Hello All, and thank you in advance for your help. I'm installing a bathroom in a basement. Approximately 3 feet below grade. My issues are with the DWV . There will be a sink, a shower, and a toilet.
First, Can I use a cross tee to connect all the drains from the three sources? The toilet is in between the shower and the vanity. The shower drain is about 3' from the toilet center and the vanity will be about 5'.
If this is possible, is it permissible to run only one vent line off the showers 2" drain? What is the minimum size pipe I can use for the vent? This vent will then connect with the vent for the toilet in the floor above. If not how should I vent these so I can do the following (if at all)
The vent pipe for the toilet on the first floor, comes off the toilets waste pipe about 12" down stream from the closet bend and runs horizontal for about 18" before going vertical into a wall. I believe that it joins up with all the other first floor bathroom vents in the attic.
Well I hope this is clear enough to make some sense out of. Lastly, I'm located in Alameda County of Calif. If it matters.

Unless your code calls for a vent on each fixture, (as you have described ) IU have a better plan using only one 2" vent (see image) If wet vents are allowed in your area thgis will be your simplest and easiest installation. Good luck, Tom