When a trap arm extends laterally through a stud wall inside of the wall, we call it a dirty arm. Check the drain center below the floor to see if it matches the center of the trap arm above or to remove the trap arm to see if the drain goes down (normal scenario) , or to the side (dirty arm scenario).Click to see full answer. Regarding this, how long is a dirty arm?dirty arm length. California – 1½”-3½ feet, 2″-5 feet, 3″-6 feet, 4″-10 feet(except a toilet is limited to 6 feet).Additionally, what is a plumbing revent? A revent pipe, also called an auxiliary vent, attaches to the drain line near the fixture and runs up and over to the main vent. It may attach directly behind the fixture or to the horizontal drain line. In the case of a tub that is close to a stack, its drain may empty into a pipe that also serves as a vent. Moreover, what is a plumbing trap arm? Basically, it’s a simple cup of water that keeps sewer air from communicating with the air you breathe. The trap arm is the portion of a fixture drain between a trap weir and its protecting vent. The trap weir is the point at the bottom of the trap arm pipe where it connects to the trap.Does a bathtub drain need a vent?Whether you’re putting in a tub, toilet, sink, or floor drain, they all need a plumbing vent on the drain to make it work properly. The fixture without a vent may drain slowly. The drain will likely make gurgling noises. The water in the trap could siphon out, resulting in a potent sewer smell.