The soil drain vent pipe runs up to the roof in the corner of our back bedroom and is connected to the toilet through the dividing wall. Recently, we've noticed a sewage smell in the bedroom when the powerful bathroom fan is on.
I un-boxed the vent pipe and removed it from the connecting pipe/boss. Peering down, I noticed that the plastic pipe does not appear to sit properly into the female socket of the clay pipe bend located in the soil below the concrete floor (see drawing).
The lower section of the access fitting is firmly cemented into the floor. To move it at all I would need to break up the cement, but only at the expense of making a load of dust and possibly breaking the fitting.
If I could possibly sleeve the fitting from the toilet branch and down into the terracotta bend, that might reduce and seepage to an acceptable minimum. I know professional drainage companies use expanding sleeves that cure into a watertight seal, but can a DIYer buy those and fit them? Though not an easy job, the offending joint is within an arm's reach, so it should be possible to do it manually.
The smell is unlikely to be coming from the toilet waste pipe system. More likely sources are if you have a shallow trap on a bath or shower and the seal is being broken by the extractor.
If when you close the bathroom quickly you feel air resistance then i suggest you cut 10mm off the bottom of the door to allow air in.
Or replace the traps with deeper ones.
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