Sink Drain under floor level

Hello, I have been told by Building Control that I have to run my sink drain in 110mm if it is under the floor, but can run 40 or 50mm if above the floor. I have a cloakroom sink which is in 32mm, and runs under the floor in 32mm to a stack. Is the Building Control man correct or has he misunderstood what I am proposing, and if so, would it be ok to run a sink waste under floor level for about 3.5m in 50mm to an outside stack (with his permission)? Thanks.

ask him again, (but make sure you are both meaning the same thing) since if some one here says X and he says Y whos fault is that?

"oh mr building inspector some one on the diy not forum said you are wrong" , dont think that will go down very well do you

a reason for it being 110mm under floor maybe due to the blockages that might form if it was of a smaller diameter, this is due to foodstuff entering the pipe. This is why the required fall from a sink is relativly steep (to clear food stuff from the pipework) and it is also the reason why you should not use a bottle trap on a sink - as they are not self cleaning.

If the pipe work is accessible (above ground) then its not an issue if it blocks and a basin is exempt as it does not deal with foodstuff flushed down the waste.


not saying he is wrong or right though!

BTW - Ive got the Part H of the building regs next to me i'll have a quick browse. - no couldny find anything related to underground waste pipework from a sink.