type and age. Everything he writes draws directly from jobs he has carried out. He does not write about treatments, products or causes he has not dealt with first-hand in the field.
