Damp proofing guarantees, and what they are really worth
A 30 year damp proofing guarantee only means something if it is insurance-backed by an independent provider, so that it survives the contractor closing down. Check who underwrites it, what it covers, what voids it, and whether the surveyor holds CSRT or CSSW qualifications through the Property Care Association.
Two very different documents
Almost every damp quote carries a long guarantee, and there are two kinds behind that word. A contractor guarantee is a promise by the firm that did the work. It is worth exactly as much as the firm's continued existence, and damp proofing has a high rate of company turnover. An insurance-backed guarantee is a policy issued by an independent provider such as GPI or QANW, which pays for remedial work if the original contractor is no longer trading. The second usually costs a modest one-off premium, often under £100, and it is the one worth having.
| Question | Good answer | Warning sign |
|---|---|---|
| Who underwrites the guarantee? | A named independent insurer or provider | "We guarantee our own work" |
| How long is the cover? | 20 or 30 years for a damp proof course, 10 to 20 for waterproofing | Vague or unstated |
| What does it cover? | Re-treatment and replastering of the failed area | Materials only, labour excluded |
| Is it transferable? | Yes, to the next owner, sometimes for a small fee | Personal to the original customer |
| Who designed the works? | Surveyor holding CSRT, or CSSW for below ground | No qualification named |
| Trade body membership? | Property Care Association member | Logos used without membership numbers |
What voids a damp guarantee
Guarantees are narrower than they look, and these exclusions are standard across the industry:
- Damp from a different cause. A damp proof course guarantee covers rising damp only. If the wall gets wet from a leaking gutter or from condensation, the policy does not respond.
- Bridging created later. Building a new patio, raising a border, or rendering down past the treated line typically voids cover for that section.
- Redecorating too early. Trapping construction moisture behind vinyl paper or oil-based paint before the wall has dried is treated as the owner's action, not a failure of the work.
- Missed maintenance. On drained basement systems the annual pump service is usually a condition. Skip it, and a claim can fail on that alone.
- Alterations by others. New openings, chased channels for cables and removed plaster in the treated zone all create arguments.
Qualifications behind the paperwork
The Property Care Association is the trade body for damp, timber and waterproofing in the UK. Its members work to published codes of practice and offer insurance-backed guarantees through recognised providers. Individual surveyors hold the CSRT qualification for remedial treatment, and CSSW for structural waterproofing design, which is the one that matters for basements and cellars under BS 8102. Where a specification names a standard and a qualified author, it can be checked; where it names neither, there is nothing to check.
What to keep
Keep the survey report, the specification, the invoice, the guarantee document with its policy number, and the photographs taken before and during the work. When the house is sold, that bundle answers the buyer's surveyor in one go, and it is the difference between a price reduction and a nod. If the guarantee is transferable, transfer it at the point of sale rather than promising to sort it later.
The honest limitation
A guarantee is a fallback, not a quality control system. The far better protection is a correct diagnosis in the first place, a specification that names materials and standards, and a wall that is given time to dry. Those are covered in the guides on telling damp types apart and 2026 damp proofing costs, and in independent damp surveys.