Free study tools · 248 CMR 10.16(10)
Trap Arm Length Checker: How Far Can the Vent Be?
Pick the trap size, enter the developed length of the trap arm, and see whether it meets the maximum distance from trap to vent in 248 CMR 10.16(10) Table 1. If it doesn't, you'll see why it matters and how to fix it.
1. Describe the trap arm
2. Result
248 CMR 10.16(10) Table 1: maximum trap arm length
| Trap size | Maximum developed length |
|---|---|
| 1-1/2 in. | 5 ft |
| 2 in. | 6 ft |
| 3 in. | 8 ft |
| 4 in. | 10 ft |
Why the trap arm has a length limit
A trap only works while it holds its water seal. When a fixture drains, the water leaving the trap can fill the trap arm and pull air behind it. If the vent is too far away, that moving slug of water creates suction on the outlet side of the trap and siphons the seal out of the fixture's own trap. That's called self-siphonage. Once the seal is gone, sewer gas can come back through the fixture.
Keeping the vent within the Table 1 distance gets air to the trap arm before that suction builds. A longer arm also collects more fall along its run, which makes it more likely the arm runs full.
How to fix a trap arm that's too long
- Move the vent connection closer to the trap so the developed length is within the Table 1 limit.
- Add a vent at the fixture (an individual vent) instead of relying on a vent farther down the line.
- Move the fixture or the drain so the run from the trap to the vent is shorter.
On the exam, read the question for the trap size, not the drain size downstream. The distance limit goes with the trap.
Keep studying
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Study aid — confirm with the current 248 CMR and your inspector. Values are from 248 CMR 10.16(10) as published by Cornell LII (amended effective 12/8/2023) and were checked in September 2026. Other rules, such as trap arm slope and fitting requirements, also apply and are not checked here. PressureTest is independent and not affiliated with the Commonwealth of Massachusetts, the Board of State Examiners of Plumbers & Gas Fitters or PSI.