Size of waste vent pipe

[Drew1000]

Is it OK to reduce a 100mm soil pipe serving my toilet bath and wash hand basin to a 50mm up into the loft and then through a vent to outside. This would save me putting in a pipe chase as I could get the 50mm vent pipe up through a stud partition.

 

[ajs heating]

no min is 75mm 3" in part h regulations.

 

[Drew1000]

I might be clutching at straws but would two 50mm pipes branching from the 100mm be OK. I would then branch back into a 100mm in the loft for ease of fiiting to outsie vent.

 

[ajs heating]

couldn't answer that one. as we don't chop and change pipes like that and never seen it done.
you can always have a word with a building inspector at the council.