Drainage and septic tanks
An inspection chamber is half full of silt and the WC flushed clear on the day. Why does a clear flush not empty the silt question?
A clear WC flush does not empty the question raised by an inspection chamber half full of silt. The flush shows that water left the pan while someone watched. It does not remove the silt, and it does not show that the chamber or the buried run is clear. Silt in the chamber is its own finding. The flush and the silt both have to stay in the report, side by side.
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?
Silt takes up space in the chamber and can mean solids are being dropped in the run. One flush on the day of the visit can find a path through that silt and still look clear at the pan. The surveyor has not jetted the line. A swirl in the pan is not a survey of the pipes that remain buried beyond the chamber.
Treat the chamber on its own facts: how deep the silt sat, and whether the invert could even be seen. A camera, if the buried run needs to be looked at, is a further instruction and is not included unless it was agreed. Do not let a successful flush talk you out of the silt that was still lying there when the cover was lifted.
What should you ask your surveyor?
- How full of silt was the chamber, and could you see the invert at all?
- Did the flush shift the silt, or did the silt stay while the water passed?
- Are you leaving the silt as an open question after that clear flush?
What are the typical next steps?
- Do not treat one clear WC flush as a clear drainage system.
- Record the silt in the inspection chamber as still unresolved.
- If the buried run needs to be seen, agree a camera instead of assuming one was used.
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 the pan emptied quickly, is the drain in good order?
No. A pan that emptied is not the chamber. The chamber was half full of silt, and the demonstration flush did not remove that silt.
Does the silt prove the pipe has collapsed further along?
No. Silt is a reason to investigate the chamber and the run. It is not, by itself, proof of a collapse in the buried pipe.