I am replacing a single sink vanity with a double sink. The current drain line in the crawl space is 1 1/2". This line empties into a 3" line after about 10 feet and has a drop of over 6". I would like to use all 1 1/2" lines in the wall and connect to the 1 1/2" line to drain for both sinks. It would be very difficult to replace the 1 1/2 " line with a 2" line because of crawl space height restrictions. Will 1 1/2" lines do the job? I've included a rough diagram.

1 1/2 will be just fine for two bathroom sinks. Also, everything looks great in your diagram, except, I would use a double fixture fitting in place of the sanitary cross.

I don't know your location, but in Ohio you can put three lav sinks on a 1 1/2" waste line. Also in Ohio even that it is a better job to use a double fixture fitting it is not legal. Code says you must use a santary cross. If you are not getting a inspection or a state that approves it, by all means use the double fixture fitting. It is illegal in Ohio because on that type fitting the vent opening is lower than the waste openings. Will the double fixture fitting work properly? Yes it will.

Good call puffmugs. Weird how codes vary form area to area. We can only use sanitary cross if the outlet is two pipe sizes above the required fixture drain size. We are not allowed to use double wye, 45 combos due to the reason you mention about the vent opening being lower than the drain, however, on a true double fixture fitting, the vent opening is only slightly lower than that of a san tee. Big difference between a double fixture fitting and a double wye 1/8 bend combo. Not sure if we are talking about the same fitting? First pic is a true double fixture fitting, which gives just a little more downward flow so one sink doesn't try to wash back into the other (trivial at the very least, imo.) The second, (not allowed under my code, as well as pic three) is the double wye 1/8 bend fitting). And of course pic three is the sanitary cross. If you have ever tried to snake a bathroom sink that is tied into a san cross, you will know the snake just shoots over to the other sink. With the double fixture fitting, you have a much better chance of getting the snake to go down vs across, (still a crapshoot either way, unless you have a drop head bit). Overall, a sanitary cross will work just fine, as will the double fixture fitting, but never use the double wye 1/8 bend fitting, as it pretty much eliminates the functionality of the vent, as puffmugs said. Take care and please let us know how things work out.

I'm installing my first vessel sinks and it is now time to rough-in the drain. Normally I would rough-in the drain at 18" to 22". The cabinet height is 34 1/2 inches. The pop-up drain will extend below the countertop 6" inches making it 28 1/2" off the finished floor. I'm trying not to use a tailpipe extension, but with a 1 1/4" p-trap you only have about 1 1/2 inches to make the vertical adjustment. I'm thinking I should just go at least 10" below the countertop (making the drain a maximum of 24 1/2" off the finished floor and use a tailpipe extension. Any suggestions would be appreciated.

Thanks for you help. I picked up a 1 1/2 sanitary cross today and as soon as I figure out the rough-in drain height for my vessel sinks (I submitted another question), I'll complete the sink rough-in. Can't wait to go back under the house. Just spent 2 1/2 hour under the house re-plumbing our new shower. At age 65 crawling under the house in not my favorite job. Thanks again.

In my area we pick up out vanities at 2". That's not to say 1 1/2" won't work just fine. We bring out stubouts out at 18" to center off the finished floor. Good luck on your project. Tom