Adding second sink - DWV question

Your help on this would be much appreciated. My husband and I are adding a second sink in our master bath (2nd floor). We are trying to connect to the existing sink's vent without adding a vent for the new sink because there is no easy way to route it around recessed medicine cabinet framing in a load bearing wall. We are wondering if using separate drain lines allows us to use the one vent. Also, is the 4 foot horizontal drain line too long?

We moved the existing vent/waste line about 9 inches (again, for the med cabinet framing). The existing sink used to connect directly to the vertical vent/waste line from the P-trap.

I hope that I am using proper terminology...

Thanks,
Linda

drain

We have no problem with the terminology. The problem is SEEING where the pipes are in relation to where the sink is going to go. What you want to do is probably possible, but there are ways to do it correctly, and just as many ways to do it wrong. It depends on the placement of the pipe and sink. Doing it according to your drawing will usually make one of the two drains either too high or too low.

HJ:
I'm not sure what you mean by too high or too low. Do you mean too high or too low to connect to the sink? We have installed only one P-trap before, but I thought that there was some play in the P-trap installation to allow for sink-to-drain line height variations (e.g. kitchen height vs. bathroom height cabinets).

I added some heights to the drawing, and realized that I had the drain lines from the two sinks entering the vertical waste/vent line in flipped order. We are trying to stay 'near' the standard 18" drain line height.

Thanks,
Linda

What you have is the top lav wet vented over the lower.
The vertical wet vent portion should be 2" pipe.
The second lav can go 42" to the vent.
I would be looking at venting the second lav, since you are too far, and you don't have the wet vent at 2".
The vent can come off near the second lav, and revent at 42" to the first vent.
The way it looks in the drawing, draining the upper lav, can pull the water from the lower trap seal.
And that is really the whole point of this right?
So, just add a few more fittings and pipe, to complete the job.

Terry - thanks so much for your response. I hope that I have interpreted what you said correctly - here is another diagram with the new vent in red. Is this ok? We could offset the new drain from the centerline a bit to reduce the run to 45 or 46 inches (instead of 48"). Oh, and I did confirm that the existing vent is 1 1/2 inches.

Thanks,
Linda