Drum Trap, Over-Pitched Trap Arm, and a Gurgling Kitchen Sink
A 1925 house near Kalamazoo has a second-floor lavatory whose 1-1/2-inch trap is a brass drum trap installed in the ceiling cavity below, with a soldered cover that has been painted over. The trap arm from the fixture to the stack is 5 feet long and drops about 9 inches, and the fixture has no vent. On the first floor, the kitchen sink gurgles loudly, and the homeowner says the trap 'empties itself' after each use. Its 1-1/2-inch trap arm runs 7 feet to the vertical stack with no vent. The owner wants to replace both fixtures without opening ceilings. He asks what has to change.
1. What is wrong with the drum trap, and what must replace it? Can the new trap be left inaccessible?
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The drum trap must go.
A drum trap is a large-volume, flat-bottomed body that cannot be cleaned properly through its small cover, and it does not scour itself on each use. Traps must be of the self-scouring (self-cleaning) type, and a drum trap does not meet that requirement: solids and hair settle in the bottom and the trap gradually becomes a grease and debris reservoir. Its cover here is soldered and painted over, so it is not even serviceable. Access to a drum trap is also a problem in its own right.
Replacement:
- A P-trap of the same nominal size as the fixture drain (1-1/2 inch for the lavatory), with a water seal of not less than 2 inches and not more than 4 inches.
- The trap must be level (the seal must be uniform) and the trap arm must be pitched toward the stack.
- No double trapping, no S-trap arrangement, and no more than one trap per fixture.
- The trap must be accessible for cleaning and removal. If the trap must stay in the ceiling cavity, install a listed access panel directly below it, or reroute so the trap is inside the vanity cabinet where it is reachable. Because the owner does not want ceilings opened, the practical answer is often to cut the drain in above the ceiling and drop the trap into the vanity - which requires opening the wall or ceiling anyway for the rough-in.
Plan a small access opening now; concealing the trap permanently will fail inspection and will cost more later.
2. Analyze the lavatory trap arm that drops 9 inches over 5 feet. What failure does that cause, and what is the correct fix?
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The diagnosis: 9 inches over 5 feet is about 1.8 inches per foot - roughly seven times the required slope.
A trap arm is limited so that the total fall on the arm does not exceed the diameter of the pipe. For a 1-1/2-inch arm that means the maximum fall is 1-1/2 inches, which occurs over 6 feet at 1/4 inch per foot. Here the arm falls 9 inches - six times the maximum - and it is also unvented.
Why it fails: when the fixture drains, the near-vertical arm dumps a slug of water and pulls a full siphon behind it. With no vent to break the siphon, the trap is emptied and stays empty, so sewer gas enters the bathroom. The steep arm also makes the trap behave like the bottom of an S-trap.
Correct fix:
- Re-pitch the trap arm to 1/4 inch per foot toward the stack.
- Keep the arm length within 6 feet for 1-1/2 inch (8 feet if upsized to 2 inch - but upsizing does not substitute for a vent).
- Vent the fixture. The vent takeoff must be within the allowed trap arm length, and the vent must rise to at least 6 inches above the flood-level rim of the fixture before it offsets horizontally.
- Replace the drum trap with the P-trap described above.
This work cannot be done without opening the ceiling or wall; tell the owner up front.
3. The kitchen sink trap empties itself and the sink gurgles. Diagnose it and give the options, ranking them.
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Diagnosis: classic trap siphonage from an unvented, over-length trap arm.
The 1-1/2-inch trap arm runs 7 feet with no vent. The maximum length for a 1-1/2-inch trap arm is 6 feet, and that limit assumes the arm terminates at a vent. Here the arm is both too long and unvented, so when the sink drains the water column siphons the trap seal; the gurgling is air being pulled back through the drain after the seal is lost.
Do not stop at the trap. Check for the usual co-conspirators: an S-trap created by the drain dropping immediately below the fixture, a partially clogged branch that increases siphonage, and a stack that is being used as a drain for fixtures above with no venting at this floor.
Options, best first:
- Add a vent within 6 feet of the trap - an individual vent rising above the flood-level rim of the sink and connecting to a vent stack, or a fixture vent returning to the stack above the highest fixture. This fixes the cause.
- Relocate the trap so the arm is shortened, if the geometry allows; a shorter arm within 6 feet still must be vented at its termination.
- Upsize the trap arm to 2 inch, which extends the allowable length to 8 feet - only valid if a vent still exists at the end of the arm. Upsizing alone does not vent anything and does not cure the siphoning.
- Any mechanical vent device (air admittance valve) must be a listed device, installed where permitted by the local authority, accessible, and with the correct orientation; it is an alternative, not a default, and Michigan acceptance should be confirmed with the AHJ before relying on it.
Tell the owner plainly: the sink needs a vent, and the trap needs to be reachable for cleaning.