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Soil pipe cross flow between sink and shower

Discuss Soil pipe cross flow between sink and shower in the Showers and Wetrooms Advice area at Plumbers Forums

Attached is a ground floor picture of the soil pipe in our home. There was originally a toilet which has now been permanently capped off and you can also see a sink waste pipe attached.

My plan is to drill out one of the 50mm sockets directly opposite the current sink waste for a new 40mm shower waste pipe, attached with a Marley Push-Fit connector (the same as pictured for sink) but I've just discovered that this would be discouraged due to potential "crossflow".

I want to check would crossflow still be a problem in this case given it's a sink/shower so no solid waste flow from those pipes, nothing to block the opposite pipe. Both pipes would be slightly angled as you'd expect and the sink has quite a sharp downwards drop so I can't see how either would cause a large flow of water up the opposite pipe but I may be wrong.

I definitely don't want to be replacing any part of the stack so would it be better to fit the shower waste opposite, move the sink waste up to the socket above where it currently is and cap off the existing sink socket? Or is this all overkill and just go opposite with the shower anyway? There is also another toilet waste near the opposite wall, worst case I can route the sink waste under the cabinets to there if the above is a terrible idea.

All much easier with the walls down so want to get it right now, thanks!


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Last edited:
Use the old toilet outlet eg 110mm-40mm etc

And swap the sink waste eg shower uses the boss sink uses the old toilet connection
The 110mm hole is going to be against some plasterboard to match the depth of adjacent cabinets so there's no chance of getting pipe in that route. The shower waste can only come in from the one side too as it exits the room, under the stairs and back in to the soil stack to remain hidden.
You can’t go opposite if one connection is higher than the other, as the higher one can feed in to the lower.

You can go opposite if they are the same height.

It’s push fit, I’d take tee out, replace with boss and slip coupling depending on higher pipe arrangement.
I ended up just going with same level opposites in the end. It looks like it's the suggested arrangement in the boss guidance and crossflow isn't a problem for opposites with 40mm. I don't think it would have been worth touching the tee in this case, there's two more floors above with 2 more toilets, bath, shower, sinks etc connected to the soil pipe so the whole lot would need to be carefully supported for little overall gain.

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