What is a P-trap?

A P-trap is the curved section of pipe located directly under your sink or basin. It gets its name from its shape, which resembles the letter P when viewed from the side. The trap holds a standing column of water inside the bend, creating a barrier that stops sewer gases and odors from backing up into your home through the drain.

The water seal in a P-trap is essential for plumbing safety. Sewer lines emit hydrogen sulfide and methane, which are both hazardous and unpleasant. The trapped water blocks these gases from entering living spaces. When you use the sink, fresh water flows through the trap and pushes waste down the line while maintaining that protective water column.

P-traps are the most common trap style in Australian plumbing because they work well under typical sinks and vanities. They come in different materials (PVC, chrome-plated brass, or cast iron) and sizes depending on the fixture and local code requirements in Perth metro areas. A dry trap, or one where the water has evaporated (often after a long vacancy), loses its seal and allows gas to escape into the house, which is why occasional use of all drains matters.

S-traps, by contrast, curve upward then down and were used historically but are now rarely compliant in new work because they tend to siphon their water seals more easily than P-traps do. If you need to install or replace a trap, a qualified plumber can ensure it meets current regulations and functions properly for your fixture.