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Single-stack drainage and the trap seal

What the trap seal does, and what keeps it or breaks it on a single stack.

Schematic of single-stack above-ground drainage. A basin and a WC connect through water-seal traps and branch pipes to a soil and vent stack that is open at the top and runs to the foul drain at the bottom, with notes on what keeps the trap seal and what breaks it.
Single-stack drainage and trap seal schematic

Every appliance connects to the soil and vent stack through a trap. The trap holds a small plug of water — the seal — that stops drain air and smells coming back into the room.

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