Drainage and septic tanks

The soil stack is inside a boxed duct the survey did not open. What is hidden?

The pipe, the joints, and any leak above the point you can see. A painted boxing is not an inspection of the stack. The survey did not cut it open, and it did not test the drains. Approved Document H does not see through plywood.

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?

Internal stacks are boxed for appearance, often in a corner of a bathroom or a cupboard. The surveyor looks at stains on the boxing, soft skirtings, and smell. They do not remove a secured panel as a standard Level 1 or Level 2 step. A clean box can hide a cracked joint. A stained box can be a leak, condensation, or an old mark. The visit cannot weigh those without opening up, which needs the seller’s agreement and is not a camera. Do not kick a hole in the duct on a second viewing.

If the stain is active, ask for the panel to come off before exchange, or accept that you are buying an unseen pipe. A plumber can look once it is open. A drainage camera does not replace that look at an above-ground joint, and a camera was not part of the ordinary survey. There is no grade number to quote. The report should say “boxed, not opened”. Upstairs WCs that flush do not prove the hidden joint. They only show that water left the pan on the day.

What should you ask your surveyor?

Where is the boxing, and was there staining or a smell?
Did you open any panel?
Are you limiting the comment to the outside of the duct?

What are the typical next steps?

Do not cut the boxing open yourself.
If a stain is damp now, ask for access before you treat the stack as sound.
Do not invent a camera result for a pipe that is above the ground.

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 new tiling over the duct mean the stack was replaced?

No. Tiles hide the box. They do not record the pipe.

Is a boxed stack the same as a macerator?

No. A boxed stack may still be gravity. A macerator is a pump. The survey should not mix the words.