4.1 Fixture Traps, Trap Seals & Direct Fixture Connections

The basic rule (SPS 382.32 (3))

"Each plumbing fixture, each compartment of a plumbing fixture and each floor drain shall be separately trapped by a water seal trap ... A fixture shall not be double trapped." A double trap creates an air lock between two seals that can stop flow or blow a seal.

Trap seal depth. Each trap must provide a liquid seal of at least 2 inches and not more than 4 inches, unless another section says otherwise. A deeper seal resists evaporation but is harder to scour clean. Specific devices, such as garage catch basins, car wash interceptors and sand interceptors, require much deeper seals, but those are interceptor outlets, not fixture traps.

Exceptions: fixtures not separately trapped

Amendments under CR 26-019, effective September 1, 2026, list five cases:

1. Fixtures with integral traps, such as water closets and many urinals.
2. Compartments of a combination fixture on one trap, if no compartment is more than 6 inches deeper than another, the waste outlets farthest apart are no more than 30 inches apart, and no compartment has a food waste grinder.
3. Storm drains as provided in SPS 382.36 (12) (a).
4. A floor drain in a garage attached to a one- or 2-family dwelling that discharges to the ground surface.
5. Local waste piping up to 20 feet long within a one- or 2-family dwelling.

Trap seal protection

Traps that lose their seal to evaporation, such as a seldom-used floor drain, must have a trap seal primer valve or another department-acceptable protection. Primer valves must meet ASSE 1018 or ASSE 1044. Barrier-type trap seal protectors for floor drains must meet ASSE 1072. A code note lists mineral oil, vegetable oil, propylene glycol and glycerin as liquids that reduce evaporation.

Design and size

• Traps must be self-scouring and may not have interior partitions unless integral to the fixture. A uniform-diameter P-trap is considered self-scouring.
• Trap diameters may not be smaller than Table 382.30-1 (section 2.1): lavatory 1-1/4 inches, bathtub and kitchen sink 1-1/2 inches, shower and clothes washer 2 inches, floor drains equal to the drain size.
• Shower replacing a nonpublic residential bathtub: the trap may stay 1-1/2 inches if the shower has one control valve, cannot run more than one showerhead at a time (a diverter to 2 heads is fine), and the showerhead flow is no more than 2.5 gpm.

Prohibited traps (SPS 382.32 (3) (f))

Setting and distances (SPS 382.32 (4))

Traps must be rigidly supported, set level with respect to their water seal, and protected from freezing and evaporation.

Vertical distance from the fixture outlet to the trap weir:

Horizontal distance from the outlet centerline to the trap inlet centerline:

A floor-mounted water closet's outlet centerline must also be at least 30 inches from a 3-inch double tee.

Worked example. A kitchen sink in an island has its outlet 12 inches horizontally and 20 inches vertically from where the trap can be set. Both distances comply (15 and 24 inches). If cabinet work moves the trap 18 inches sideways, the horizontal limit is exceeded and the trap must move under the outlet.

Direct fixture connections (SPS 382.32 (5))

Except for the indirect-waste cases in SPS 382.33, every fixture and appliance discharging waste connects directly to the drain system.

• Floor drains must be accessible for cleaning. One that receives garbage-can wash water must be at least 3 inches.
• Kitchen sinks: the horizontal drain from a kitchen sink trap may not join a vertical drain through a double sanitary tee where a food waste grinder or pumped-discharge appliance is installed.
• Water closets discharge through a drain or fitting at least 3 inches in diameter:
  • Floor-mounted wall-outlet closets: 4-inch or 4 x 3 inch closet collar, or a horizontal or vertical carrier fitting.
  • Floor-outlet closets: 4-inch or 4 x 3 inch closet collar.
  • Wall-mounted wall-outlet closets: carrier fitting.
  • Two closets back to back on a 3-inch vertical drain: a 3-inch double wye pattern fitting for floor-outlet closets; a double wye or carrier fitting for wall-outlet closets.
• Blowout-type fixtures are installed as the department approves.

Exam traps

• A 2-inch seal minimum and 4-inch maximum is a frequent "which trap complies" item.
• "Trap the dishwasher separately" is wrong; a residential dishwasher discharges through indirect waste (section 4.3).
• A sanitary cross (double sanitary tee) is not allowed for a kitchen sink with a disposer.