I'm renovating my kitchen which used to have soffits. We've pulled out all the soffits and framing of the soffits.

The vent stack for my sink goes up the wall and comes out of the wall just before the top plate - then it goes into the ceiling where it runs in between two joists. The wall is an exterior 2x4 wall.

If you saw it from the side it would look something like this:

enter image description here

I'm wondering - can I reduce that 2" vent to 1.5" vent and drill through the top plate and feed the 1.5" PVC through and then enlarge it back to 2" inside the ceiling?

I'm thinking something like this:

enter image description here

Is that possible? Are there any other safe ways to get that vent through that top plate and into the ceiling?? This would be in the state of IL if that matters.

  • if it's just for the sink i.5" is perobably enough vent, but I don't know if code allows narrow sections in vent lines
    – Jasen
    Commented Oct 20, 2020 at 9:41

1 Answer 1


Depends what it's venting, which may be just the sink, or may be more than the sink, depending how things are presently arranged.

Per ICC, a kitchen sink (alone) is 2 DFUs, so if this is a dry vent serving ONLY the kitchen sink, if could be as small as 1-1/4" (which is suitable for JUST 2 DFUs, no more) or it could be 1-1/2" (suitable for up to 8 DFUs.)

However, your plan to expand to 2" after making the corner (as drawn) fails 905.2:

Vent and branch pipes shall be so graded and connected as to drain back to the drainage pipe by gravity.

since the lower part of that upper 2" pipe will not drain into the 1-1/2" pipe because reducers are centered, not offset. At least all the reducers I've met are - could be someone makes them that way.

  • Here is a good URL for the table 906.1 vent sizing up.codes/s/vent-pipe-sizing and this text also points out a couple of other relevant limitations, in case the OP has other fixtures connected. Commented Oct 20, 2020 at 14:00
  • Thank you for this information! So I was able to see that same vent pipe goes into my basement - eventually going down into the concrete floor (2" all the way - there is an unused toilet flange in the basement - probably connected to it I'd imagine). So my guess is that it wouldn't be a good idea to reduce because it's not code and we may put a toilet in the basement some day. What is the best way to get that 2" pipe through the top plate in this case? Is there a way or am I hosed?
    – Hanny
    Commented Oct 20, 2020 at 14:01
  • Leave as is, or change it to a pair of 45's (still not through the top plate) and put a plumbing chase around it (box it in) so you don't have to look at it. Being a kitchen, put a cabinet (for rarely used items, give it will be up high) around it is one option.
    – Ecnerwal
    Commented Oct 20, 2020 at 14:10
  • Now you know why your kitchen had soffits. :( You could put up large crown molding (using 2 45° elbows would allow the pipe to hide behind the crown better) instead of the soffit, if you wanted. Or soffit just along this wall, or make a tray ceiling so it's a "feature"! That will give you the added benefit of soffits all the way around to hide stuff behind.
    – FreeMan
    Commented Oct 20, 2020 at 16:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.