Damp, roofs, knotweed, movement and drains: the defects surveyors are most often sued over

Most negligent-survey claims come down to a handful of defects. The law is the same for all of them — a duty to inspect with reasonable skill and care within the level of survey, and to follow up or flag what could not be resolved — but the practical questions differ. This guide takes the five most common in turn. For what each level of survey involves, see Level 1, 2 or 3: what your survey had to find.

1. Damp

The commonest complaint. Rising damp, penetrating damp and condensation each leave signs: tide marks and salts on plaster, blistering paint, black mould in corners, decayed skirting boards, a smell. A competent surveyor at Level 2 or 3 will use an electrical moisture meter at ground-floor level and around suspected areas, note readings that are high, look at external ground levels and damp-proof courses, and check for blocked air bricks and defective rainwater goods.

Where readings are high or signs are present, the report should say so, offer a likely cause where it can, and recommend a specialist damp and timber report before exchange. What it should not do is give a "fair" condition rating to a wall with a visible tide mark.

Evidence that matters: the surveyor's own damp readings (or the absence of any), your photographs of the affected areas, a specialist damp and timber report, and anything showing the damp was present at the time — old salts, historic staining under new paint, neighbours' accounts.

2. Roofs

A Level 2 surveyor inspects the roof from ground level with binoculars and looks into the roof space from the hatch. A Level 3 surveyor enters the roof space where it is safe. Neither is expected to walk the roof. What both must do is report what could not be seen — a rear slope hidden by a neighbouring building, a flat roof with no access — and describe the condition of what could.

Signs a competent surveyor should pick up: slipped, cracked or missing tiles; sagging ridges or rafters; degraded or torn underfelt visible from inside; daylight through the covering; staining on rafters or ceilings; inadequate ventilation causing condensation; poor flashings at chimneys and abutments. Where the covering is old and near the end of its life, the report should say so even if it is not yet leaking.

The typical dispute is scope: "the rear slope was not visible from the ground". The question then is whether the report said that, and whether it recommended further inspection. A report that describes "the roof" as satisfactory, without recording that half of it was unseen, has not done its job.

3. Japanese knotweed

Knotweed affects value and mortgageability, and it is a defect surveyors are specifically expected to look for when inspecting the grounds. RICS guidance on knotweed and residential property was revised in 2022, moving from a rigid distance-based rule to an assessment of management and impact, but the inspection duty is unchanged: look, record what is seen, and where knotweed is identified or suspected recommend a specialist survey and management plan.

Knotweed is easiest to see in summer and hardest in winter, when it dies back to canes and rhizome crowns. A surveyor inspecting in January is not negligent for failing to see growth that was not there — but should be alert to dead canes, and to a neighbouring property with visible infestation.

Damages in knotweed cases include the cost of a treatment plan and the residual reduction in value that can persist even after treatment. The Court of Appeal considered knotweed and diminution in Davies v Bridgend County Borough Council (a nuisance claim between neighbours rather than a surveyor claim), and the Supreme Court in 2024 confirmed that only loss caused by the defendant's breach is recoverable — a reminder that causation has to be shown, not assumed.

4. Structural movement

Cracking is not the same as subsidence, and a competent surveyor is expected to know the difference. Old, stable cracks from historic settlement are common and often harmless; progressive movement — cracks that are fresh, widening, diagonal, wider at one end, mirrored inside and out, or accompanied by sticking doors and sloping floors — needs investigation. Surveyors commonly classify crack widths by reference to the categories in BRE Digest 251, from hairline to severe.

The surveyor's job is to identify cracking, describe it, give a view on whether it is likely to be historic or ongoing, and where there is doubt, recommend a structural engineer's inspection and, often, a period of monitoring before exchange. Failing to mention visible cracking, or describing significant diagonal cracking as "minor settlement" without qualification, is where claims arise.

Evidence: photographs of the cracks with a scale, a structural engineer's report addressing when the movement is likely to have started, and any insurance or monitoring history for the property.

5. Drains

Below-ground drainage is largely invisible, and no standard survey includes a CCTV drain survey. What a Level 2 or 3 surveyor should do is lift accessible inspection-chamber covers where it is safe, look for signs of a problem — standing water, blockages, cracked or collapsed pipework visible in the chamber, damp or subsidence near the drain runs, smells — and recommend a CCTV survey where there are signs or where the property's age and construction make problems likely.

The typical claim is a collapsed or root-damaged drain that a CCTV survey would have found. The question is whether there were signs that should have prompted the recommendation. If the covers were lifted, the chambers were clear and there was nothing to see, the surveyor may have done all that was required. If the report says nothing about drainage at all, or the covers were never lifted, the position is different.

Two questions for every defect

Whatever the defect, a claim depends on the same two questions, and you will need an independent surveyor to answer them: was it there at the time, and was it visible — or were there signs — within the scope of the survey you paid for? The guide on independent reports and retrospective valuations explains what to ask for; the guide on diminution in value explains how the loss is measured once negligence is established.

What a solicitor will want from you

• The survey report and terms of engagement, including the level of survey.
• Dated photographs of the defect as found, and of the same areas at the time of the survey if you have them.
• Specialist reports: damp and timber, roofing, knotweed, structural engineer, CCTV drainage — whichever applies.
• Repair quotes and any invoices for work already done.
• Your complaint to the surveyor and their response.

This guide is general information about the law of England and Wales. It is not legal advice. CasePoint does not cover personal injury, including injury caused by a building defect.