Bathroom clean waste layouts

Hello All
Old bathroom has bath, shower, bidet and basin all draining connected to a common 32mm waste. When the bath is emptied, basin and bidet gurgle loudly and the shower tray fills with water.
I'm replacing all this with 40mm but need some advice for layouts. All ground floor. Drop to exit about 6" below floor, easy access from above.
I think I can stop gurgling by replacing the piddly slimline S-traps with beefy P-traps, does that sound like it'll work? AIUI, it gurgles because the outrush of water from the bath causes a vacuum. I've never seen one, but it occurs to me that an air inlet valve (in 40mm, basically a flappy bit of rubber) might help? Or am I completely wrong?
Is it realistic to expect no water back into the shower tray? Is putting a seperate drain just for the shower a good idea?
Not looking for shortcuts. Don't intend to move so want this done right.
Thanks!
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Simon Avery, Dartmoor, UK
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In an earlier contribution to this discussion,

Sounds like you need non-return valves in the outlets - particularly the shower. I don't know whether they exist in a form compatible with 40mm waste pipe.
It will help if you separate the bath and shower outlets so that, ideally, they have separate routes to the stack. If you *must* join them, do it as far downstream as possible.
The reason it is happening is because the bath is higher than the shower and, when you release a lot of water all at once, it's easier for it to flow up into the shower than along into the drain. You need an easy exit path for the bath water - with as little back pressure as possible - to ensure that it doesn't flow the wrong way.
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Set Square
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wrote:

They do exist, plumbers merchants stock them, but they are regarded as a last resort for an existing installation. Need cleaning, for one thing. With a new installation good layout should avoid problems. AAVs if absolutely neccesary.
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Niall

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Hello Set

That would prevent the shower flooding, but I don't think it would fix the gurgling. I'm pretty much resigned to having the shower on its own drain now. Seems simpler, and I've got to do something with all this 40mm pipe I over-ordered...
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Simon Avery, Dartmoor, UK
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Hi Simon.
The main problem with your existing layout is that is was done in 32mm pipe.
Although not ideal, you may be able serve all of these appliances on a common 40mm run as it is likely that you will only discharge one appliance at a time. the recommended max run in 40mm is 3 meters if you're not exceeding this then fine. My guess is that water came up in the shower because you were discharging the bath down a 32mm pipe which is undersized for a bath outlet. you should be able to solve any gurgling problems by fitting anti-vac traps or HepVos
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Hello BillP

Well, strictly speaking there's at least three different sizes, two imperial I think. The only good thing about the existing is that it doesn't leak inside the house. (The outflow completely misses the drain though)

I am. The long run to the bath is about 4-5m with a ~140mm drop below floor.
What I'm looking at now is shower seperate, then bath and basin on /straight/ run (90 bend at joint for basin). The current one goes up and around joists and all sorts, more elbows than straights which can't help. The shower is going to be coping with a lot of water (I hope, if I've got the input right), arm-waving guess of 20-25l/min, so if I don't want a flooded floor, that one's got to be great also.

I think this might be the way for the basin. Thanks.
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Simon Avery, Dartmoor, UK
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I think that's what anti-syphon traps are meant to fix, they have a flapped air inlet to break the vacuum without drawing through the trap. Just had a sook on one (new & clean :-) & this is what it does.

As set sq said, it's all about levels (+ woosh factor). I think 40mm should help, but separate runs (or part runs), each with their own fall, would really nail it. I'd prefer to avoid non return valves, just another thing to go wrong/get clogged.

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On 20/01/2004 Simon Avery opined:-

I don't know about your other ideas, but I would be inclined to move the points where the 3 connections join, as far away as possible combined with being as low as possible.
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Harry (M1BYT) (Lap)
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For a start the pipe is far to small a diameter to take all those appliances and would need to be in the region of 50mm to do the job properly. Are you sure it is only 32mm (1 1/4 inches) ? It would be easy enough to run a length of 50mm behind all the appliances and use reducer tees to connect the different things onto it. Also, if you don't have an air intake point at the furthest point from the main stack, or even a loop venting pipe configuration, then the appliances are all going to suffer from having their water sucked out of the traps.
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