SANITARY DRAINAGE

Trap seals and water seal depth

Every fixture trap holds a small body of water that blocks drain air from coming back up into the room. That water seal is the whole reason the trap exists, and if it drops below the minimum depth the barrier is gone.

What the water seal does

A trap is the bend under a fixture that always holds water. That trapped water is a plug. It lets waste pass through but stops the air in the drain, which carries the smell and the gases, from coming back up into the room. No seal, no barrier, and the drain vents straight into the building.

The 50mm minimum

The seal is measured as the vertical depth of water the trap holds, and the established minimum is 50mm. That depth is what gives the seal enough water to survive the normal pressure swings in a working drainage system. Let it sit shallower and ordinary use can pull or push the seal out, which is why the minimum is fixed rather than a guideline.

Why seals fail

• Evaporation. A fixture that is not used dries out over time and the seal simply disappears. Common on floor wastes and rarely used basins.
• Self-siphonage. The fixture's own discharge fills the pipe and acts like a siphon, dragging the seal down the drain behind the water.
• Back-pressure. A surge somewhere in the system pushes air back against the seal and blows it up and out of the trap.

The link to venting

Two of those three failures, self-siphonage and back-pressure, are pressure problems, and venting is how the standard deals with them. A correctly vented branch lets air in and out so the discharge does not turn into a siphon and a surge does not blow the seal. So a trap with the right seal depth and a missing or undersized vent is still at risk. When you check a trap, check the vent that protects it.

Photograph the trap fitted and accessible, and the vent arrangement, before anything is closed in. Elemetric keeps that with the job.

Common questions

What is the minimum trap seal depth?

The established minimum water seal depth is 50mm. That is the vertical depth of water held in the trap that keeps drain air out of the building. Below that the seal can be broken too easily.

Why does a trap seal fail?

The three common causes are evaporation when a fixture sits unused and the water dries out, self-siphonage where the fixture's own discharge pulls the seal down the drain, and back-pressure from the system pushing the seal back up. Proper venting deals with the siphonage and back-pressure causes.

General information for licensed tradespeople, not legal or regulatory advice. The licensed plumber remains solely responsible for compliance. Refer to the current AS/NZS 3500 standards and the Building and Plumbing Commission (formerly the VBA) for authoritative requirements.