Occasional Unwanted Odour

10 Sep 2017
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United Kingdom
Late Edit......

Please now see this thread where I have posted more accurate information https://www.diynot.com/diy/threads/trying-to-understand-my-waste-and-soil-pipe-plumbing.578193/ Thanks.


Our house is a bit less than 20 years old. It has a bathroom at the front with a downstairs toilet immediately below. There is a boxed-in section in the corner of the bathroom that follows down through the downstairs toilet. I assume that this is the soil pipe.

At the back of the house, opposite the main bathroom, there are two en-suites that back on to each other. There is no evidence of a soil pipe in either of them.

Between the two en-suites and the main bathroom is a stair-well and landing.

Over the past few weeks we have noticed an occasional sewage-type odour in one of the bedrooms with an en-suite (although not normally in the en-suite itself) and also in the area of landing outside of that bedroom.

Q1. Is it possible that a waste pipe runs under the floorboards from the bathrooms at the back of the house to the soil pipe at the front of the house?

If so, to avoid the stair well, it would have to run along the back wall and then turn to come through under the bedroom and landing (where the odour is) to join with the soil pipe at the front.

If I go outside and look up, the soil pipe does not protrude through the roof and I can see no other obvious vent.

Q2. How does the soil pipe vent?

This odour may be there once a day or so and then 30 minutes later it's gone.

Q3. Am I on the right track thinking this odour is coming from a waste pipe running under the floor from the back to the front of the house and that there is a problem with the venting of the pipe?

Q4. Can any qualified plumber deal with this problem or do I need some kind of specialist?

We have not seen any problems with toilets flushing away or sinks draining.

Sorry for all the questions but as you can imagine, this is an unpleasant and unwanted problem.

Any advice would be very welcome.

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If there’s no soil pipe visible, then it could be concealed, so could terminate through the loft and roof. Failing that, you may have an air admittance valve sticking.
Thanks for your reply, CBW. The only evidence of a soil pipe is in the main bathroom at the front of the house. I've just been reading about AAVs so I guess if I have one it would be fitted at the top of this soil pipe, perhaps in the loft space. Either that or it's concealed in the tiled boxed-in bit in the bathroom, which wouldn't be so good.

If the AAV is sticking then I would expect the odour to come from a sink plug hole. This odour is in the bedroom and landing but not normally in a bathroom. I think there must be a waste pipe running from the back bathrooms to the soil pipe in the front bathroom. Is it even possible there's a waste pipe under the floorboards upstairs and if there is could an odour come from it into the room space above?

Fitting a new AAV seems to be a relatively inexpensive thing to try as long as it's accessible and not glued on to the pipe.

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