Drain Vent Clogged

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.

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. drain vent clogged

That vacuum is the enemy.

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. Plumbing codes solved this 100 years ago with the vent stack

Before you ladder up to the sky, do the "Bucket Test." Go to the roof vent

If you flush the toilet and see bubbles rising in the bowl, or if the water level fluctuates wildly, the vent stack is failing to equalize the pressure. Common Causes of Clogs Why would a pipe on your roof get blocked?