Reduction of roof vent from 4" to 3"

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Reduction of roof vent from 4" to 3"

Author: sum (FL)

I am in the process of redoing my entire bathroom.

It has one toilet (4" CI), a lav sink (1.5" CI) and a shower (2" CI) all tied to a 4" CI stack that runs all the way up the roof.

I will be changing the stack to 3" PVC, the toilet to 3" PVC (but with a 4X3 closet bend), lav 1.5" PVC and the shower 2" PVC.

On the roof is one of those old lead flashing where it wraps around the 4" CI and folds over the top of the 4" CI. When I remove the 4" CI and insert a 3" PVC, there will be a difference of 1.5" in ODs.

I can try and see if I can unfold the lead wrap and refold it onto the 3" PVC but I don't know if I will break it in the process.

A second option is to use a short piece of 4" PVC just for the last 18" or so to go through the roof and have a 4X3 in the attic but I know this is not to code to decrease in size going downstream even if it's for the first 18" of a vertical vent?

or I can tear out the surrounding concrete roof tiles and waterproofing membrane and put in a new vent new membrane new flashing etc for a 3".

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Re: Reduction of roof vent from 4" to 3"
Author: bsipps (PA) In my area you can absolutely reduce the vent to 3” as long as you have another vtr atleast 1.5” I would just leave the CI through the roof and connect 3” pvc to it unless you want to pay a roofer to switch the water seal
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