Drainage and septic tanks

Decking covers the gulley the wastes should reach. What could the survey not see?

The trap, the grate, and whether the wastes still discharge there. Boards over a gulley hide the fitting a surveyor would otherwise look at. The visit did not lift the deck, did not prove a blockage, and did not camera the pipe. You have a covered gulley, not a drainage test.

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?

Decks are built across the back of kitchens and hide the gulley that took the sink. The surveyor sees the deck, perhaps a waste pipe vanishing under a board, and any damp at the wall above. They do not crowbar the deck on a standard inspection, just as a Level 1 or Level 2 survey does not remove chamber covers as standard. Smell is a clue. It is not a trace. Approved Document H does not see under the boards, and it is not a camera report. Do not invent a grade for a pipe that stayed in the dark.

Before exchange you can ask the seller to lift a board so the gulley can be seen. If they will not, you are buying an unseen outlet. A drainage firm needs that opening before they can say the trap is blocked, disconnected, or fine. Do not assume the deck was built because the gulley had failed. People deck over gullies because the boards were simpler. Also do not assume it is sound because the kitchen sink ran. Water can leave a sink and still not reach the gulley you cannot see. Keep this separate from a gulley in the middle of an open patio. That one was visible. This one was not.

What should you ask your surveyor?

Where does the waste pipe disappear under the deck?
Did you see the gulley at all?
Are you recording a limit, not a proved blockage?

What are the typical next steps?

Ask for a board to be lifted if you need the gulley seen before exchange.
Do not cut the deck up on a viewing.
Do not camera-grade a fitting nobody uncovered.

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 deck mean the drainage was renewed underneath?

No. The deck is timber or composite. The gulley may be the old one. The survey did not see it.

Is this the same as a channel drain across a drive?

No. A channel drain is a slot you can see. This is a gulley hidden under boards.