Basements and cellars
A basement tanking membrane bulges as if water sits behind it. What system behind the membrane was not inspected, and what pump was not run?
The system behind the membrane was not inspected, and the pump was not run. The basement tanking membrane bulges as if water sits behind it, which is an appearance the survey could see. What lies behind, the drained cavity, the channels or the wall itself, was not opened. Any pump that should take that water away was not operated. The inspection has recorded a bulge. It has not confirmed standing water, not tested the system, and not declared the tanking a success or a failure beyond the bulge it could see.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
A cavity-drain membrane is meant to stand off the wall and let water fall to a drain and often a sump. A bulge can mean the membrane has come away, or that water is trapped, or both. Looking at the bulge does not show the fixings, the perimeter channel or whether a pump would start. The survey did not cut the membrane and did not switch a pump on. Cutting it yourself to look would damage the system the report has not yet understood.
Ask a basement specialist to inspect the system behind the membrane and to run the pump the survey left idle. Tell the solicitor the finding is a bulge, not a tested system. Do not describe the basement as dry because the floor you stood on was dry. The membrane is telling a different story, and that story was not opened. Keep the pump test for the person who will actually run it.
What should you ask your surveyor?
- What bulge did you see, and did you open the membrane?
- Was the system behind it left uninspected?
- Which pump, if any, was not run?
What are the typical next steps?
- Read the bulge as a sign, not as a tested waterproofing system.
- Do not cut the membrane open yourself.
- Ask a basement specialist to inspect behind it and to run the pump.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
If there is no water on the floor, is the bulge harmless?
Not on this visit. Water on the floor would be one sign. A bulge as if water sits behind the membrane is another, and the system behind it was not inspected. The pump was not run. A dry floor does not explain the bulge and does not test the drainage behind the sheet.
Does the bulge identify the maker of the tanking?
No. A bulge is a shape in the sheet. It does not name a system, show a guarantee, or prove what is behind the membrane. The survey has not inspected that system and has not run a pump. Papers for the tanking, if they exist, are still a separate file.