Short definition
A drum trap is a cylindrical canister trap installed flush with the floor, with the inlet at the bottom and the outlet at the top — once standard under clawfoot tubs and in shared kitchen-laundry runs. Modern code bans them for new work because they vent poorly and trap solids. In Washington’s pre-1940 housing stock they’re still common and need replacing during any bathroom remodel.
What it is
Where a P-trap is a U-bend, a drum trap is a vertical canister, typically 4 inches in diameter, set into the floor with a removable brass or chrome cap flush with the finished surface. Water enters near the bottom and exits at the top, so the canister is full of water at rest — a deep, generous trap seal. The design was popular before the 1940s for tubs, and was sometimes used to capture solids from kitchens and laundries before disposers and laundry standpipes became standard.
Two structural problems doomed it:
- The internal geometry can’t be vented in a way modern code accepts. Drum traps create aspiration that pulls trap seals on shared waste lines.
- Solids settle in the bottom of the canister and can’t be reached with a snake from upstream — the inlet is below the outlet.
Why it matters to a homeowner
If you live in a pre-1940 Seattle or Tacoma home and you’re planning a bathroom remodel, the floor under your existing tub probably has a drum trap. The Uniform Plumbing Code (UPC 1004.1) limits drum and bottle traps to “special conditions only” and requires their replacement when remodel work cuts into the existing piping. Existing legal-non-conforming installations are usually grandfathered in if they’re still functioning, but as soon as a permit pulls back the floor, replacement to a standard P-trap becomes part of the job.
A persistent sewer-gas smell from an upstairs bathroom in a pre-1940 home, paired with a slow tub drain, is a common drum-trap symptom — the seal is failing or the trap is improperly vented (or both).
When you’ll encounter this term
- A bathroom remodel in a pre-1940 Seattle bungalow or Tacoma craftsman; plumber finds a drum trap under the tub and includes replacement in the quote.
- DIY tile-floor refresh stops short when a brass disc shows up in the floor — that’s the cap.
- Persistent sewer-gas smell from an upstairs bathroom and a slow tub drain.
Common variants / not the same as
- Drum trap vs. P-trap. Drum is a canister; P is a U-bend. Drum holds more water but vents poorly and traps solids.
- Drum trap vs. bottle trap. Bottle trap is the UK-style decorative version under sinks; drum trap is the floor-flush canister under tubs.
- Drum trap vs. house trap (running trap). Drum is at the fixture; the house trap is at the building exit.
Common failure modes
- Cap gasket dries and the trap leaks under the floor. Symptom: subfloor rot, ceiling stain in the room below.
- Canister wall corrodes — old lead bodies and brass caps crack with age.
- Sediment buildup blocks flow — and you can’t reach the buildup with a snake from upstream.
- “Smell can’t be located” — the source is the drum trap, because its seal is failing or its vent is missing.
Washington note
Pre-1940 Seattle (Wallingford, Ballard, West Seattle bungalows; Capitol Hill apartments) and pre-1940 Tacoma (Stadium District, Old Town) homes commonly have drum traps under their original cast-iron tubs. The Washington-adopted UPC follows the standard rule: existing drum traps are typically left alone if they’re still functioning, but any remodel that cuts into the line forces replacement with a properly-vented P-trap. If you’re scheduling a bathroom remodel in a pre-1940 home, expect the plumber’s quote to include a drum-trap replacement line item — that’s the right answer, not an upcharge.