Do P traps need to be vented?
P-traps need vents
1 – They give the sewer gases a place to vent, so they do not build pressure inside your sewer lines. 2 – They prevent siphoning of the water out of the trap. Does an air admittance valve go before or after the P-trap? Air admittance valves are typically placed between the P-trap of a fixture and the drain line. They're usually mounted on one leg of a sanitary tee, with the other leg going to the drain.