Reducing soil vent pipe

[Luke246]

We currently have a 110mm cast iron soil vent pipe blocking part of the bathroom window and coming out through the roof.

Is it ok to reduce the middle section down to 40mm/50mm pipe using reducers either end?

 

[DAZB]

Without seeing it this is just a suggestion but you could cut it off lower down and use a Durgo valve which lets it vent but stops odours escaping. Cutting the cast iron is the difficult bit but you could possibly take it down to the next joint and fit the Durgo into that. A photo would help you get a more accurate reply.

 

[Exedon]

Air admittance valves can only be used if drains are adequate vented elsewhere.

 

[Bod ...]

Pictures would be good.
Be careful with AAV's. If too many of your neighbours have done the same thing it make problems. Don't do if you are end of Terrace