Drainage and septic tanks

A soakaway takes the roof water. Why is that not a septic discharge, and what was not measured?

Roof water to a soakaway is the ordinary use of a surface-water soakaway. It is not a septic tank emptying into the ground. The survey did not measure the crates or the pit, did not test how fast the ground takes water, and did not prove the soakaway is large enough. Seeing a downpipe disappear towards the lawn is not that design check.

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 soakaway for rain is a hole or a set of crates that lets roof water disperse. Foul drainage should not be sent there. The survey can follow a downpipe only as far as it remains visible. Once it goes underground, size, wrapping, and silt are hidden. Approved Document H is the building-regulations document for drainage when work is controlled. It is not a calculation of this crate, and the surveyor has not reproduced one. Do not invent a cubic metre from the area of the lawn. Do not call the roof soakaway a failed septic tank. They are different systems unless a foul pipe is clearly in the same pit, which the survey would have to have seen.

Soggy ground after rain is a clue, not a capacity test. A dry day proves nothing. If the lawn floods, a drainage firm can advise whether the soakaway is silted, undersized, or simply overwhelmed. That advice is not a camera grade, and this visit did not film the pipe. Keep septic rules for the foul system, if the house has one, in their own papers. This question is only the roof water. Ask the seller whether a drawing exists. A drawing is still not a measurement the surveyor repeated on the day.

What should you ask your surveyor?

Which roofs appear to discharge to the soakaway, and where does the pipe vanish?
Did you measure any crate or pit? Please say if you could not.
Did you see foul waste in the same system, or only rainwater pipes?

What are the typical next steps?

Do not treat the roof soakaway as a septic discharge.
Do not invent a size from the flower bed.
If the garden floods, ask a drainage firm to look at this soakaway. Give them the survey limit.

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

Does a new plastic crate lid mean the soakaway was designed for this roof?

No. A lid shows something was buried. The survey did not check the volume.

Is a soakaway that takes only roof water a misconnection?

Not from that fact alone. The concern would be foul water in it. The survey has not proved the crate size either way.