Envelope and protection · Sealants & Membranes
Chemical Injection Damp-Proofing (Retrofit Damp-Proof Course)
Describes the injected retrofit damp-proofing material as one element of a remedial scheme, distinct from the course built into new walling and from the water-repellent cream applied to a facade.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What chemical injection damp-proofing is
Chemical injection damp-proofing is a water-repellent active — usually a silane, a siloxane or a blend of the two — carried in a cream, a gel or a low-viscosity fluid and introduced into a line of holes drilled into an existing wall, most often into a mortar bed. Once in the masonry the carrier releases the active, which migrates into the pore structure and lines the pore walls so that the surfaces within the material become water-repellent while the pores themselves stay open.
It is not the same object as the course built into walling as it is constructed. The published damp-proof course is laid in as the wall goes up and cannot be added afterwards without taking the wall apart; the injected route is the only material in this family that goes into a wall that already exists. Nor is it the facade treatment applied to an outer face against wind-driven rain: the same broad chemistry is sold for that purpose and for this one, and the two are not interchangeable. One treats a surface against rain arriving on it; this one is introduced into a horizontal line within the thickness of the wall.
It is one element of a scheme rather than a treatment in itself. The hole pattern, the bed chosen, the condition of the masonry and the mortar, the removal and renewal of plaster carrying salts, and a period in which the wall is allowed to dry are all part of the same work, and the replastering and the waiting are usually the larger part of the disruption. A wall can also be damp for reasons an injected line does not touch at all — a raised external level, a bridged cavity, a defective gutter or downpipe, a blocked subfloor vent, a leaking service, or condensation arising from how the building is used. Which of those applies is a diagnosis made on the building by a qualified professional.
The library's published position on the wider subject applies here without modification: a damp-proof course of any kind is not a product that solves rising damp. Whether a course is the issue at all, and where one would need to be, is a question about the specific building rather than about the material chosen.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Injection damp-proofing
- Damp-proofing cream
- Silane injection damp-proofing
- Remedial injection damp-proofing
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Remedial damp-proofing schemes in solid masonry walls where an existing course has been bridged, has broken down or was never built in.
- Lines introduced into a mortar bed in an existing wall as one element of a scheme drawn up after investigation.
- Treatment at the base of internal partitions and party walls built off a floor construction without separation.
- Work carried out alongside the removal and renewal of plaster that has taken up salts.
- Renovation work where a wall is to be lined or insulated afterwards and the design calls for the base of the wall to be addressed first.
- Localised work where an earlier treatment stopped short of a corner, a return or an abutment.
Consideration
Where it is commonly considered
- Where a qualified professional has investigated the wall, identified the source of the moisture, and set out injection as one element of the remedy.
- Where the masonry and the mortar have been examined and the material is one the manufacturer's documentation describes for that construction.
- Where external ground levels, drainage and rainwater goods have already been looked at, so the wall is not simply going to be re-wetted.
- Where the programme allows a drying period between the work and the finishes going back.
- Where the wall is solid masonry and a built-in course could not be introduced without taking the wall apart.
- Where the disruption of removing plaster, skirtings, services and decoration has been understood before the work is agreed.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where nobody has established why the wall is damp; rising damp, penetrating damp, plumbing leakage and condensation present similarly and are routinely confused.
- In below-ground or retaining walling, where waterproofing is a specialist design discipline rather than a material choice.
- Where the masonry is rubble-filled, of variable construction, or contains voids that no injected line can follow.
- In historic and listed construction, where drilling and the introduction of a chemical are conservation decisions taken with specialist advice.
- Where the moisture is being supplied by a raised external level, a bridged cavity, a defective gutter or a leaking service, none of which an injected line addresses.
- Where the wall carries a finish, a panelling or a decoration whose removal and renewal has not been considered as part of the same work.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The material lines pore walls so that liquid water is less readily drawn through them, while leaving the pores open. What that achieves in a given wall depends entirely on where the moisture is coming from and on what else is being done at the same time.
- Substrate dependency
- Mortar type, unit type, wall thickness, rubble fill, voids and any salt already present all change how a line behaves. The wall is the variable, and it is the one nobody can see into.
- Installation dependency
- The outcome is decided by the hole pattern, the bed chosen, the operator's judgement on the day and the manufacturer's procedure — and none of it can be seen once the holes are made good.
- Documentation
- Because the work disappears, the contractor's record, the drawings and the warranty paperwork are the only later evidence of what was done and where.
- Repairability
- Nothing can be inspected afterwards, so remedying a poor installation generally means going back over the same wall rather than finding and fixing a point.
- Maintenance
- The material itself is not maintained. What continues to matter is everything around the wall: ground levels, paving, drainage, gutters and how water is kept away from the base of the building.
- Appearance
- Made-good holes, salt staining that emerges while a wall dries, and the replastering that follows are the visible consequences of the work, and they outlast the disruption itself.
Exposure
Exposure and environmental conditions
- Where is the water actually coming from, and who has investigated the building to establish that?
- Have external ground levels, paving or planting risen against this wall since it was built?
- Are gutters, downpipes and surface drainage discharging clear of the wall rather than against it?
- Is there a cavity, and if so has anything bridged it — debris, a later insulation, or an alteration?
