2 toilets, 1 soil pipe

[MrDavid]

Hi all

 

Looking for some advice on soil pipes, any help would be much appreciated! Drainage isn't my strength 

I'm renovating a 4 bed flat, looking to add a 2nd bathroom and rearrange the existing bathroom and kitchen.

It's a 1920s ex LA building, has one external SVP and one internal WP.

 

Some fundamental q's

Can I connect 2 toilets to the same soil stack using 1 soil pipe?

The existing stack was shown as 3.5" on the original architect's drawings but seems bigger in reality (I will check).

And do I need AAV's for each toilet? The SVP vents to atmosphere above eaves level and this flat is on the top floor.

 

Assuming that's all fine, does anyone have any comments on the following two options?

 

 

Option 1 is to run a soil pipe in a service void below bathroom 1 with a raised FFL.

Creates some Part K issues with the level changes, which are resolvable if not necessarily desirable.

May bump up the hallway FFL so height difference is only 1no. 220 rise.

 

 

Option 2 is to flip bathroom 2 layout and try to run the WC2 soil pipe behind the bathtub.

However I am concerned this creates a lot of bends, especially in the connection back to main stack.

 

I stumbled on this forum a few weeks ago as I was researching for my flat and am a big admirer of this community.

After this I have a question on to combi or not to combi for 2 bathrooms, which I know is a favourite question for buildhub.

 

Thanks

 

[PeterW]

8 hours ago, MrDavid said:

Can I connect 2 toilets to the same soil stack using 1 soil pipe?

Yes

 

8 hours ago, MrDavid said:

The existing stack was shown as 3.5" on the original architect's drawings but seems bigger in reality (I will check).

More likely 4” iron

 

8 hours ago, MrDavid said:

And do I need AAV's for each toilet? The SVP vents to atmosphere above eaves level and this flat is on the top floor.

Potentially on 1.08 but could be on the sink waste

 

8 hours ago, MrDavid said:

Option 2 is to flip bathroom 2 layout and try to run the WC2 soil pipe behind the bathtub.

this is fine but watch your levels as you need about a 40mm drop from WC to stack on that and you may find you need a WC with a high outlet to do that

on the furthest WC so you don’t end up under the level of the second WC outlet. Needs to be a Y / swept T into the main run.

 

Your bigger issue will be the shower in 1.08 as it will be near floor level - the drains to the left from the kitchen seem to go to a different stack..?

[MrDavid]

Amazing thanks for the feedback Peter. Noted on the shower level, the drainage levels here may prove the driving constraint rather than WC soil, if I want to go for level shower tray.

 

Do you see any issues with Option 1? A difference between the two options I neglected to mention is that Option 2 squeezes 1.08 by 100mm to return some wiggle room into the service area behind 1.07. There is an existing VP by the sink here that needs to be negotiated.

[TerryE]

AKAIK, option 2 is bad news from a blockage PoV you have a left 90° bend, followed by a right 45° followed by a right 90° all without rodding access.   I can't see the BInsp being happy with this if he picks it up.  This is a shit-show waiting to happen.  Option 1 is a far cleaner roddable route.  In general you need an accessible rodding access at any 90° bend. 

[saveasteading]

15 hours ago, MrDavid said:

Drainage isn't my strength 

 

Some basics to remember, which some plumbers and groundworkers don't seem to check, or understand.

 

Downhill is essential, and avoid droops by using supports.

Corners are bad. Make them as few and as smooth as you can.

Think through, the passage of the worst that might reasonably go along that pipe. Will it reach the end?

It is assumed that 2 wc's will not be flushed at the same moment.

If it does block, is it easy to clear using a snake or rod? Will the snake get round the corners?

 

Once each section is built, test it. Not with water but a golf ball, A snooker ball is ok but runs too well. For small pipes use a marble or ball bearing.

you can test each section and finally the whole of each length. No throwing allowed.

Finally test with water. You can buy an air test kit. Alternatively bung the outlet end (£5 for a bung) and fill with water. Then rest easy.

 

With all that thought through, you shouldn't ever have a blockage anyway.

 

Specifically,

why do you have 2 drains at kitchen to bathroom wall? can't you use just 1 ?

Presumably all the 90 degree bends are shown indicatively, and will have smoother turns/junctions.

 

 

[MrDavid]

On 12/01/2023 at 15:21, TerryE said:

all without rodding access

Fair enough! Will put in RE w/ access from kitchen at that 90° at end of bath tub, are there any limitations on RE branch lengths?

Option 1 is also my preferred route, but I have 2.6m existing FTC and am contemplating 100mm ceiling buildup to fit in recessed downlights, maybe more if I want to conceal duct route back from 1.07 to external wall. So all combined that squeezes 1.08 to 2.2m FTC which is not ideal.

 

On 12/01/2023 at 15:43, saveasteading said:

why do you have 2 drains at kitchen to bathroom wall? can't you use just 1 ?

two branch connectors already exist going into that WP at 160mm AFFL and 440mm AFFL, so would like to split drainage flow between them to avoid blockages

and yes this is schematic only, am going for 200mm bends on the 90s, does need some planning.

testing is sensible!

 

Thanks all for comments