That source is often the other P-traps in your house.
You need a ladder, a flashlight, and a strong stomach. After locating the vent pipe (a small, gray or black PVC or cast-iron stub poking out of the shingles), you shine the light down into the abyss. If you see daylight, you’re fine. If you see darkness, or a mat of squirrel nest, you’ve found the culprit. vent stack clogged
In severe cases, the drain speed becomes glacial. Water can’t flow downhill if a column of trapped air is pushing back up from below. Your morning shower becomes a 45-minute wait for a muddy puddle to disappear. That source is often the other P-traps in your house
You’ll hear the —the sound of your sink’s trap being siphoned dry by the force of a shower drain two rooms away. You’ll see the burp —a sudden bubble of sewer gas erupting from a toilet as the pressure equalizes violently. You’ll smell the stench —that distinct rotten-egg aroma of hydrogen sulfide rising from the empty traps meant to block it. If you see daylight, you’re fine
When the vent stack clogs, your home’s plumbing has an asthma attack. It can’t inhale. So, when a toilet flushes or a sink drains, the rushing water creates that vacuum. With no air to fill the void, the water has no choice but to pull from the nearest available source.
You don’t have a clogged drain. You have a clogged vent stack.