Soil stack venting

[hurridriver]

In a two storey house with a single soil stack can the "vent" portion of the stack above the highest inlet be reduced to 50mm? Have seen so many conflicting views when trawling the internet but no definitive answers. A customer wants a bathroom changed which is no problem but a soil stack is in the way. No room to fit a 110mm AAV but a 50mm pipe could be squeezed in and routed into the loft with a 50mm AAV topping off. Any advice gratefully accepted.

 

[justlead1]

Hi. I think 75 mm is the smallest to comply with building regs. However 50 mm was the vent size when anti syphon was employed on one pipe systems. Good Luck