Hi group,

I would like to install a bathroom sink against a wall that is common to another bathroom. The 1-1/2" drain of that bathroom sink runs horizontally for about two feet where it enters a sanitary tee of a two inch vertical line. This is behind the location of the new sink so it would be very easy to cut that two foot drain line and install a sanitary tee but I know that a sanitary tee should not be installed on it’s side.

The new sink will be installed at least four inches higher than the existing sink in the adjoining bathroom so the drain should get a small assist from gravity.

Can a sanitary tee be installed horizontally if the tee faces up?

If not, how would you suggest the new sink drain be tied into the existing horizontal drain. Thanks for any suggestions.

no, the 'run' of a sanitary 't' must be vertical

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

In that case, would it be okay to use a wye?

the right way to do it would be cut a 2 x 1 1/2 san tee into the vertical 2 inch.

This bathroom is on the ground floor of a four story building so it will not be too easy to cut into the 2 inch vertical.

the problem is that one sink draining could syphon the trap of the other.
i suppose that if you managed to get an AAV under the new sink that would be OK.
you'd be looking at something like this.
[archello.com]

One thing I didn’t mention earlier is that the “new” sink used to be on an adjacent wall, and the drain pipe in the adjacent wall is only a couple of feet from the sink in the other bathroom, so, if I hook up that sink to the drain on the adjacent wall and the “new “ sink to the drain line of the other bathroom sink, each sink will drain into it’s own two inch vertical drain, and problem solved.