branch in to soil stack

[andy1020]

Hi folks

I'm redoing the downstairs bathroom. The bath and basin currently drain in to an old cast iron branch going in to a cast iron rainwater pipe (from the gutters). 32mm is connected in by badly done wrapping. I reckon the rainwater cast iron joins the main drain underground but seeing as redoing it all I'd prefer to connect via boss in to the main (pvc) soil stack.

See diagram of soil stacks. Thinking about connecting 40mm waste either in at point A (under the floor branched in to the internal soil pipe (from upstairs en suite) where it flows horizontally (slightly sloping obviously) under the floor before going out the wall and joining the main stack) or point B in to the main stack.

Questions...

1. I know you can't connect below 450mm of the base of a stack. Does that mean point A is not OK cos it's not above 450mm of the base of the en suite stack? Or does that horizontal bit running under the floor not count as the base of the stack because actually it carries on and connects as a branch in to the main stack outside which has a much lower base underground? (This would be easy connection running my 40mm waste under the floor from bath and basin straight in to the soil pipe with a boss near the top half of the horizontal pipe. It's only about 2m away which is ok with 40mm I think?)

2. If I can't do A, then I need to run my 40mm waste under the floor and core drill through the wall and then connect outside via boss to the exernal stack. (I've got 1.2m plenty space for leaving the minimum distance from another connection and 450mm above base at ground level). Anything wrong with this?

I know both would work it's just what meets building regs. (By the way I'm in Scotland. I'm looking at part h which is English but struggling to find anything detailed and reckon it's same idea in all of uk right?!) Don't want a headache later as plan to sell house sometime and all it takes is a fussy lawyer or surveyor....

Thanks all...!

 

[John D v2.0]

You can connect at a because it's a branch pipe rather than a stack at that point. However you are supposed to connect smaller pipes into the upper part of larger pipes, for obvious reasons. Connecting at the side would be best avoided, as high up as possible. Another option is to have a branch in 110mm into the pipe at A, and then connect your smaller pipe into the end of the 110mm.

Just understand the faster things are moving the more pressure waves it causes as it rushes past. This is less of a concern when all branches have an open vent, but that's not the case here.
With ours I put cling film over the other outlets while flushing the toilet, to assess any pressure wave at the branch at the bottom of the stack. Turned out it was minimal let alone enough to pull the traps.

 

[andy1020]

magic, cheers John.
Yes I was planning using a boss with 45 degree elbow on the 40mm pipe so it's as high on the 110mm pipe as it can be.
I get the pressure wave yes and that the AAV will be slower to relieve that than an open vent.
I'll test that but yes reckon it'll be fine. The upstairs wc is only a couple of metres up and I'm not using any minimum level traps in new bathroom either. Thanks for the advice.
Saves a core drill through the wall

 

[John D v2.0]

I get the pressure wave yes and that the AAV will be slower to relieve that than an open vent.

Indeed also the aav is air admittance only, so it only lets air in and not out.

I'm not using any minimum level traps in new bathroom either.

Minimum if you're going into a shared pipe with other appliances would be 3 inch ie 76mm, the smaller depths would be ok into a gulley or similar air break.

Haven't got the doc handy but that's from memory