First floor bathroom / en-suite wastes

[Rishard]

Hello, just scouring the web / forum for similar ideas relating to first floor waste runs. I’m trying to work out the best way to route my waste pipes back to my stack. 
 

Is it best to try group the en-suite shower/basin/w/c into one 110mm pipe and back to the stack, or keep w/c separate from the basin and shower? From my research, these could step up to 50mm pipe from the basin/shower and back to the stack. 
 

OR 

 

Should I be trying to group say the w/c from the en-suite and the bathroom on the same 110mm run to the stack? Keeping the en-suite basin / shower on one 50mm run and do the same for the bathroom? 
 

I have posijoists which I can route most of these runs in. The w/c near my stack could go through the wall if beneficial. 
 

I’d also take some advice on good way to fix all this pipework to the posi joists. I thought about using some rips of ply to use as a side wall which I could clip / fix pipes to (on a fall) but if there are any other good bracket fixings people have used which would take up less space I’d be interested to know. 
 

Has anyone used much dampening on the larger soil pipes to reduce noise? Just a thought as the stack is currently coming up in the corner of a bedroom. This could be adjusted to come up in the bathroom if it makes more sense. 
 

Thanks for any insights. 
 

Stack highlighted red
 

 

[MPx]

Will be interested to see the "right" answers to this, but my layman take is:

 

Have the soil stack in the bathroom rather than bedroom if poss - it will make a noise but can be lessened with boxing in and sound deadening insulation.  You only need one 110 pipe with a suitable fall running from the top right corner of the En-suite to the stack in the corner of the Bathroom.  The toilets will need their own branch in at 110, but all of the other elements can be run in their individual 40mm/32mm pipe to the nearest point on the 110 and connect in there with an adaptor  Of course they all will need their own individual trap near their outlets.  The stack needs to run up and out above roof level for venting - and if it was me I'd put an air admittance valve above the outlet of the basin in the en-suite (the furthest element from the stack) and that will stop any gurgling - but many don't seem to bother with AAVs and accept gurgling.

[Rishard]

I agree, main soil stack can be dog legged to come up in the bathroom which makes sense. Are you saying the 2 toilets should have their own 110mm run rather than joining together before joining the stack. 
 

Once the en-suite toilet waste is in the joists, is it ok to join on to this run with say the shower waste? Would I use a strap boss onto the 110 or is there a better method? 
 

 

[MPx]

No not separate runs, just one pipe right across the back wall into the stack.  Each toilet will need its own local branch.  The other wastes can all individually go into the nearest point on the main 110 pipe using adaptors. Don't really like the sound of a dogleg on the stack - didn't realise that was the implication of moving it into the bathroom.  I'd probably leave it straight ... but its no different really to your 110 pipe feeding across from the en-suite so would work.

[Rishard]

Great. This makes much more sense than trying to run multiple individual wastes back to the stack. 
 

I want them all to be ran in the floor void to keep the rooms as clear as possible. Would I be right in thinking all these bends / connections want to be solvent welded connections? 

[MPx]

Not necessarily - horses for courses.  The 110 more often than not will be rubber seals push fit. Adaptors are often solvent welds.  Traps most often compression fittings.  The rest can be any of the above.  Think in terms of future maintenance and any potential long term changes.  No change hard to get at places - solvent weld for permanent trouble free fixing.  Everywhere else consider the other options - especially if you might need to move something to get at something else (eg to replace a tap).

[Rishard]

Ok that makes sense. I think I’ll try dog leg the stack so it comes up in the bathroom using 2 x 45° bends. Just enough offset to get through a 140mm block wall and come up on the otherside. It’s doesn’t look too upsetting. 
 

My en-suite w/c is 2.8m from the stack so I can just get the fall within my joist webs. Do you think if I wanted to move my w/c to where the basin Is in the en-suite. Could I do a stub stack in the corner where the w/c is currently located and gain the fall above floor level in boxing? 
 

Because of where the joist webs are my 110mm pipe is about 200mm from the wall. A swept bend might be needed if I was to do the stub stack option. I see in the regs that it should be avoided where possible but can be used if large radius bends or 2x45° are used. Would be using an aav. 

[MPx]

What fall are you targetting?  I understand just 25mm would be enough over the 2m that it will be in the En-Suite (or 60mm total from the stack - 4.8m away) - have you not got a 190 void?  I think if you break out the soil pipe into the above floor en-suite, you're going to have to be very imaginative with how you box in and disguise it  - but I guess it could be done.  

[Rishard]

I’m going for 12.5mm per m. This works to the original plan as sadly I only have 50mm adjustment within the web of the joists. 110mm pipe inbetween the 160mm webs. Based on this lack of fall to move the en-suite w/c to the new location using the floor void to get the fall I may have to settle with its original location. 
 

