Do I actually need an injected damp proof course?

An injected damp proof course is a chemical cream or fluid pushed into a line of holes drilled along a mortar joint, which cures to line the pores of the masonry and stop water climbing through them. It can work. The problem is not the chemistry, it is the diagnosis: the product is very often sold to cure a symptom that is being produced by something else entirely, and no chemical can stop water that is arriving by a route it does not sit in.

What you'll need

• Torch
• Tape measure
• Hygrometer (optional)
• Independent surveyor's report (optional)

Step by step

1. Check the height first.Water cannot climb much more than about a metre through masonry against gravity. Damp above that is not rising, whatever anyone says, and the diagnosis has to change.
2. Check whether it follows rain.Mark the edge of the patch in pencil on a dry week. If it grows during heavy rain and shrinks afterwards, it is penetrating damp and the fix is on the outside of the building.
3. Check whether it behaves like condensation.Black spotty mould, worse in winter, worse in the morning, worst on cold corners, reveals and behind furniture, no salts. That is condensation and it needs ventilation and warmth, not chemicals.
4. Check every bridge.Ground level, render carried past the barrier, internal floor higher than it, cavity debris, planters and steps. This is the check that catches the largest share of misdiagnosed walls.
5. Wait out a heating season after removing any bridge.A wall that has been wet for years does not dry in a month. Judging the result too early is how people end up buying the treatment anyway.
6. Only then get an independent opinion.If all four checks are genuinely clear and the wall is still wet, pay for a surveyor who sells no treatment. A paid report that says 'you do not need this' is the cheapest money in the whole exercise.
7. Understand what the work actually involves if you do proceed.The injection itself is the small part. The specification almost always includes hacking off plaster to a metre and replastering, and it is the replastering that costs the money and makes the mess.
8. Read the guarantee before you value it.A long guarantee is only worth the trading life of the company issuing it, and most exclude the thing that actually went wrong. Insurance-backed guarantees exist and are worth more, but check who the insurer is.

Tips

• Injection is largely ineffective in rubble-filled walls, random stone walls and very wet masonry, because the chemical cannot form a continuous line through a wall that has no continuous mortar bed.
• If a company diagnoses rising damp from a hand-held meter reading alone, that is not a diagnosis. See what a meter actually measures before you accept a number.
• A genuinely failed barrier does exist — a cracked slate course after settlement, or bitumen destroyed by later building work. It is just far less common than the number of injections carried out.

Common mistakes

• Accepting a diagnosis from the company that will do the work — The survey is free because the treatment is the product. Even honest surveyors working on that basis are structurally biased toward finding a treatable fault.
• Injecting a solid stone or rubble-filled wall — There is no continuous mortar bed for the chemical to form a line in, so it cannot produce a continuous barrier however much is used.
• Treating the wall and leaving the salt-contaminated plaster on — Hygroscopic salts pull water out of room air indefinitely. The wall dries, the plaster stays damp, and the homeowner concludes the treatment failed.

Alternatives

• Remove the bridge: Lower the ground, cut the render back above the barrier, clear the cavity, deal with the internal floor level. Cheap, permanent, and the correct answer for the majority of walls quoted for injection.
• Let the wall breathe: In a solid-walled older house, replacing cement render and gypsum plaster with lime allows moisture to evaporate rather than accumulate. This is the traditional answer and it is often the right one.
• A physical damp proof course: Cutting a new barrier into the mortar bed in short sections, or driving in stainless plates. More disruptive and more expensive than injection, but it is a real barrier rather than a treatment of the pore structure.
• Do nothing except monitor: If the damp is mild, stable, and not damaging anything, recording it with dated photographs for a year is a legitimate and free option. Many walls turn out to be drying from something that was already fixed.

Questions people ask