Venting a drainage system is one of those conceptual things that first-time plumbers often miss. The idea of maintaining a barrier between the sewage system and the house is an essential element in a sanitary plumbing system. Without a trap to seal off plumbing fixtures, nasty things such as explosive methane gas, vermin and hideous smells can find there way into the house.
Every fixture requires a properly sized vent.
No's fig.p8 1. Use sanitary tee instead. Sanitary tee on its back not a drainage fitting. Horizontal wet venting.