7.1 Trap Design, Seals & Prohibited Traps

7.1 Trap Design, Seals & Prohibited Traps

Quick Answer: Under 248 CMR 10.08 (effective 12/8/2023), each fixture trap holds a liquid seal of not less than 2 inches and not more than 4 inches, unless a special condition requires a deeper seal. Venting under 248 CMR 10.16 must keep that trap from seeing more than 1 inch of water pressure differential. Give each fixture its own water-seal trap as close as possible to the outlet, with the listed combination-fixture, three-bowl, integral-trap, and dishwasher exceptions. Do not install moving-part, bell, crown-vented, interior-partition (except integral), or full S traps. PSI puts only two items in Traps. The Board still expects the whole 10.08 trap rule.

This is not a courtesy P-trap under a lavatory. A trap is a public-health device: a water seal that stops sewer gas, vermin, and odor from entering occupied space. Massachusetts writes the numbers in 248 CMR 10.08, not in the International Plumbing Code. Candidates who grew up on IPC Chapter 10 lose easy points by quoting a 1¼-inch lavatory trap, a 24-inch “trap arm” mix-up, or an IPC house-trap ban that is not how 10.08 treats building and running traps. Independent OpenExamPrep teaching for the Massachusetts journeyman plumber exam uses the 12/8/2023 Uniform State Plumbing Code.

One trap per fixture — and the four exceptions that actually exist

248 CMR 10.08(2)(a) starts with a bright line: individual plumbing fixtures shall be separately trapped by a water-seal trap placed as close as possible to the fixture outlet. The vertical distance from the fixture outlet to the trap weir shall not exceed 24 inches (10.08 Examples 1 and 2). That 24-inch figure is outlet-to-weir, not a free pass to stretch a trap arm across a room. Trap-arm developed length to the vent lives in 248 CMR 10.16 Table 1 and is a later chapter.

The separate-trapping rule has four exceptions, and the exam loves to mix them:

1. Integral traps. A water closet, a pedestal urinal with an earthenware trap standard, and similar fixtures that already contain the seal do not get a second P-trap on the outlet.
2. Combination plumbing fixtures. One trap may serve a combination fixture when one compartment is not more than 6 inches deeper than the other and the waste outlets are not more than 30 inches apart.
3. Not more than three single-compartment sinks or lavatories. They must be immediately adjacent and in the same room. When three fixtures share the trap, the trap is centrally located. Center-to-center measurement of the waste outlets shall not exceed 30 inches.
4. Domestic dishwasher. The waste may be separately trapped, connected to the manufactured inlet-side opening of a food-waste disposer, or connected between the kitchen-sink outlet and the trap inlet using a wye fitting. Do not dump a dishwasher into the sink tailpiece with a saddle or a drilled hole; 10.05 later forbids saddles and tapping of drainage pipe.

Scenario. A Springfield double-bowl kitchen sink has bowls 4 inches different in depth and outlets 28 inches apart. One 1½-inch trap is legal as a combination fixture. If the second bowl is 8 inches deeper, that exception is gone — two traps, or change the bowls. Three lavatories on a 36-inch center-to-center gang waste in the same restroom fail the 30-inch limit even if they share a room.

Minimum trap size is a Massachusetts table, not an IPC memory

248 CMR 10.08(2)(b) says the trap shall drain the fixture rapidly and shall be no smaller than Table 1. The fixture trap shall also be the same pipe size as the drain into which it discharges. Exposed traps of copper-alloy tubing are a minimum of 17 gauge (10.08(1)).

The exam trap is treating a Massachusetts common lavatory as a 1¼-inch trap because that is what many IPC jobs use. Table 1 lists 1½ inches for a common lavatory. A residential shower stall is 2 inches, not a 1½-inch “because it is a house.” A clothes-washer standpipe trap is 2 inches. Do not size traps from an IPC fixture table on this closed-book exam.

Prohibited traps and self-scouring design

248 CMR 10.08(2)(c) names five prohibited types. Memorize the list as written:

1. Traps that depend on moving parts to maintain their seal.
2. Bell traps.
3. Crown-vented traps.
4. Separate fixture traps that depend on interior partitions for their seal.
5. Full S traps.

