40mm Waste into existing vertical sewage pipe

29 Jul 2015
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United Kingdom
Hi all,

Renovating my bathroom at the moment and discovered that the previous plumber had put the waste pipe for the sink/bath/shower into the vertical sewage pipe above the floor level (meaning all of the wastes were ultimately flowing uphill - which explains the poor drainage in the room!).

I want to get the replacement 40mm waste into the downpipe at the lowest possible point, but ideally want to avoid replacing the whole stack as it would mean ripping out some boxing in the room below that’s only just been decorated. Other than drilling a 40mm hole in the downpipe and sealing it with putty, is there any product out there that would give me a better seal? I know you can get compression rings for pipes that clamp around them for this purpose but the setup here would not allow me to do it as there is no clearance and a lot of other pipes fouling the area. I’m thinking of some sort of semi-circular plate that can be glued to the vertical pipe with a 40mm inlet on it. Does anyone know of such a product?


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You might just get a solvent weld boss adaptor below the existing hole but you'll only gain about 50mm at best.
You've just discovered one of the snags with doing a house room by room- all those compression fittings and shoddy pipework in that boxing are a concern, ideally you'd pull it all out and start again but understand your reluctance to do so.
Presumably it's the bath that's the problem- you could always put it on a plinth?
The shower is the issue, want a low-profile tray in there so the waste outlet will be below the floorboards before it’s even started.

I’m going to try and put a boss adaptor below the lip of the bottom pipe so it’s right at the bottom of the joist, the boxing below is fairly generously dimensioned so I can cut some more of the plasterboard out in-front of it. Solvent weld boss… should have guessed! SOmetimes its just a case of knowing what to ask for.

I do intend to replace the compression joints, but the rest of the 10mm pipe work is from when the House was converted to central heating and combi-boiler, and the whole system is a mess, the supply for the kitchen hot tap goes from the ground level, all the way up to the attic then back down to the ground floor kitchen - it takes about a minute to get hot water through. But that’s not a job for today.
I thought originally that section dropping into the plasterboard was curved but apparently not- yes if you can hack some of that plasterboard out to make space you should be able to fit one below the collar (don't try and fit it on the collar, the pipe radius will be different to the adaptor and it may not weld completely).

EDIT Does your proposed shower waste run across any joists? If yes remember there are limits to where and how deep you can safely notch joists....
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It does, but thankfully it’s only a 2m span (5 joists) so I believe I can get the drop required with holes within a reasonable tolerance of the joist centreline. I’ll brace it witch some steel strapping above and below also.
Only way it will work is re-positioning that copper so it is neat and tidy out of the way.

If you are running a basin, bath and shower into that waste I would recommend you increase to 50mm at the first branch into the stack. Add an anti-vac trap on the basin which will help vent the run and keep everything free flowing.
The bath is the opposite side of the room, thankfully. The holes for the waste are by far not the most concerning holes/notches. These 8”wide notches cut by whoever did the central heating are right in the middle of the 3m joist span. By a small mercy they align right in the centre of a 60cm floor board so I intend to add some extra screws through the board into the joists each side of the notch which should help with the compressive load. Would you also consider strengthening the joist here too? What would you do - ply-board both sides of the joist glued and screwed?

The notches for the 22mm pipe and the empty notches are no longer needed so I will fill them back in.


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Yup, I would .... make up sister joists to go either side of the notches and bolt/washer from opposite sides and make a plan of the floor with measurements where the pipework runs, for future reference.
I’ve picked up this boss collar which is more slender than others I‘ve seen, I think with a bit of effort and swearing I should be able to fix it around the pipe. Question… what do I seal the inside edge with? Solvent weld glue? Are all soil pipes compatible with solvent glue?


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I’ve picked up this boss collar which is more slender than others I‘ve seen, I think with a bit of effort and swearing I should be able to fix it around the pipe. Question… what do I seal the inside edge with? Solvent weld glue? Are all soil pipes compatible with solvent glue?

You could also use a mechanical boss.
Google it as there's a few to choose from.
Well, it was a bastard of a job as expected, had to re-route all the pipe work, reinforce most of the joists in two places (once for the large notch in the centre, once around the new waste). In the end the waste boss turned out to be rather easy, I couldn’t get the clip around so I just cut it off and solvent glued the boss to the pipe over a 54mm hole. It seems secure enough and it passes the sniff test. Job done.

Cheers all.




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