Soil pipe cross flow between sink and shower

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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" with the sink opposite.

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. I'm unable to use the old toilet connection as all of this is being boxed in to the adjacent cabinet depth so I wouldn't have space for a pipe there.

The second screenshot seems to indicate I should just go opposite and NOT staggered.

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

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Touching a soil stack. shortening/lengthening pipes is covered by building regs, I kid you not.

So you should really be following guidance from the appropriate approved Regs document and you really should apply for building regs. Not many do, hence why you did your post for advice.
 
Touching a soil stack. shortening/lengthening pipes is covered by building regs, I kid you not.

So you should really be following guidance from the appropriate approved Regs document and you really should apply for building regs. Not many do, hence why you did your post for advice.
Thanks for the advice, it is quite surprising too as I've not seen it mentioned anywhere yet! As you say I can't imagine many follow it and I won't be either. I've realised this evening I can likely cap off that sink waste pipe and have both shower and sink go in from the sockets on the other side, eliminating the what I think is minor case of potential cross flow. I'll need a couple of bends but I can make it!
 
The ideal would be to run them both in on the same side of that double boss branch, each on their own boss one above the other.

Alternatively, run a 50mm into the lower boss, then fit a 50mm tee and have the shower run straight in with a 50>40mm reducer and the basin tee runs into the top of it with a 50>32mm reducer.

Fit an antivac trap on the basin will allow the basin run to vent and stop the shower pulling on the trap,
 
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Thanks! There's no reason I can't run both into the same side, it just has to be the opposite from the current entry point so in that case I'll run the shower into the bottom one to keep the pipe low with good slope and have the basin into the top one, same side. Cap off the current open socket.

Thanks for both suggestions but if it's best have both enter the boss I'll go that route, just wanted to know before I make the journey tomorrow to the only store that sells Marley connectors anywhere near me!

I still don't really understand how it would be a massive problem with them opposite though, surely the shower flowing across into the sink waste exit would just flow immediately back out given the massively steep incline up to the basin. Especially with the antivac basin trap....?
 
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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" with the sink opposite.

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. I'm unable to use the old toilet connection as all of this is being boxed in to the adjacent cabinet depth so I wouldn't have space for a pipe there.

The second screenshot seems to indicate I should just go opposite and NOT staggered.

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

What you are proposing will be fine.


waste connections.jpg
 
Thanks Jim, I'm not sure why there's conflicting information when I've tried googling it but that diagram makes it very clear and looks like my original plan is back on. It'll involve less pipework bends from the basin if they can go in opposite sides vs the one side so that's the ideal setup.
 
We were always taught never to have the same level connections as there can always be a risk of crossflow - Whether it is permitted or not isn't the issue IMO - it's whether there is a risk of cross flow.

That being said that was with branch connections, as per Doc H, not necessarily around boss connections but I still wouldn't take the chance if there are any other viable/easy options.
 
We were always taught never to have the same level connections as there can always be a risk of crossflow - Whether it is permitted or not isn't the issue IMO - it's whether there is a risk of cross flow.

That being said that was with branch connections, as per Doc H, not necessarily around boss connections but I still wouldn't take the chance if there are any other viable/easy options.
Who is "We" and who told you this?, and only out of interest how long have you been dabbling in plumbing jobs Madrab?.
 
Had to make it personal didn't you ... surprise surprise

'We' being me and the 3 other apprentices who were trained by a very experienced (master) plumber with Glasgow City Council, too many years ago now thanks ... and hmmm lets see, I've been dabbling (I wish) with my own plumbing company, successfully, for oh .... over 20 odd years now and I certainly don't need any patronising plumbing forum 'pro' trying to have a go, thanks very much.

So please give it a rest and go and annoy someone else, try and leave the forum to others that have at least some manners and a semblance of respect for others on here, when they don't agree with something.
 
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We were always taught never to have the same level connections as there can always be a risk of crossflow - Whether it is permitted or not isn't the issue IMO - it's whether there is a risk of cross flow.

That being said that was with branch connections, as per Doc H, not necessarily around boss connections but I still wouldn't take the chance if there are any other viable/easy options.

The other option doesn't seem ideal - I might be completely wrong there though and it might be fine, definitely possible. Pic attached, blue is with the pipes opposite and is where I was planning for the basin to go from, red is what I'd need to do if going in the same side. So would be a couple of 90 degree bends to get past the rad pipes and over the soil pipe over to the basin location.

I might be missing something obvious but if it's just a bit of tap water splashing up the opposite shower pipe and falling back again, how big of an issue is it in reality? I understand the big issue two toilets opposite could cause....
 

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Yes, you are correct @burg from a design and standards point of view then they are of the view that with boss connections specifically on the same horizontal plane, there is no risk of crossflow. Where 32/40mm connections are concerned they have decided that there wouldn't be enough flow to cause a crossflow issue.

As suggested the BR's I was referring to are for branches and not specifically boss connections, I hadn't seen the other boss specs and as they say every day can be a school day.

1685626999968.jpeg
Also when it comes to being pragmatic, if there are specs that say it's ok and that is the best approach for you then no reason not to IMO.
 
Thanks for the advice, much appreciated and puts my mind at ease when the walls are back up. Saves me having to drive 40 minutes to the nearest place selling the Marley connections for another too!

Good to know about the branch v boss differences but I don't think I'll ever be getting to the point where I end up fitting a branch myself!
 

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