Alternative option would be to exit the w/c waste out the wall and create a new stack to serve this externally on the house. Not a fan of this. 
 

or box in above floor level in the en-suite . Also not much of a fan of this. 
 

Should have gone for the larger joists… ah well. Maybe next time. 

Edited December 16 by Rishard

[Nickfromwales]

Over 2m it can be virtually flat. It’ll have the velocity of the flushed water to get you well clear of the first straight running pipe and off down south.

 

I did an en-suite where a Saniflow was constantly blocking / breaking down, and the clients asked me to find a route to the manhole. Due to other services the 5m run had to lay pretty much completely flat, with a very slight fall for the last 1.5m; when I say slight, almost no distinguishable movement of the bubble on the level. 
 

Works perfectly, in my mates FIL house, and has been doing so for the last 15 years.

 

Regs are wonderful, when life’s wonderful, but if you ask a BCO for a deviation, and demonstrate that it flushes perfectly 10 times in a row, they’ll just say “fine”.

 

If you have b regs involved? 
 

If not, fit the 2m run with the absolute max fall you can introduce, and move on to the next problem. It’ll be fine.

[Nickfromwales]

On 12/12/2025 at 20:47, Rishard said:

Great. This makes much more sense than trying to run multiple individual wastes back to the stack. 
 

I want them all to be ran in the floor void to keep the rooms as clear as possible. Would I be right in thinking all these bends / connections want to be solvent welded connections? 

Solvent weld if under floors etc is a must, for all smaller bore stuff. 
 

Soil / foul 110mm is perfectly fine as push fit.

 

I use all round patent band to hang the soil pipes and it’s easy to do the falls this way too. You want to avoid connecting the soil pipe to the lower chord of the joists.

 

 

For a previous clients build where I fitted a bathroom over a bedroom I used dynamat (Killmat) to directly sound-deaden the pipework, and another layer of the acoustic rockwool immediately above the downstairs ceiling (between the bottom chords of those posi joists) for as good a result as possible. 

[Rishard]

If I was to run the en-suite w/c the full distance it would need to travel 5.6m to get to the stack. I’ve got 50mm at the very most for the fall. Do you think that’s possible? Would be great if it was… 

 

I’ll have a building inspector but if it’s the same guy, I doubt they will give it a seconds glance. More for my own piece of mind. He didn’t care a dot about the tons of mesh Il lapped to specific distances and tied beautifully all to the spec of my engineer. More interested in talking about how crap the company he works for is. 

 

I’ll try some of that banding and mating. Looks the business. 

[Nickfromwales]

41 minutes ago, Rishard said:

If I was to run the en-suite w/c the full distance it would need to travel 5.6m to get to the stack. I’ve got 50mm at the very most for the fall. Do you think that’s possible? Would be great if it was…

Is it a straight run? With 50mm fall I'd say it would work. There, I said it.

 

I'll get a kicking here from most for saying that, but then they'd have to shut up and buy the drinks when it is demonstrated and proves to actually be reliable and functionable.

 

The energy / velocity of the flushed water is quite significant, and when you add in the fact this drops vertically first, gaining further momentum, you'll see this will be shifting at a rate of knots when you flush the WC (or "bog" if you're from my neck of the woods).

 

If it really matters to you then set up a WC in the room loose, run the pipe, see how it performs, decide if you're happy. Proof will always be in the pudding, regardless of what's on paper.

[Temp]

Most bathrooms need some sort of storage. One option is to build storage against a wall and run the waste in the bottom.

[Rishard]

22 hours ago, Nickfromwales said:

Is it a straight run? With 50mm fall I'd say it would work. There, I said it.

 

I'll get a kicking here from most for saying that, but then they'd have to shut up and buy the drinks when it is demonstrated and proves to actually be reliable and functionable.

 

The energy / velocity of the flushed water is quite significant, and when you add in the fact this drops vertically first, gaining further momentum, you'll see this will be shifting at a rate of knots when you flush the WC (or "bog" if you're from my neck of the woods).

 

If it really matters to you then set up a WC in the room loose, run the pipe, see how it performs, decide if you're happy. Proof will always be in the pudding, regardless of what's on paper.

It is a straight run, apart from a 45° where it joins into the stack. I’m happy to make it the designated site bog while the build progresses. That should put it through its paces. 
 

I did think about having a high level traditional cistern. Would this add any extra umphh to it? 
 

the storage solution is also a good idea. I’ve done plenty of boxes out skirtingboard plinths in the past for customers. Feels like a bit of an after thought, a cupboard sounds more intentional. 

Edited Wednesday at 21:25 by Rishard

[Nickfromwales]

On 17/12/2025 at 21:24, Rishard said:

I did think about having a high level traditional cistern. Would this add any extra umphh to it?

Yup.