Code Applied by the app plumb.trap.arm-length
Trap arms
How far a fixture's trap may sit from its vent along the drain, by trap size: 5 feet at 1-1/4 inch, 6 feet at 1-1/2 inch, 8 feet at 2 inch, 12 feet at 3 inch, 16 feet at 4 inch.
| When | The app uses |
|---|---|
| 1-1/4 in trap, at 1/4 in per foot, at most | 5 ft |
| 1-1/2 in trap, at 1/4 in per foot, at most | 6 ft |
| 2 in trap, at 1/4 in per foot, at most | 8 ft |
| 3 in trap, at 1/8 in per foot, at most | 12 ft |
| 4 in trap, at 1/8 in per foot, at most | 16 ft |
- Source
- IPC · 909.1, Table 909.1 · read the public text
- