Can you route a 1 1/2 lav drain down vertically to the floor joist level to the soil stack

Hi Curious49

The answer to your question depends on the number of plumbing fixtures connected to that 3" soil stack, on how many different floors of plumbing are involved with the 3" stack, and on what state you live in, amongst quite a few other things!

Give us some more information, OK? The more information you provide, the better we can help...

Thanks... Mark