Buying a House With Damp
How worried to be, what it costs to fix, and how to use it in a negotiation.
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Why Damp
Proofing Fails
A significant share of our work is re-doing damp proofing somebody has already been paid for. Six failure modes account for almost all of it, and every one is visible before the contract is signed if you know what to ask.
Last reviewed September 2026 by Tony Murphy, Founder & Managing Director. We update these guides in place rather than letting them go stale.
The Six Reasons
- The diagnosis was wrong — the commonest cause by a wide margin.
- Contaminated plaster was left in place.
- The external cause was never addressed.
- The wrong system was specified for the construction.
- Decoration went on before the wall had dried.
- The guarantee was worth less than it looked.
1. The Diagnosis Was Wrong
This is the big one. A wall is damp, a contractor attends, a meter reads high, and a chemical damp proof course is quoted. But the wall was damp because a raised patio was bridging the existing DPC, or a downpipe had split behind a bush, or the room had no working ventilation.
The injection is carried out competently. It achieves nothing, because there was never anything wrong with the damp proof course.
The question to ask: "What did you rule out, and how?" A surveyor who has genuinely diagnosed the problem can tell you why it is not condensation and why it is not penetrating damp. One who cannot is guessing.
2. Contaminated Plaster Was Left in Place
Where rising damp has been present for years, groundwater carries nitrate and chloride salts into the plaster and leaves them there as the water evaporates. Those salts are hygroscopic — they pull moisture out of the air indefinitely.
Install a perfect damp proof course, leave the old plaster, and the wall keeps reading damp, keeps showing tide marks and keeps ruining decoration. The client concludes the DPC failed. It did not; the job was only half done.
The question to ask: "Does the price include removing the contaminated plaster and replacing it with a salt-inhibiting render system, and to what height?" Industry practice is typically 300mm above the highest evidence of damp. See our damp proofing page for how we specify this.
3. The External Cause Was Never Addressed
Internal work on an externally-caused problem is money spent holding back a tide. If ground levels are above the damp proof course, if the gutter is still overflowing, if the pointing is blown on the weather elevation, the water keeps arriving.
Some contractors genuinely do not look outside. Others look, see work that belongs to a different trade, and quote only for their own.
The question to ask: "What did you find outside, and is any of it included?" If the answer is that they did not go outside, get another quote.
4. The Wrong System for the Construction
Solid-wall period buildings were built to breathe — moisture moves through lime mortar and plaster and evaporates. Seal that with dense cement render or an impermeable coating and the moisture does not stop arriving, it just comes out somewhere else, often higher up or in the next room.
Similarly, cementitious tanking on a friable substrate under real water pressure will fail, and no amount of good workmanship changes that. It was the wrong specification before anyone opened a tub.
The question to ask: "Why this system rather than the alternatives?" A specifier should be able to answer in terms of your construction, exposure and water pressure.
5. Decoration Went on Too Early
New render carries construction water and needs to dry before it is sealed with paint. The rule of thumb is roughly one month per 25mm of thickness, which in practice means four to six weeks before decorating, and a breathable mist coat first rather than a vinyl silk.
Decorate at two weeks with a non-breathable paint and it will blister. The wall is fine. The paint is trapping water that has nowhere to go.
The question to ask: "What drying time do you specify, and what should I decorate with?" It should be in the paperwork, not a doorstep conversation.
6. The Guarantee Was Worth Less Than It Looked
A twenty-year guarantee from a contractor is worth exactly as long as the contractor trades. In a sector with high company turnover, that is a real risk rather than a theoretical one.
Check three things: whether it is a contractor guarantee or an insurance-backed one, whether it is assignable to a future owner, and what it is conditional on. A drained basement system whose guarantee requires annual servicing is void the moment you skip a service.
The question to ask: "Is it insurance-backed, is it assignable, and what voids it?" Ask for the actual document before you commit.
Before you sign anything
Get the diagnosis in writing, separately from the quote. A document that says only "supply and install chemical DPC to front elevation — £2,400" is a price, not a diagnosis. You want to see what was found, what was ruled out, and why this treatment follows.
What Good Looks Like
A properly run job produces a written report that names the cause and the evidence for it, a specification that addresses external causes as well as internal symptoms, plaster removal and replacement where salt contamination is present, stated drying times and decoration guidance, and a guarantee document handed over on completion rather than promised.
None of that is exotic. It is simply the difference between diagnosing a building and selling a product.
Questions
Failed Damp WorkFAQs
Possibly, and it depends on whether the guarantee is insurance-backed, whether the original contractor is still trading, and what conditions attach. Start by finding the guarantee document and the original specification. If the failure is because the diagnosis was wrong rather than the installation defective, many contractor guarantees will not cover it — which is exactly why the diagnosis matters more than the guarantee length.
A correctly diagnosed and correctly installed chemical damp proof course with proper replastering should last the life of the building — guarantees of 20 years are common and reflect genuine expectations. If remedial damp work fails within a few years, the overwhelming likelihood is that the original diagnosis was wrong, not that the product degraded.
The three usual explanations are that contaminated plaster was left in place and is drawing moisture from the air, that the real cause was never rising damp so the DPC was irrelevant, or that the wall is still drying — which legitimately takes weeks to months. A survey will distinguish between them, and the answer determines whether you have a claim or a drying period.
On anything over a few hundred pounds, yes — and specifically from someone paid for the survey rather than for the work. Compare the diagnoses, not just the prices. Two quotes for the same treatment at different prices tell you little; two different diagnoses tell you that at least one contractor has not established the cause.
Keep Reading
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Read the guideDamp and Mould in Rented Property
The legal position for landlords and agents, and what evidence you need.
Read the guide
Next Step
Get the Cause Found & Fixed
Book a survey and you get a written report: what is causing the damp, what it takes to fix it and what it costs. No pressure, no guesswork.
Survey fee deducted from the cost of works if you proceed