Opinions please - Soil and waste design

[Fireball40k]

I could desperately do with some help in finalising a new soil and waste arrangement for a domestic extension.

I need to get two bathrooms into a single exernal stack. The bathrooms are side by side - the stack is on the far side of one of them. Bathroom 1 has a WC, bath, shower and basin. Bathroom 2 has a WC, shower and basin. The WCs will be back to back either side of a 9" wall and this has caused me one of the biggest headaches trying to avoid any crossflow between them while sending them both into a single horizontal soil pipe that spans just over 3m to the external stack.

The solution that I am hoping will solve this is to use a 45 degree branch and a 92.5 degree branch in the horizontal plane to combine the two WCs in a safe manner as they combine and then run along the single horizontal 4" to the stack. I also need to put a basin into the boss of the 92.5 branch.

Just after the two soils join, I then intend to use two 45 degree bends to send the flow down and through 90 degrees to the main horizontal flow. I need to do this because I am limited for space and height - two 90 degree bends would result in the main horizontal starting too low (the 3m span of the main horizontal needs to occur totally within a 7" floor void, so to get the right fall I need every millimetre).

As the main horizontal emerges into daylight I intend to run it round a 90 degree bend with access and then into a 92.5 degree branch on the vertical stack. Meanwhile, a 40mm waste from the bath will drop onto that external run before it enters the stack through a boss.

The 40mm wastes from the two showers will emerge separately and then combine (again into 40mm) and run down to a boss on the stack (at a safe distance below the WC input).

The diagram of all this is (hopefully) shown below, but can also be found in JPG or PDF format at 'http://www.ctxd.com/picture_library/soil1.jpg' or http://www.ctxd.com/picture_library/soil1.pdf'.

One of my main questions concerns the minor vent stack. I want to help protect the traps of the WCs and the basin - is this a) necessary in this design and b) in the right place? If it needed to be placed after the two WC soils merge (as a stub stack) then I may well have height problems in getting the main horizontal high enough at that end to give the appropriate fall.

The other question relates to the merging of the two shower wastes - is this acceptable and is 40mm large enough for both of them - considering that one of them will have travelled 4m (would a vent next to the trap of this long run shower be useful/necessary)?

I would really appreciate any feedback from people who know about these things because I'm at the far limit of my understanding here and I would rather sort the problems out in theory on paper before hacking the walls apart and failing inspections!

Please give me your thoughts.

Many thanks in advance. Steve

 

[Softus]

It all looks very well thought out, Fireball40k, but in answer to your rather intelligent questoins...

One of my main questions concerns the minor vent stack. I want to help protect the traps of the WCs and the basin - is this a) necessary in this design and b) in the right place?

a) Yes.
b) Yes, but there's a risk of pulling the water seal from the bath trap, but you could put a 40mm anti-syphon unit between the bath trap and the soil branch.

The other question relates to the merging of the two shower wastes - is this acceptable and is 40mm large enough for both of them - considering that one of them will have travelled 4m (would a vent next to the trap of this long run shower be useful/necessary)?

No - that distance is too long for 40mm, so you need to use 50mm.

...I would rather sort the problems out in theory on paper before hacking the walls apart and failing inspections!

I have nothing but respect for that approach. And perhaps your understanding is better than you realise

 

[Fireball40k]

Thanks Softus,

I appreciate your advice.

One other thing, would you suggest going for push fit or solvent on the waste side of things? I'm pretty sure that I'll opt for push fit on the soil stuff, but (after having experienced a basin waste coming apart and flooding the kitchen) I'm seriously considering the solvent approach.

Thanks again

Steve

 

[Softus]

Personally I don't have a hard-and-fast rule about solvent weld and push-fit, but generally I favour push-fit for inaccessible joints.

I wouldn't recommend a solvent weld joint purely on the basis that it will resist strain better than push-fit (aka ring seal) - you should never install plastic waste such that it's supporting its own weight, because in time that's likely to result in it fracturing.

Another consideration is thermal expansion; a long run of pipe should have room for longitudinal expansion, which you can give it with ring seal (by deliberately cutting the pipe short) but not as easily with solvent weld. I've once had to fix a 110mm internal soil stack bent like a banana between floors in a block of flats, simply because it was installed in the winter and given no expansion room. Just after the two year warranty was up one boss cracked under the strain and started a campaign of weeping its putrid tears down the soil stack to collect behind the kitchen units on the ground floor flat.

 

[Nige F]

Installed with no expansion room must`ve been done by transco guy doing a moonlighter

 

[Softus]

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