248 CMR 10.08(2)(d) then tells you what a legal trap looks like. Fixture traps shall be self-scouring and shall have no interior partitions except where the trap is integral with the fixture. That is why a water-closet trapway is legal and a drum trap with a hidden wall is not. Slip or ground joints that are part of a trap must meet 248 CMR 10.07(6) — slip joints only on the inlet fixture side, ground joints allowed within the trap seal. Each fixture trap shall have an accessible cleanout plug of ample size protected by the water seal, except:

• traps cast integrally or in combination with the fixture where the seal is readily accessible, or a portion of the trap is readily removable for cleaning; and
• traps for floor-outlet and similar fixtures, including bathtubs and showers, where a trap cleanout would not be accessible. In that case a cleanout goes in the nearest accessible location.

A “self-scouring” trap is one whose interior is washed by ordinary fixture discharge so solids do not sit in a chamber. A bell trap fails that test: a raised bell in a sump depends on a cover remaining seated. A crown-vented trap puts the vent at the top of the seal so the vent can siphon or foul. A full S trap is two 90s that invite self-siphonage. Drum traps and other partition traps that are not integral with the fixture are the interior-partition ban.

Trap seal, pressure differential, and why 10.16 belongs in this answer

248 CMR 10.08(2)(e) is the number pair PSI is hunting:

• Liquid seal not less than 2 inches and not more than 4 inches, except where special conditions require a deeper seal (Example 3).
• Protection from siphonage, aspiration, momentum, oscillation, back pressure, evaporation, or capillary action is accomplished by soil or waste stacks, vents, re-vents, dry vents, wet vents, loop vents, circuit or continuous vents, or combinations thereof, installed to 248 CMR 10.16, so that at no time shall the trap be subjected to a pressure differential of more than 1 inch of water.

A 1½-inch seal is a violation even if the P-trap “looks fine.” A 6-inch seal on an ordinary lavatory is also a violation unless the job is a special-condition deeper seal the inspector can defend. The 1-inch water-column limit is why you do not skip the vent and hope the stack “breathes enough.” Momentum from a nearby water closet, a long unvented arm, or a poorly placed wet vent will pull or push that seal. 10.08 sends you to 10.16; it does not reprint the vent-to-trap feet table here.

Trap setting. Traps shall be set level with respect to their water seals and, wherever necessary, protected from freezing (10.08(2)(f)). A trap rolled so the weir is high on one side is not a 2-inch seal on the low side.

Building traps, running traps, open parking garages, acid traps

248 CMR 10.08(2)(g) does not copy an IPC “no building trap” slogan. Building and running traps shall not be installed unless, in the opinion of the Inspector, they are deemed necessary. The freeze exception is specific: where a trap that otherwise complies with 10.08(2)(d) would sit in freezing temperatures, a running trap may be installed in an area not subject to freezing, as close as possible to the fixture it serves. Each building trap, when installed, gets an accessible cleanout and a relieving vent or fresh-air intake that need not be larger than one-half the diameter of the drain to which it connects.

Open parking garages (openings in exterior walls on two or more sides of each level of not less than 20% of the total perimeter wall space on that level):

• Floor drains on the open level(s) need not be trapped.
• A running trap goes on the sanitary drain before it enters the separator or containment system required by 248 CMR 10.09(1)(a), with an accessible cleanout and a 4-inch vent run independently through the roof.
• Stacks on that system are installed at intervals not exceeding 60 feet and to 248 CMR 10.16(3)(a).

Acid-resistant traps. Where a vitrified-clay or other brittle-ware acid-resistant trap is installed underground, embed it in concrete extending 6 inches beyond the bottom and sides (10.08(2)(h)).

Do not size, ban, or “except” traps from an IPC table. Massachusetts trap seals, weir distance, Table 1 sizes, prohibited types, and building-trap language live in 248 CMR 10.08. IPC Chapter 10 is a warning, not a study sheet.

Official resources

• 248 CMR 10.08 Traps and Cleanouts (Cornell LII) — 12/8/2023 trap seal, Table 1, prohibited traps, 24-inch weir
• 248 CMR 10.08 (UpCodes) — same section with examples
• 248 CMR 10.07(6) Slip and Ground Joints — trap-side slip and ground joints
• 248 CMR 10.00 Uniform State Plumbing Code (Mass.gov) — official 12/8/2023 posting