Drain surveys for subsidence: agree the investigation brief

RICS identifies leaking drains or water mains as one possible way that supporting ground can be softened or washed away. It also describes other causes of subsidence. That makes drainage a possible line of investigation, not the automatic explanation for a crack or a sticking door.

If you have noticed unexplained changes, seek specialist advice rather than choosing a drain repair from an online checklist. RICS advises getting help for unexplained signs of movement. Give the adviser the history you know, including when changes were noticed and whether drainage work has already taken place. Keep observations separate from a diagnosis.

Sources: RICS.

Ask the person coordinating the investigation for a written brief before requesting quotations. It should identify the area of concern, the pipes to be considered and the outputs needed. Where the route is uncertain, ask whether establishing the route is part of the task or a separate instruction.

Ask who will interpret the results and whether that person needs to speak with the contractor before attendance. A general homebuyer package may not answer the particular question being investigated. Request a list of exclusions, including any parts of the system that the proposed equipment or available access will not cover.

Sources: RICS; RICS.

Water UK's published design and construction guidance treats visual inspection, air testing and water testing as separate procedures. That document concerns sewer adoption, not the specification for your house. It is a useful reminder to ask which type of evidence the specialist actually needs rather than assuming a camera recording answers every question.

Do not use this guide to carry out a pressure or water test. Ask a competent specialist whether any additional test is appropriate, who should perform it and how the findings will be interpreted. A recorded defect, evidence of leakage and an explanation of building movement are different conclusions. Ask the report writer to make those distinctions explicit.

Sources: Water UK; RICS.

RICS advises checking buildings-insurance cover and notes that insurers can recommend specialist advice. Contact your insurer about the proposed investigation and ask what it wants you to do next. Do not assume that a privately booked survey or repair will be paid for under your policy.

Keep any insurer reference, written instruction, contractor scope and report together. Ask whether the insurer wants original recordings, a plan, test results or a particular report format. If emergency action is necessary, follow the relevant professional advice and retain a record of what was done; do not treat this booking guide as a reason to delay urgent help.

Sources: RICS.

When the report arrives, send the complete findings and limitations to the specialist overseeing the investigation. Ask whether the inspected pipes answer the original brief and what remains unknown. If repairs are proposed, request the evidence for their scope and ask how their completion will be documented.

A proposal to repair a drain does not by itself establish that the building investigation is finished. Ask who will decide whether further assessment or monitoring is required. Keep the drainage findings within the wider case record so that later advisers can distinguish the pipe work from the conclusions about the property.

Sources: RICS; Water UK.

• Who is coordinating the building investigation?
• Which question should the drain inspection resolve?
• Which pipes, access points and report outputs are included?
• Is visible condition enough, or does the specialist require other evidence?
• Has the insurer agreed the instruction where a claim may be involved?
• Who will assess the results and decide what happens after any repair?

Find and check a drainage contractor when you are ready to ask for quotes.