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.

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.

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.