Is is permitted for basin to discharge into w/c branch pipe

18 May 2012
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United Kingdom
I'm installing a shower room in a new extension that has a suspended floor over the 110mm soil pipe that's part of a single stack system which also serves a shower room with w/c macerator and bathroom with w/c upstairs to the open vented stack. Also, the stack takes discharge from two sinks on the ground floor. The soil pipe also has a 110mm AAV stack at this level.

The new ground floor shower room has 170 x 45 mm joists at 400mm centres. The problem is that to install the wash basin I will need to find a way to cross 3 of these joist to reach the soil pipe. This pipe only provides for about 75mm free head space in which to install a 40 mm bend and boss strap which looks impossible. I could run holes through the 3 joists to provide extra head room of some 120 mm to reach to top of the soil pipe in the void between the 3rd and 4th joists, but would need to put in brick piers under each hole to retain joist integrity.

I will also be installing a w/c which will join into the soil pipe through branch pipe entering a push fit y junction. The basin and the w/c both initially drop into the same joist void. If I could run the basin discharge within this void, I would have some 100mm plus head room to connect into this 110mm branch pipe.

The 110mm pipe run from the w/c to the y piece is around 1.5m, whilst the distance to this branch pipe is around 0.9m, enabling me to achieve the required 1-in-55 gradient all in 32mm.

I have a similar, although a little less demanding issue regarding the shower. I want to keep the tray at floor height. This is located on the opposite soil pipe side with the waste discharging one joist away from the soil pipe. Rather than cut through the joist, I could attach a boss strap to this at 2 o'clock rather than a 12 o'clock to the soil pipe.

My 2 key questions are, is it permitted within the regs to:

1. plumb the basin waste into the w/c discharge pipe within 200mm of the w/c drop?

2. connect a 40mm waste into a soil pipe at 2 rather that 12 o'clock (also a possibility for the basin)?
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Thanks Nige. I also spoke with the guy at Osma who tells me that bosses between 10 & 2 o'clock is compliant.

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