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.
Plumbing codes solved this 100 years ago with the vent stack. This is a vertical pipe (usually 2–3 inches thick) that runs from your sewer line, up through your walls, and punches out through your roof. Its job is singular: to bring outside air into the system to break the vacuum.
If only the toilet clogs, it's a poop knife problem. If only the sink clogs, it's hair. But if everything on the same floor or side of the house drains slowly at the same time? That is a vent issue. The entire branch is suffocating. drain vent clogged
Without it, you are playing Russian roulette with atmospheric pressure. With it, you sleep soundly, listening to the silent, satisfying whoosh of water that knows exactly where to go. If you have tried everything—the plunger, the snake, the enzyme treatment—and your drains still weep instead of roar, look up. Look at the sky. Look at that pipe sticking out of your shingles.
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. Most vent clogs are "saddle clogs
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 vacuum is the enemy.
It stops leaves, birds, and squirrels. It stops the lint. It stops the mystery.