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Sewage smells in both new bathrooms

We recently did a loft conversion which included fitting a loft bathroom and refurbished the 1st floor bathroom with a new toilet and decommissioned the old toilet on the 1st floor which was in an adjoining room. The 1st floor toilet and the loft toilet now share a new soil pipe. However the builder routed the venting in such a way that the old cast iron soil pipe for the old toilet on the 1st floor is now the vent for both bathroom toilets. Ever since we moved back in with the new bathrooms configured this way we have had constant sewage smells in both bathrooms, the builder told us this is normal for toilets? The old cast iron pipe has been blocked up from the inside where it used to exit the old toilet but the connection out of the wall from the old toilet to the cast iron pipe is still there. The new waste pipe connection joins the old cast iron pipe well below where the old toilet waste used to exit. I'm not sure what to make of it all but seems not quite right, any advice?

I did some unscientific tests.

I closed the window and ran the shower and sink tap at the same time and flushed the toilet (with no content), there was some gurgling from the sink but no smells detected.

Later on, one of the kids sent a number 2 down the toilet, upon flushing there was an immediate sewage smell coming into the room from the toilet bowl. I opened the window and that just made it worse as the breeze just came in and wafted the smell around the bathroom even more.

In summary the sewage smell is isolated to whenever a No.2 is flushed down the toilet. I am now pretty sure it is a soil pipe and venting problem, especially given the weird running of the soil pipe and the reuse of a section of the old soil pipe as a vent. Plus I believe the plumber employed by my builder for the job is not of a high enough quality to tackle more complex wastage requirements like vacuums and venting as his skills are very basic, this is over his head. I am sure a more skilled plumber will immediately rectify the problem.

4 Answers from MyBuilder Plumbers

Best Answer

The running of the pipework is very important not only for the water but also for the successful venting to stop either vacuum problems or back pressure problems. Either can allow foul smells to enter living areas. Vacuum problems can suck the water out of the traps and back pressure can blow foul smells past the water trap.
Also you would have to be sure the old soil pipe has been properly sealed.
I would lean toward it being a problem of inadequate open venting for the pipes connected to the new soil pipe allowing a build up of back pressure especially when flushing a w-c.
It could actually mean rerunning all the waste water pipework to correct the problem.
Sorry I know it's not what you would like to hear and it's not easy to find someone that actually understands all the pitfalls of not doing the job right but that's what you need to do.
I have been known to get around these problems by adding vent pipes into the pipework and piping to a high discharge level preferably back into the stack above water entry points. This is an old school solution that many years ago was done quite commonly. The venting stops both vacuum and back pressure.


Answered 18th Jun 2013

You maybe getting self syphonage from your new installation. This is when the traps on your bath, basin & showers are emptying. Ex. when you empty the bath you may get a gurgle from your basin plug hole this is due to the vacuum created in the pipe work, the vacuum sucks all water oiut of your traps, thus allowing the smell of sewage to come up the vent pipe in to your bathrooms. Check by sticking a small thin green plant support stick into the waste traps in your bath basin & shower, get back to me with the results. This may not be the answer, but plumbing is all about elimination sometimes. Also get a qualified plumber round to do a pressure test on the stack itself, this will quickly identify the problem. Additionally, why did the builder re-use a previous part of soil pipe for venting's bound to smell!!!


Answered 19th Jun 2013

Sounds really bad and the work done not right! You may be able to get away with it ie not tearing the stack down by fitting HEP/VO self sealing traps to your basins and baths. They do work but this may be a remedy and not the cure. Bottom line though it needs to be done a.s.a.p or it will make you and your kids ill!


Answered 19th Jun 2013

I would put antivac traps where possible also check the hight of the soil a dergo valve would stop any orders omitted from the soil stack but allow air into the stack . Sounds like some of the time the traps are being pulled by the w/c this is very unhygienic also gives vermin the chance of entering the property


Answered 3rd Jul 2020

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