Combined rainwater and soil stack?

[ZoeZoe]

Hi, we have moved our upstairs bathroom so need to put a new soil stack on the outside wall. In the place where we want to put it is a rainwater pipe with a "bucket style" opening at the top, (sorry I don't know the correct term), because it is a flat-roof property so it catches the water from the gully on the roof into the bucket type opening and goes down the pipe.
Our rainwater and sewage all join to the same area underground, so would it be possible to combine our rainwater and soil stack into one pipe? Rather than having that rainwater pipe plus a new soil stack pipe next to it too.
So basically we want to replace the rainwater pipe with a soil stack with the "bucket style" opening on top. So any smells come out of the "bucket" above the windows, but also the rainwater still flows into it from the roof. So both rainwater and soil will go down the one pipe and under the ground.
Is this pipe combo allowed? Or do they have to be kept separate above ground?
Thank you very much for any help.

 

[exedon2]

Vent pipes must terminate 900mm above any opening into building (window ect) within 3 mts of vent pipe building regs H1 section 1.23

 

[snowhead]

The Bucket aka Hopper, would be the lowest open part of the stack so would have to be 900mm above any opening, as above.

The regulation about distances is there to prevent the possibility of foul smells entering the building.

 

[ZoeZoe]

The Bucket aka Hopper, would be the lowest open part of the stack so would have to be 900mm above any opening, as above.

The regulation about distances is there to prevent the possibility of foul smells entering the building.

The hopper would be more than 900mm above the top of the window. So would that make it ok to combine the two?

 

[Ric2013]

What used to be called a two-stack system, I think, was formally quite common. You'd discharge waste from showers and baths and basins into a hopper (sometimes shared with rainwater) and down to a trapped gully at the ground level via a 2.5"ish drainpipe (where the underground drainage is the old combined system). This form of system was already considered old-hat in the 1969 Readers Digest DIY manual and I'm not not sure if converting a previously rainwater only downpipe to combined use is really acceptable (I think such systems run on grandfather rights only), but in the circumstances of a combined drainage system, I'd struggle to say it's really wrong, though I would encourage you to consider a sustainable urban drainage approach and to discharge your rainwater to some form of ground infiltration system if at all possible.

However, and it is a big however, the old-fashioned two-stack always had the soil from the WC as a separate 4"/110mm pipe. What is sounds like you are proposing is a bit different in that you'd be using the single stack system of a 110mm soil and vent and then run the vent to the edge of the roof and discharge into the top of it via a hopper. I suppose if you can comply with Approved Document H 1.23, that only leaves the issue of 1.31 which requires a perforated cover. I can't think how you can guarantee the hopper won't block when you are applying a cover over it and so I can't see how you can guarantee the stack is, in fact, vented. I also have my concerns about how you can possibly do this and not make it look like a bodge. Unless you are trying to be alternative and willing to have a regulatory argument now, or at at some point in the future, I'm inclined to say you need a separate pipe for your rainwater.

Either way, if you're on a combined system, any soil connection to the underground drain needs to be without the trapped gully you most likely already have for the rainwater downpipe, so you have some underground pipework to modify.

 

[ZoeZoe]

Thanks for your replies