Drainage and septic tanks
The boxed stack ends at an air-admittance valve in the airing cupboard. Why is that not the same as a vent opening above the roof?
Because an air-admittance valve in the airing cupboard is a local valve on the end of the boxed stack, not a vent pipe opening above the roof. The survey can record the valve it saw. It cannot rename that valve as an open terminal in free air, and it has not proved how the rest of the stack is ventilated. A valve admits air at the fitting. An open vent is a pipe that finishes above the roof. They are different things. Ask a plumber to confirm the ventilation, rather than calling the cupboard valve a roof vent.
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?
Stacks need air so that traps are not siphoned. An open vent provides that by finishing above the roof. An air-admittance valve is a one-way fitting, often where a boxed stack ends inside a cupboard. It should open to let air in and stay shut against smells. Seeing it tells you a valve was there. It does not tell you a pipe continues out through the roof, and it does not prove this valve is a suitable arrangement for the whole stack.
Ask a plumber to look at the valve, the cupboard, and whether any open vent still leaves the roof. Do not tape over the valve or enclose it more tightly because you dislike it. If the survey never recorded a roof terminal, do not add one in your own notes. The useful next step is to identify how this stack actually gets air. Pretending the cupboard fitting is the pot you did not see would only confuse the solicitor later.
What should you ask your surveyor?
- Was the air-admittance valve fully in view, or only the boxed end of the stack?
- Did you see a soil pipe leave the roof, separate from this cupboard?
- Any smell, staining or a damaged cap on the valve itself?
What are the typical next steps?
- Do not describe the cupboard valve as a vent opening above the roof.
- Ask a plumber how this boxed stack is ventilated.
- Leave the valve untaped and uncovered until that visit.
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 you saw no pot on the roof, has the stack failed?
Not from that alone. No pot is not the same fact as a valve in the airing cupboard. The survey has not proved how this stack is ventilated.
Can you swap the valve for a screw cap to stop a draught?
No. Do not cap the valve. It is there to admit air. Blocking it changes the stack, and it is not a job for a viewing.