Swiftly reaching the limits of my knowledge when it comes to drainage, I have realised. The layout below is what we currently propose but I'd be grateful for any input/critique and experience of what to look out for

I'm intending to avoid soil stacks going through the roof and want to do the lot with stub stacks and AAV's where required - this should be possible since we have a bungalow arrangement.

Looking at the levels, there'll be several items (bath and MVHR condensate drain) which will need to enter the main stack at a lower level than the main WC pipe (float?). I understand that certain manufacturers offer a collar boss arrangement that is designed to allow this without needing the standard 200mm separation to avoid backflow.

This is appealing, since we only have 50mm screed, then RC slab on insulation so I'm keen to keep as much of the horizontal runs above the FFL as possible. However I realise there may be horror stories with this so please share any ideas.

Likewise, draining the island unit dishwasher and sink is a challenge. What I've ultimately come up with is a separate run below slab, meeting in the nearest inspection chamber so everything can be rodded.

In terms of sizing, I've assumed 40mm pipe to everything but the WC's. 110mm stacks and whatever we need below ground. This is mainly to keep things 'simple' rather than mixing 32 and 40mm pipe but again if this is overkill it would be good to know.

Any opinions on solvent vs. push-fit?

Thanks in advance.

I'm not an expert either. That's one reason I used a water board-approved plumber, as well as to avoid an extra inspection. So I have questions as much as advice.

(1) I don't think you'll need AAVs on the sinks in the kitchen, but check with your plumber.

(2) I think the AAVs on the soil stacks need to be 110 mm units. Are they?

(3) Note that the 'WC manifold' will be somewhat lower by the time it gets to the second WC, reducing the height available for extra connections to the stack below the WC connection. Check carefully how it will all fit together - I ended up with the waste pipe from one WC on the first floor passing above floor level in my plant room since there wasn't space to put it in the floor void. Not a disaster but not very elegant either!

(4) Where is your vent pipe? Mine is a 3" steel rainwater pipe up the outside of the house.

You may be able to get away with HepVO trap as an air admittance valve.

I would have an external vent pipe at the head of the run

I would run the kitchen sink shower and bath all to the same external gulley

I would run the two WC's on a good fall pipe into an external vent pipe collecting sinks and basins on the way

No drains under building, one pipe in screed to island sink

Thanks guys,

I've asked our BCO about the vent pipe at the head of the run and the durgo valve / HepVo etc.

Tony, are you suggesting a single external gulley 'pot' that receives separate pipes out from kitchen sink, bath etc.? That sounds like a lot more holes in the outside wall. I was hoping to just use the inspection chamber from the drain as a rodding point, so I can't see what advantages the gulley brings.

Agree on avoiding the drains under buildings, but we only have 50mm screed + 10mm (?) finish. Even with 32mm pipe that's not a lot of cover. Do you think it's doable or would the screed just crack?

My kitchen sink and my downstairs shower run straight into separate drains; I don't see what the problem is and neither did my plumber or BCO. I don't have any waste pipes through my walls at all; they all go under the slab.

Posted By: Doubting_ThomasShower is intended to be walk-in wet room type, with a 'bottom outlet' gulley (plus bell trap) embedded in the slab - like a commercial kitchen. Outlet from gulley drops straight down to drain below. I can see how this might need to be vented as there's no stack to this branch, but will wait to see what BCO says.

My downstairs shower is also a wet room with the gulley/trap let into the slab, which is also a raft. We had reduced the depth of the slab in the area to make room for the trap but I still had to chisel out some more in the precise few square inches (after checking with the SE) because the depth wasn't quite enough.

I went for a side outlet because I wasn't confident that the vertical 110 mm waste pipe stub would be precisely under the waste position in the shower base former. The trap outlet is led by a short length of 40 mm pipe straight onto the top of the piece of 110 mm poking out of the slab. There's no need for a vent.

Again though, why risk it when Tom's original plan appears fine?

Why not run under the slab?

Thanks for the input rocconut.

Would be good to know from others whether there are similar failures with solvent systems of a similar age - I'm guessing they'd go brittle too, just with general plastic aging?

A standard ridge vent is unlikely to be airtight enough for us, but I'm still doing the research...