Which fixture does not require an external trap?

Which fixture does not require an external trap?

• A. Water closet
• B. Lavatory
• C. Sink
• D. Floor drain

Answer: (A)

The key idea is whether the trap is built into the fixture itself or must be added separately in the drain line. A trap is the U-shaped bend that holds water to block sewer gases from coming back into the building. A toilet has an integral trap formed by the toilet’s own waste outlet and siphon design. That means the water seal is built into the fixture, so no separate external trap is needed or allowed to be added on the line after the toilet. Lavatories and sinks, on the other hand, rely on a separate trap, typically a P-trap, installed immediately beneath the fixture and connected to the wall drain. Floor drains generally have their own trap within the drain assembly in the floor, but the point remains that these fixtures don’t rely on an internal trap like a toilet does. So the fixture that does not require an external trap is the water closet.

The key idea is whether the trap is built into the fixture itself or must be added separately in the drain line. A trap is the U-shaped bend that holds water to block sewer gases from coming back into the building.

A toilet has an integral trap formed by the toilet’s own waste outlet and siphon design. That means the water seal is built into the fixture, so no separate external trap is needed or allowed to be added on the line after the toilet.

Lavatories and sinks, on the other hand, rely on a separate trap, typically a P-trap, installed immediately beneath the fixture and connected to the wall drain. Floor drains generally have their own trap within the drain assembly in the floor, but the point remains that these fixtures don’t rely on an internal trap like a toilet does.

So the fixture that does not require an external trap is the water closet.