Traps and floor waste gullies

A trap only works while it holds water.

The vent article covered water being pulled out of a trap. This one is about the trap itself: what it does, how else it loses its water, and why a floor waste gully, the trap that sees the least water, needs looking after.

• What does a vent do?

The water in the bend keeps the smell out.

Every fixture has a trap, in the same room and close to the fixture. The seal is the depth of water it holds: nominally 75 mm for a waste water fixture and 50 mm for a soil fixture such as a WC.

G13/AS1 3.1.1, 3.2.1, 3.3.1

• Traps, on the G13 page

A vent cannot help a trap that gets no water.

A basin is used every day, so its trap is filled again and again. A floor waste may wait years for a spill. The answer is to keep water coming, and both documents ask for that where a floor waste runs straight into the plumbing.

G13/AS1 3.4.3 b), 3.4.6 · AS/NZS 3500.2 4.6.8.1

The floor's own trap.

G13/AS1 calls it a floor waste. AS/NZS 3500.2 calls it a floor waste gully and treats it as a fixture trap. It is for water, not for human waste.

G13/AS1 3.4.1 · AS/NZS 3500.2 4.6.7.1

• Floor wastes, on the G13 page

Daily use keeps it full, and saves vents.

G13/AS1 sends a fixture's pipe to a gully trap or a stack, so this is a design to AS/NZS 3500.2.

• Gains (our reading): one trap kept full by daily use, and fewer vents.
• Costs (our reading): fixtures stay close, and the floor needs depth: a riser from the water to the floor of ≥ 100–200 mm by its size and entry, ≤ 600 mm.
• A basin or drinking fountain has its own trap as well.

G13/AS1 4.5.1 · AS/NZS 3500.2 4.6.7.2–4.6.7.4, 4.6.7.7, 8.2.2(a)

A charge pipe does what daily use would.

These are for a trapped floor waste that runs straight into the plumbing. A tundish leaves an air gap, so the drain can never reach the water supply. AS/NZS 3500.2 has much the same list.

G13/AS1 3.4.3 b), 3.4.6 · AS/NZS 3500.2 4.6.8.1

The same idea, in a different place.

A gully trap is the same water door outside the building, where waste pipes discharge into its dish.

G13/AS1 3.2.1, 3.4.1, 3.4.2 · G13/AS2 3.3.1 · AS/NZS 3500.2 4.6.7.9

• Gully traps, on the G13 page

E3 asks by what an overflow could reach.

Whether a room needs a floor waste follows the fixture's position, not the room's name. A household kitchen sink or laundry tub can be exempt with an integrated overflow ≥ 0.25 l/s. G13 then says how the floor waste is built and where it goes.

E3/AS1 2.0.1, 2.0.2 · G13/AS1 3.1.2, 3.3.1, 3.4.3 · AS/NZS 3500.2 6.5.3

• E3's overflow rules