3" pipe.

[plumro]

Is it normal practice to reduce from 4" to 3" when venting a soil pipe out through the roof? If so why is 3" pipe and fittings so hard to come by. I cannot get a 3" vent cowl anywhere!

 

[GreenArmy]

No its not normal practice.

 

[Bernie2]

No its not usual practise but you can reduce vents, there is an equation somewhere for it.
But anyway it won't help you with your cowl will it.

Perhaps a wire balloon as used on cast iron soil pipes may do it, just press the tails in a bit.