toilet waste pipe

[jonnyp1]

i am replacing a 2nd floor bathroom suite, in 1970s 3 storey house . the existing waste pipe appears to be 4" upto toilet below but because it goes up at angle from there between roof it is 3" up to 2nd floor. i thought the rule was minimum of 100mm, if so what should i do i dont know but not sure if 100mm will fit up where the 3" pipe goes, will it work on 3" should i just close it up and leave?? The basin and shower drain separately down to 4" pipe so just toilet.

 

[Nige F]

From your wc down it should be 4 inch.......but a vent can be smaller.........was passed by building control when built .....probably because the bigger pipe wouldn`t fit