Fitting advice for 2-in-1 Combination toilet/basin

[Ric2013]

You need some kind of branch tee off the soil pipe which will involve cutting into the stack, I'm afraid, with a 110mm branch and then as Shaun said.

 

[davhill]

Good one Ric, thanks,

The soil stack has a collar at the base and seems to plug
straight into the ceramic pipe heading to the septic tank.
Just been searching and found a Big Boss in various sizes
but none as big as 110mm. If there's a strap boss with a
110mm stub and the tiny fall, that'd be perfect.
Funny thing is, the ID of the 68mm strap boss I have is
less than 15mm less than the ID of the toilet outlet so
the step would only be 7mm. This is why I thought it'd work.

 

[Ric2013]

You really do need to cut in. It isn't really possible to make a 110mm strap boss as you can't really drill a 110mm hole into a 110mm pipe (think about it), hence they don't exist.

I do see your point, though. Your pan ID is likely to be around 80mm, so the restriction would indeed be only a few mm (were it a water supply pipe the restriction would be less of an issue and I have myself gone from 22 to 15 to 22 to get through an existing hole I did not want to enlarge due to the presence of asbestos). The problem is it would be a step in an upward direction with a waste pipe that is designed to be but partially filled, and you have solids in that pipe. So it would only be a matter of time before a bit of paper or a lump of faecal matter sticks on the step and then you inevitably have a regular blockage where it is least desireable.

Stick some photos up of your soil pipe and I'm sure we can advise you if you get stuck.

 

[Ric2013]

If this is ground floor, there could be possible issues relating to how close your intended tee is to the underground bend where the stack becomes horizontal. There is a minimum recommendation of 450mm, but I have seen that rule broken without any obvious symptom. As I say, if you really want to DIY this, some photos and a sketch would help us help you.

 

[SimonG]

I take it that there was nothing there previously? If so, are you aware that the installation will be notifiable?