Vent Clogged ^hot^ | Drain

When you flush a toilet and the shower drain burps air at you, that is air being sucked through the shower’s P-trap. The vent is supposed to supply that air, but it can't. So the system cannibalizes air from the nearest sink or tub, sucking the water seal dry. That seal is the only thing stopping sewer gas from entering your lungs. Lose the seal, lose the war.

But here is the nuance: You can't just clear the top. drain vent clogged

Most vent clogs are "saddle clogs." They sit at the bottom of the vent stack, right where it turns horizontal to join the main sewer line. Water never washes that area. Waste solids and grease sneak up over time, creating a hard, calcified shelf. When you flush a toilet and the shower

But here is the dirty secret most homeowners never realize until they’ve cut a hole in their ceiling: Your drain isn't the problem. Your drain’s breath is. That seal is the only thing stopping sewer

A plumber will shove a garden hose down the roof vent and turn it on full blast. If the water backs up instantly and overflows the roof pipe? The clog is near the top (bird nest). If the water runs for 30 seconds, then backs up? The clog is deep (grease shelf).

Go to the roof vent. (Safety first: don't do this on a wet or icy roof). Shine a flashlight down the hole. If you see a solid wall of black sludge or a nest, you found your problem.