I currently have a 2" vent for a full 2nd fl bath going through the roof. This vent has a 2" vent for lav/tub/laundry WYEd into a 2" vent for the toilet(this connection is in 3rd fl attic) which then extends up through roof.
I am adding a 3rd floor half bath. I want to wet vent the 3rd fl toilet via a 2" vent off the 3rd fl lav. Here's the question: Can I tie this 2" 3rd fl lav vent in to the existing 2" vent from 2nd fl bath, then increase vent size through roof to 3". Thing is, this connection(WYE), 3rd fl vent to 2nd fl vent, would leave 18" from the tie in to where the vent penetrates the roof.I thought I read somewhere that such a tie in needs to be a min. ~47" from where the vent penetrates the roof. Thanks for any info.
Bob

Hi Rmmacaluso:

You can do exactly what you proposed. Connect the 2 two inch vents into a 3"x2"x2" wye fitting (may need to bush a 3" wye fitting (may need to bush a 3" wye down)... just make sure to install the wye so the flow of the fitting allows rain water to drain back toward the drain, i.e. make sure to install the wye fitting upside down... ;) Then penetrate the roof with 3" as suggested.

No issue with the 47" wye down)...just make sure to install the wye so the flow of the fitting allows rain water to drain back toward the drain, i.e., make sure to install the wye fitting upside down...;) Then penetrate the roof with 3" you mentioned no problem!

Good luck...

MARK

Here's the question: Can I tie this 2" 3rd fl lav vent in to the existing 2" vent from 2nd fl bath, then increase vent size through roof to 3".
Mark nailed it! This is called "reventing" and you invert the wye,(turn it upside down) and slope back to the fixture or fixtures that are being vented.
Good luck, Tom