Can size of soil pipe be reduced to vent

[eddiewarden]

I have a 110mm diameter soil pipe from the wc to the sewer, this pipe is vented through the roof.
Can I reduce the diameter of the pipe venting the system from above the wc.
Any info would be appreciated.

 

[compheat]

The answer is yes you can size it down to 50 mm or use a Durgo air admittance valve inside the building (roof space)

 

[JimCrow]

The dry section of a 4" vent pipe can only be reduced down to 3" (75%)
so your 110mm svp could be reduced to 82mm pipe.

 

[Breesey]

And an AAV is only an option if the system is vented else where