A drum trap is an old cylindrical drain trap shaped like a can, once installed under bathtubs in homes built before the 1960s. It holds a large pool of water as its seal. Drum traps clog easily, are hard to clean, and are no longer allowed for new work under modern code.
Before the P-trap became standard, many homes used a drum trap. It is a wide cylinder that looks like a large soup can set into the floor. You still find them under old bathtubs, mostly in houses built before the 1960s.
How a drum trap works
Like any trap, a drum trap holds a plug of water called a trap seal. That standing water blocks sewer gas from rising into the room. The drain pipe enters low on the drum, and a second pipe leaves higher up. Water fills the can and stays there. Because the drum is so wide, it holds a much larger pool of water than a modern trap does.
Why drum traps cause trouble
That big pool of water is also the problem. Waste and hair settle inside the drum instead of washing through. Over time this leads to slow drains and stubborn clogs. Cleaning is a chore too. The lid sits on top and often rusts in place, so it is hard to open. A modern P-trap is self-scouring, which means each rush of draining water scrubs the trap clean on its own.
Drum traps and code today
Drum traps are not up to code for new plumbing in most areas. When an old bathroom is remodeled or a house is repiped, the drum trap usually gets swapped for a P-trap. If you own an older Phoenix home and a tub drains slowly no matter what you try, an aging drum trap may be the cause. See why a bathtub will not drain and what is a P-trap.