I'm installing a half bath in my attic. There is a 2" vent from the downstairs bathroom sink that would be very easy to tap into for the upstairs sink. My question is, can I use this vent as a wet vent for the upstairs sink?

Hi Jack...

Sorry... you can't connect a waste line from another floor to a vent from below. The velocity that the water drops at would create a DRAW/siphon effect on the PTRAP being served on the lower floor and will cause bubbling/gurgling and will allow sewer gasses to enter the home... a health hazard!

I would run a 3" drain up there for the toilet and then connect the sink into that drain pipe. The vent from the sink can connect into the vent from downstairs at a minimum of 42" off the finish floor... at least that is a bonus!

Hope that all made sense...

MARK