The short answer
A damp-proofing guarantee is the contractor’s promise to put right a specified treatment if it fails within the guarantee period — often 20 or 30 years — but it is only as good as the firm behind it. If the contractor stops trading, an unbacked guarantee is worthless. Insurance-backed guarantees (IBG) survive that. Guarantees are usually limited to the exact work done, exclude condensation and new defects, and are checked closely when a house is bought or sold.
Long guarantee periods are a major selling point in damp-proofing — “30-year guarantee” sounds reassuring. But the value of any guarantee depends on what it covers, what it excludes and, above all, whether it survives the firm going out of business. Here is how to read a damp-proofing guarantee so that it actually protects you rather than just selling the job.
Damp guarantees at a glance
- What it is Contractor’s promise to remedy specified failure
- Typical length Often 20–30 years
- Key protection Insurance-backed guarantee (IBG)
- Usually excludes Condensation and unrelated new defects
- Matters most When buying or selling the property
What a guarantee actually covers
A damp-proofing guarantee covers the specific treatment carried out — for example a chemical damp-proof course to a named wall and its associated replastering — for a set period. If that treated area fails because the work itself was defective, the contractor returns and rectifies it at no charge. Crucially, it is a workmanship promise on a defined job, not blanket damp insurance for the whole house. Read the certificate carefully to see exactly which walls, which treatment and which period are covered, because anything outside that scope is not protected.
Why insurance backing (IBG) matters — and what happens if the firm closes
A guarantee is a promise from a company, so it is only worth as much as the company’s survival. Many damp firms are small, and a 30-year guarantee easily outlives a business. If the contractor has ceased trading when a fault appears, an ordinary “company-backed” guarantee is effectively unenforceable — there is no one left to honour it. An insurance-backed guarantee (IBG) solves this: it is a separate policy, taken out with an independent provider at the time of the work, that steps in to fund the remedial work (or a replacement contractor) if the original firm is no longer trading. Without an IBG, an impressive-sounding 30-year guarantee can evaporate at exactly the moment you need it.
Common exclusions
- Condensation — almost always excluded, because it is a ventilation and lifestyle issue, not a failure of the treatment.
- New or unrelated defects — a later roof or gutter leak causing penetrating damp is not covered.
- Untreated areas — only the specific walls actually treated fall within scope.
- Failure to maintain — blocked airbricks, raised external ground levels or bridged DPCs can void cover.
What guarantees mean when buying or selling
A damp-proofing guarantee usually has two layers, and the difference matters most at the worst moment. The first layer is the contractor’s own promise to put right any failure of their work, typically for ten or twenty years. That promise is only as good as the company standing behind it: if the firm ceases trading, merges or simply disappears, a company-only guarantee becomes worthless because there is no one left to honour it. The second layer is the insurance-backed guarantee (IBG) — a separate policy, written by an independent insurer through a recognised provider, that steps in to fund remedial work if the original contractor is no longer able to. Because damp work is often guaranteed for decades, the chance of the firm closing within that window is real, which is why surveyors and lenders treat a genuine IBG as essential rather than optional. Always confirm the IBG exists as a distinct policy document, check who underwrites it, and read what it actually covers and excludes.
| Feature | Company guarantee only | With insurance-backed guarantee (IBG) |
|---|---|---|
| Who honours a claim | The original contractor | An independent insurer if the contractor cannot |
| If the firm ceases trading | Effectively unenforceable | Cover continues for the policy term |
| Document to keep | Guarantee certificate | Certificate and separate IBG policy |
| What buyers’ surveyors expect | Often treated as insufficient | Generally accepted as proper protection |
Guarantees are scrutinised at sale. A buyer’s surveyor and solicitor will ask for the original certificate and the IBG policy, and check whether the guarantee is transferable to a new owner — many are, sometimes for a small administrative fee. Missing paperwork can stall a sale or trigger a fresh damp survey, and a lender may even make release of funds conditional on it. Keep the survey, specification, invoice, guarantee and IBG documents together in one place. For how this fits a purchase, see mortgage retention for damp. This page is general information, not legal advice; read the specific terms of any guarantee and verify the insurance backing directly with the provider.
Unsure if your damp guarantee is worth anything?
An independent survey can confirm whether past treatment was right and whether the cover still protects you. The enquiry is free and no-obligation.
Frequently asked questions
How long do damp-proofing guarantees last?
Often 20 or 30 years, but the length matters less than whether the guarantee is insurance-backed and what it excludes. A long guarantee from a firm that may not survive offers limited real protection.
What is an insurance-backed guarantee (IBG)?
An IBG is a separate independent insurance policy taken out at the time of the work that pays for remedial work if the original contractor has ceased trading when a covered fault appears.
Does a damp guarantee cover condensation?
Almost never. Condensation is a ventilation and lifestyle issue rather than a treatment failure, so it is a standard exclusion, along with new unrelated defects and untreated areas.
Can I transfer a damp guarantee to a buyer?
Many guarantees are transferable to a new owner, sometimes for a small fee. Keep the original certificate, IBG policy and specification, as buyers’ surveyors and solicitors will ask to see them.
Sources & further reading
- Property Care Association (PCA) — guarantees and insurance-backed guarantees
- RICS — Home Survey Standard and treatment of guarantees during sale
- gov.uk — Housing Health and Safety Rating System (HHSRS): damp and mould
This guide is general information, not a site-specific survey, medical advice or legal advice. Damp and mould should be assessed by a qualified surveyor, and health concerns discussed with a GP or the NHS.