If I move a shower stall to the other side of the room (about 8' away), can I leave original P trap in place, run a section of pipe to the new location, and add another trap at the new shower drain? Do I need to worry about the existing venting working properly?

No, cannot double trap. What about the vent?

Hi Sierra

Like Brian (Ma0641) said, you don't double trap. Instead, eliminate the old PTRAP and install the new PTRAP directly under the new shower strainer and keep the vent fitting within 5 feet (maximum) of the new PTRAP. You can connect this new vent back to the old shower vent as long as you pitch the vent so any rain water or condensation that gets in the vent falls back toward the drain via gravity!

Mark

Thank you both for the good input. What problem will I have if the current vent connects to the line more than 5' away?

I am sure new shower will be against at least one wall. Cut open that wall and run new vent up the wall and to the ceiling. Once inside the ceiling, run it across to meet with the original shower vent and connect to it. Maintain 1/4" per foot up-slope for horizontal sections of the vent.

Absence of proper venting will cause slow drainage. Also, drain may make gurgling noises as the draining water it will fight for air.

Hope that helps / Back to you / Milo

Very helpful. Thank you.