Educational Diagram
Drain, Sewer & Vent System
Your drain system carries wastewater out of the home while the vent system allows air into the plumbing system so drains can operate properly.
Tap a part to learn what it does.
Overview
Your drain system uses gravity and air. Every fixture has a trap (the U-shaped pipe) that holds water to seal out sewer gas. Branch drains carry waste to the main building drain, which connects to the building sewer — the line to the street. Vents let air in so water drains freely and traps don't get siphoned dry.
How It Works
When you drain a sink, water flows through the P-trap, into the branch drain, and to the main building drain. The vent pipe lets air enter the system so water flows smoothly — without air, the trap would siphon dry and let sewer gas in. The main drain slopes downhill to the building sewer, which carries waste to the city sewer line under the street.
Key Components
P-Trap
The U-shaped pipe under every fixture. Holds water to block sewer gas.
Branch Drain
Carries waste from a fixture to the main building drain.
Main Building Drain
The largest horizontal drain in the house. Collects all branch drains.
Building Sewer
The line from the house to the street connection (sewer main or septic).
Vent Pipe
Runs from the drain system up through the roof. Lets air in so traps don't siphon.
Cleanout
A capped access point for clearing blockages. Usually near the building sewer.
Common Issues
Multiple drains slow
Shared downstream restriction — could involve a branch, building drain, or sewer lateral depending on the layout. Further diagnosis may be warranted.
Read the guideMaintenance Tips
- Don't pour grease down drains — it solidifies and causes clogs.
- Use drain strainers in kitchen and shower drains to catch debris.
- Run water in rarely-used fixtures (guest bathrooms) to keep traps from drying out.
- If you have a cleanout, know where it is — it's the access point for clearing main line clogs.