Last Updated on August 25, 2026 by Umar Farooq
If you have searched this already, you will have been offered an injected damp-proof course by roughly everyone. It is worth knowing that the British code of practice covering that treatment says it does not apply to your wall.
The exclusion nobody quotes
BS 6576 is the code of practice for diagnosis of rising damp and installation of chemical damp-proof courses. Its own scope statement says it
Below ground level, and under hydrostatic pressure. That is a basement wall, described twice.
An independent heritage source states the mechanism behind the exclusion in plainer words. Chemical impregnation will prevent rising damp but will not stop water under pressure, so impregnation techniques cannot be used where there is a hydrostatic head such as may occur when tanking a cellar or basement.
A chemical DPC creates a water-repellent band that blocks capillary transport. It has nothing to push back with when water is being pressed through the wall by the weight of saturated soil outside. The governing standard for below-ground walls is a different one, BS 8102, which deals in barrier, integral and drained protection rather than in injection.
This does not make injection a scam. It makes it the wrong scope for the part of your wall that is underground. If your damp is on a ground floor wall above the retained level, you are back inside BS 6576 and the usual conversation applies.
Is it actually rising damp?
Start here, because the misdiagnosis rate is the reason this subject has the reputation it does. In a basement there are four candidates and only one of them rises.
| What you see | What it usually is |
|---|---|
| A horizontal tide mark at a consistent height, with salt deposits along it, present in dry weather | Genuine capillary rise. The tide line is where evaporation balances supply |
| Damp appearing after rain, patchy, at any height | Penetrating damp or surface water. Not rising |
| A wet band at the base of the wall that follows the weather and pools on the floor | Water arriving at the wall-floor joint |
| Even, cold-feeling dampness on the wall face, worse in summer | Condensation. The commonest misdiagnosis by a wide margin |
A heritage charity puts the broader point bluntly: in older buildings, moisture rising in walls is usually not a problem where the construction can breathe, allowing evaporation. The damage generally starts when someone stops it evaporating.
The tape test costs nothing
Tape a plastic sheet over the damp patch, seal all four edges, leave it a day or two. Moisture on the room side of the sheet is condensation. Moisture on the wall side is water coming through. That one test separates the commonest misdiagnosis from everything else before anyone quotes you.
The salt is the damage, not the water
Water evaporating out of a wall leaves behind whatever it was carrying. Those salts crystallise inside the pores of the masonry, and expanding salt crystals within porous materials may result in fractures forming in the material and spalling of the surface.
Two consequences follow, and both are counterintuitive.
Blocking evaporation concentrates the damage
Where evaporation is restricted to localised areas, such as defects in an impermeable paint finish, salt deposition is concentrated, forming thick crystalline deposits. The brickwork decays exactly where the paint film fails, because that is where the salt is crystallising.
So an impermeable coating does not stop the damage. It gathers it into whichever few square inches are still able to breathe.
Decay is not gradual
That is why a wall can look stable for decades and then deteriorate quickly. The salt load has been building the whole time. It also means treating the damp and ignoring the salt leaves the decay running, because salts left in the wall continue to cycle in and out of solution with changes in humidity.
Two costs nobody mentions
The heating bill
A wet wall is a poor insulator, and the effect has been measured at full scale. Rising capillary water increased wall heat transfer coefficients by 74% in solid walls and 84% in cavity walls. The heating and cooling loads of the solid-wall buildings rose by 18.5% and 29.6% respectively.
No page selling damp treatment carries a single number on this, and for a basement you are heating it is a real part of the case for fixing it.
The floor timbers
There is a specific, photographable failure at the base of ground floor walls. Salt-laden plaster debris falls behind the skirting onto the floor plate, attracts moisture, and spreads it to the timber plate and the joist. In one documented case the wall plate underneath was found mostly rotten with little sound timber remaining.
The same debris makes a bridge across the DPC, which sits out of sight directly behind that floor plate. So the rubble both causes the rot and defeats the damp course, and nobody looks because it is behind a board.
What the independent evidence says about treatment
There is a body of independent research on whether these treatments work, and it is not cited by anyone selling them. A European research project was set up specifically because of the gap. Its own summary of the state of play:
- Many different solutions available, with no clarity about the effectiveness of solutions and a lack of scientific research.
