Sink waste to soil stack or not?

18 Mar 2007
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United Kingdom
I'm having a sink installed in the downstairs loo and wondered if it's ok for the waste to go out the wall and into the soil stack 50cm away? The alternative is a 250cm run to the same outlet as the kitchen waste.

I've been warned that going into the soil stack could cause smells to come out the sink if the trap dries up. The alternative just means more external piping.

I know a strap on boss is what I need for the stack.

Any thoughts?
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The 'basin' ;) will enter the soil stack about 50 cm above the toilet waste from the same direction.

The bathroom loo enters the stack on the floor above so 200 cm above the basin entry.

Do those figures look okay? I understand that there are minimum distances from loo entry of the stack and waste. Like don't have them facing each other.

Do you get any suction from the toilet flushing all that distance above and passing the boss. ( The images of this are making me chuckle )

I also have to go past the soil stack to the kitchen basin waste so I'd prefer it to be a correct job.
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Likewise, I can’t see any problem. Being an external stack it should be open vented & this will (should) prevent the trap siphoning; I have a very similar situation with the connection to a new downstairs utility room via a boss strap from an upstairs soil stack. Any connection must be a minimum of 450mm from the drain invert (the horizontal bit in the ground) but that’s not usually a problem; if you want more details, look in Part H of the Building Regs. here.

The soil pipe manufacturers also do useful guides based on Building Regs requirements; to be found on their web sites.

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