Short definition
A house trap (or running trap, or building trap) is a U-shaped fitting installed near where the building drain leaves the house, designed to block sewer gas at the property boundary. It was standard in pre-1950 US construction and is now generally not allowed under modern UPC and IPC. Existing house traps are usually grandfathered in until renovation work cuts into them.
What it is
Where a P-trap protects one fixture, a house trap protects the whole house with a single U-bend at the building drain’s exit. Pre-1950 construction relied on it because individual fixture venting was less complete; the house trap stopped sewer gas at the boundary regardless of what individual fixtures had upstream.
Modern code took the opposite approach: every fixture has its own properly-vented trap, and the building drain is left fully open to the public sewer so air can flow up the stack vent and equalize pressure. A house trap defeats that design — it blocks atmospheric air from entering the building drain from the public-sewer side, which reduces stack capacity and induces siphonage on lower-floor fixtures.
A house trap is identified visually in the basement near the foundation wall: usually two cleanout caps in the floor, several inches apart, sitting over the U-bend.
Why it matters to a homeowner
In a pre-1950 home, a hidden house trap is a common cause of chronic whole-house slow drains that no fixture-level snaking ever solves. The U-bend itself accumulates sediment, and the single trap becomes the choke point for the entire system.
When a plumber recommends removing the house trap during a side-sewer or building-drain replacement, that’s almost always the right call — it improves flow, restores proper venting, and aligns the system with modern code. The catch: jurisdiction policy varies. Some WA jurisdictions allow house-trap removal as routine, others want to see it on the permit drawing, and a few require AHJ sign-off. Confirm with your inspector before the job starts.
When you’ll encounter this term
- Pre-1950 Seattle Wallingford / Ballard / West Seattle bungalow or Tacoma Stadium District home — house trap near the foundation wall.
- Whole-house slow drains despite cleaning every fixture and snaking each branch — house trap is the suspect.
- Plumber’s quote for a side-sewer replacement includes “remove house trap.”
Common variants / not the same as
- House trap vs. P-trap. P-trap is at each fixture; house trap is one trap at the building exit.
- House trap vs. drum trap. Drum is a canister at a fixture (typically a tub); house trap is a U-bend at the building drain.
- House trap vs. intercepting trap. Same concept; “intercepting trap” is the engineering term, used more in commercial and large-system installs.
- House trap vs. backwater valve. Backwater is one-way mechanical (allows out, blocks back). House trap is a water seal that’s always partially obstructing flow.
Common failure modes
- Sediment buildup in the U-bend — chronic re-clog of the whole house, no matter how many fixture-level snakes are run.
- Cracked cast-iron house trap — sewer leak in the basement.
- Cap stuck or seized — can’t access the trap for cleaning.
- Modern remodel adds an upstream fixture without addressing the house trap — venting math fails and aspiration shows up.