- Assessment of effectiveness is only rarely performed, and seldom by an independent party. Where it is assessed, it is usually by the producer or seller of the method, or the contractor.
- “High degree of users’ satisfaction does not always mean that the problem has been solved.”
The peer-reviewed write-up notes the abundance of commercial solutions coupled with limited scientific evidence regarding their performance. A related review is blunter about the gap between laboratory and site: despite positive results when applied under controlled conditions in the laboratory, these interventions do not always deliver the desired result in practice.
Being fair, a twelve year laboratory study did find silane-based treatments the most effective of those tested, and injection is not without merit above ground. The honest summary is that the products can work and the field evidence for them is thin.
The mechanism that makes a DPC relocate the damage
This one deserves its own section, because it is entirely absent from the commercial pages and it changes how you read a quote.
Everything that was evaporating over a metre of wall now has to leave through the bottom eight inches. That band takes five times the salt loading it did before, and the masonry below the new DPC may begin to decay rapidly as a result.
There is a second effect on the same page, and for a basement or ground floor it matters more. Where installing a DPC reduces evaporation, the risk of fungal rot and insect attack to floor timbers increases, because underfloor humidity goes up. Rot conditions start around 20% moisture content in timber.
The same source is willing to say the thing no contractor will. Where salt levels and evaporation are low and the site is well drained, it may be appropriate not to install a DPC, and instead to manage the ongoing salt attack and rising damp using sacrificial treatments.
The one thing you must not do to a cellar wall
This is the most directly relevant sentence in the literature for anyone reading this page:
The water does not stop. It goes further up, into the ground floor structure, where the timbers are.
Hard cement render at the base of walls does the same thing at a smaller scale. Because these renders are relatively impermeable they prevent evaporation of rising damp, and at best it is a short term solution before the damp rises and causes decay above the render.
Things sold for this that do not work
| Method | The finding |
|---|---|
| Passive electro-osmosis | There is no scientific basis to it. It is the movement of water through the porous medium that creates the electrical potential, not the other way round, so earthing the charge does not affect capillary suction. It gained a notorious reputation in Australia in the 1960s and 70s, with most claimed successes attributable to other works done at the same time |
| Active electro-osmosis | Holds one national appraisal, but similar fit-for-purpose assessments have not been made by the UK Building Research Establishment or the British Board of Agrément. The trade body concedes it lacks British third-party accreditation |
| Ceramic tubes and atmospheric syphons | Tests have shown an empty hole is just as effective at drying the surrounding wall, and in high salt conditions the ceramic tube is rapidly destroyed by salt attack |
| Damp-proof mortar additives | Grouped with the above as methods that have proved unsatisfactory and should no longer be considered |
| Oil-based paint on a drying wall | Alkyd paints are less vapour permeable than acrylics and should not be used where walls remain damp or are still drying out after DPC insertion |
What to do first, and it is not a treatment
The sequencing statement below is the most useful sentence in the whole subject, and it comes from a government heritage guide rather than from anyone with something to sell.
Inserting a DPC should only be contemplated after undertaking the housekeeping of Key Step 2.
The housekeeping is maintenance, site drainage, underfloor ventilation, and dealing with concrete floors and paving. The trade body agrees on the ordering, saying that bridging from ground levels, render or cavity debris should precede other treatments.
The measurements
- Ground falling away from the wall. The first metre should fall about 25 mm, which is 1 in 40, and where possible the low point should be one and a half to two metres from the building.
- A sterile zone at least 300 mm wide against the walls, with garden beds pulled back and sprinklers replaced by drippers.
- Coarse gravel in that zone. It limits rain splash while still allowing soil moisture to evaporate.
- External ground two bricks, 150 mm, below the damp course where there is one. Remove any mortar bridging it, repair leaking gutters, and lower the path if you can.
- Air drains where feasible. Excavating a trench against the building to expose the bottom of the walls lowers the evaporative zone and reduces underfloor moisture, which in turn reduces the rot and insect risk to timbers.
And then stop and look, before spending more. It is important to take the time to assess the effectiveness of the treatments to date before more invasive and costly work is considered, because that may be the best outcome, removing the need for the expensive insertion of a damp-proof course.
