Internal Soil Pipe

plumb1_2

grumbler said:

plumb1_2 said:

Norman_Castle said:

grumbler said:

Norman_Castle said:

Can narrower rectangular pipe be used to reduce the area the pipe takes. The type often used for extractor fan venting? Is there similar watertight pipe for plumbing?

Why rectangular, not round - especially as adaptors for ducting are 100mm and soil pipe is 110mm? If you mean rectangular with the same cross-section, then the difference in size will be minuscule - 11%.

I am no expert, but what I see is

Replacing 100mm round pipe with 50mm x X wide giving the same area would be a 50% reduction in  depth. 

If a plumber did that they’d be shouting of cowboys

Who they and why?

Who? Customers when they get water coming through the ceiling 
Why? Because it’s a bodge job, they do make a Sq to round adapter but it’s for rainwater.They are not designed to be glued, simple pushfit with no seal. And if trying to glue them the rounded corners will not be water/air tight. If the soil pipe backs up you’ll only need a pinprick not sealed to spray foul water underneath the bath.
They do make 110x82 adapter for 82mm pipe in either pushfit or solvent.
If you or the OP want’s to bodge crack on.

grumbler

As I  said above, the suggestion was for the vent pipe only, not for the soil pipe under the bath. 
I don't think that an AAV is designed to be watertight, and it can be just 200mm above the highest point of water entry to the system.

Section62

Norman_Castle said:

MikeJXE said:

It's not just a vent pipe it carries solids from the loo downwards into the drain 

Above the height of any toilet or sink or bath drain it is just a vent so why does it need to be either round or of the same size as pipework which carries waste water? The question was does this exist which the answer seems to be no although its not clear why not. I understand why the soil pipe is 100mm but why does the vent pipe need to be the same size?

It doesn't have to be 100mm, other sizes are possible, but the BR rules on stacks and ventilation of sanitary/foul drainage are sufficiently complicated that most of the time people just use 110mm uPVC pipe for the whole job to keep it simple and avoid either having to check what they are doing is compliant with the regs, or the hassle of sourcing other sizes.

For example, Osma Soil is available in an 82mm (nominal 3") size, but the range of fittings is much more limited than 110, isn't generally stocked by the places builders use, and isn't necessarily cheaper than the 110mm equivalents (i.e. due to production volumes and stocking).

In principle a toilet pan with an outlet diameter <80mm could be installed on a stack with a nominal pipe diameter as small as 75mm, but in practice if you need to use pipe less than 110mm to form a stack then there is probably something wrong with the design approach. There could be edge cases where a slightly smaller diameter stack pipe could make a significant difference (e.g. the OP's situation with allowing a full-size bath where a short one was originally installed), but at the same time there may be other ways to achieve the same outcome.