How can I run 2" sink vent stack through top plate?

Question

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:

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:

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.

Ecnerwal · Answer

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.

How can I run 2" sink vent stack through top plate?

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