There is a caution on drainage too. Poorly maintained French drains become French ponds in a wet climate, so a trench that is not maintained is worse than none.
If you do have work done
The replastering
Salt-contaminated plaster has to come off, to a minimum of 300 mm above the last detectable signs of dampness. What goes back on matters as much as what came off.
The materials should be similar to the original. Soft, flexible lime plaster for old flexible masonry, with a stronger cement-lime plaster only for newer stiffer walls on rigid footings. Where salt may remain in the wall, the new render should act sacrificially, with porous particulates in the mix to give the salt somewhere to go, finished with limewash so it can breathe.
Be wary of the waterproof render that usually accompanies an injected DPC. The objection from the independent side is that re-plastering treated masonry with a salt-proof and waterproof mixture is to cover up the potential problems rather than resolve them, and a heritage charity warns that salt-resistant plasters may temporarily disguise continuing damp problems. If the plaster is dry and the wall behind it is not, you have bought a quiet decade rather than a fix.
One more trap. Do not put lime render straight over existing cement render. And be aware that where a wall was previously cement rendered it will hold a high salt content, so new lime render may initially draw salts outward and cause localised premature decay, especially at wall bases.
The guarantee
A practitioner inside the damp industry states the position more honestly than the marketing does: if the contractor fails, for whatever reason, the guarantee is worthless. There is no statutory duty for specialist contractors to insure their guarantees against their own business failure.
The trade body’s own description of an insurance-backed guarantee confirms the failure mode. It exists to honour the terms of the long-term guarantee originally issued by your contractor if that contractor has ceased trading. So ask for insurance backing, check who underwrites it and whether they are regulated, and keep the paperwork somewhere you will find it in fifteen years. Even insured, documents get lost, and some policies cover only the cost of the materials used.
Note also what the guarantee covers, which is recurrence of dampness in the treated area. Not the damp problem, the treated wall.
The survey
For a traditional building, ask for a survey compliant with the joint position statement on investigating moisture in traditional buildings, produced by the surveying institution with the heritage body and the trade association. It exists precisely because diagnosis in older buildings had become a sales process.
Why mortar decides how high it goes
Useful if you are wondering why your neighbour’s wall is fine and yours is not. Testing with identical bricks and different mortars produced rise heights of 80 mm in one case and over 480 mm in another. The mortar joint, not the brick, is the highway.
Disclosure, because it matters here: that research was published by a damp-proofing manufacturer. It is consistent with why injection targets the mortar bed, and it also explains why irregular rubble and random-coursed basement walls are the difficult case. Thick walls of irregular rubble may be difficult to fully impregnate, and effectiveness in natural stone is highly variable because the hydrophobic material follows the lines of least resistance.
FAQ
Can I inject a damp course into my basement wall?
Physically yes, and it falls outside the scope of the code of practice that governs the technique, which excludes walls below ground level and walls under hydrostatic pressure. For the below-ground part of the wall, BS 8102 rather than BS 6576 is the relevant standard.
How high can rising damp go?
Commonly around a metre, and the ceiling is set by where evaporation balances the rate of supply. It depends far more on the mortar than the brick, and it rises higher wherever evaporation is blocked.
Is rising damp a myth?
No, but it is diagnosed far more often than it occurs, and the commonest thing it is confused with is condensation. Capillary rise is real physics. The dispute is about frequency and about treatment, not about existence.
The damp came back after treatment. Why?
Several possibilities worth working through in order. The salt was left in the wall and is still cycling with humidity. The damp course was bridged, often by plaster debris behind a skirting or by a raised external path. The impregnation was not continuous through the wall thickness, which the operator has to judge and thick rubble walls make hard. Or it was never rising damp.
Do I need to lower the ground outside?
It is on the list rather than at the top of it. The recommended order is to remove any mortar bridging the damp course, repair leaking guttering at roof level, and lower the path if possible. Two bricks below the damp course is the target where lowering is practical.
Is dehumidifying the answer?
It manages the symptom in the room and does nothing about water entering the wall. On a wall that is actively wicking it can make matters worse, because drying the air pulls moisture through faster and brings more salt with it.