- Is the internal environment adding moisture through ventilation patterns, heating and occupancy in a way that would show as damp regardless?
- Is any part of this wall retaining ground, which changes the discipline that applies entirely?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The continuity of the mortar bed, and the permeability of the units either side of it, govern whether an injected line can be made at all.
- Rubble cores, random construction, voids and earlier repairs all interrupt the path the material is expected to follow.
- The junction at the base of the wall with the floor construction is part of the same continuity question and is detailed, not assumed.
- Existing finishes, linings, skirtings and services in the way have to be removed for access and reinstated afterwards.
- What already exists in the wall — a slate course, a bitumen strip, dense brick, or nothing — is established by opening up rather than by inference.
- Corners, returns, party walls and abutments are where a treated line has to be carried through and where it is most often left short.
Appearance
Appearance and finish considerations
- A line of made-good holes can remain visible in fair-faced or exposed masonry, and matching mortar colour and texture is rarely straightforward.
- Salt staining appearing while the wall dries is a consequence of moisture that was already there rather than of the material introduced.
- Replastering is the visible part of the work: skirtings, architraves, services and decoration all come off and go back.
- Externally, a drilled and repointed line may read in the elevation for a long time before it weathers in.
- A wall that is drying looks and feels different from one that is dry, and finishes applied too early tend to show it.
- Where a wall is to be lined or insulated afterwards, the sequence decides whether any of this is seen at all.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What does the manufacturer's documentation describe for the masonry and mortar actually present in this wall?
- How was the line set out, and how was continuity maintained at corners, junctions and changes of construction?
- What happens where the line meets a floor construction, a return wall or an abutting property?
- What could change the wall's behaviour later — new paving, a raised bed, an extension, or a render that stops the wall drying outwards?
- Was any part of the wall left untreated, and is the reason recorded?
- Has anything been done about the source of the moisture, or only about the wall?
Upkeep
Maintenance and repair considerations
- What record exists of where the line was made, in which bed, and with which material?
- What does any warranty issued actually cover, who issues it, and what conditions attach to it?
- Who would investigate if damp reappeared, and on what basis would they start?
- Are external ground levels, drainage and rainwater goods being maintained so the wall is not re-wetted?
- Was a drying period allowed before finishes went back, and what informed the decision that it was long enough?
- Is the replastering specification recorded, so a future repair can match what is there?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- The work depends on the operator, the hole pattern and the manufacturer's procedure, and none of it can be inspected once the holes are made good.
- Existing plaster, linings, skirtings and services usually have to be removed for access and reinstated afterwards.
- The condition of the masonry decides whether holes can be drilled cleanly and whether the material stays where it is put.
- Continuity at corners, returns, party walls and at the junction with floor construction is set out before work starts rather than improvised.
- The replastering specification, the drying period and the sequence of finishes belong to the same scheme rather than to separate trades.
- Access to both faces of a wall is not always available, and which face the work is carried out from changes how it is done.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's information for the product actually used, including the constructions it is described for.
- The investigation report that identified the source of the moisture before any material was chosen.
- A drawing or written record of the treated line, showing where it begins, where it ends and in which bed it runs.
- The replastering specification forming part of the same scheme.
- The terms of any warranty offered, including exclusions and who stands behind it.
- A record of what was found when plaster and finishes were removed.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What investigation was carried out, and what did it establish about the source of the moisture?
- Why is injection part of the answer for this wall rather than something outside it?
- What is already built into this wall, and how was that established?
- How will the line be kept continuous at corners, at party walls and where floors meet the wall?
- What replastering forms part of the scheme, and why that specification?
- How long should the wall be left before finishes go back, and what will inform that decision?
- Who carries out the work, and what record will be handed over afterwards?
- What does the warranty cover, what does it exclude, and what happens if damp reappears?
Blind spots
Commonly overlooked points
- A damp-proof course of any kind is not a product that solves rising damp; whether a course is the issue at all is a question about the building.
- The injected material is one element of a scheme, and the replastering and the drying are usually the larger part of the work.
- A great deal of remedial damp-proofing is carried out on walls whose actual problem is a raised external level, a bridged cavity or a defective gutter.
- Nothing about the work can be inspected after the event, which puts unusual weight on who carried it out and on what they recorded.
- Salts left in old plaster go on drawing moisture from the air, which is why plaster is renewed rather than simply redecorated.
- The same silane and siloxane chemistry is sold as a facade cream for a completely different purpose, and the two products are not substitutes for one another.
What this page does not do
- Damp in an existing building must be investigated on site by a qualified professional. Rising damp, penetrating damp, plumbing leakage and condensation are routinely confused and cannot be separated from a description.
- This reference states nothing about where, whether or how a retrofit damp-proofing line should be introduced into any wall; that is diagnostic and design work for the specific building.
- Below-ground and retaining walling involve specialist waterproofing design rather than the selection of a remedial material.
- No injected material makes a wall dry on its own. Drainage, external levels, ventilation, finishes and drying time act together, and warranties offered by contractors are commercial documents whose terms have to be read.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Related entries
Often used alongside
Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Sealants & Membranes
Reference entries for the thin control layers — joint sealants, damp-proof courses and membranes, vapour control layers, breather membranes, tanking systems, tapes and foams.
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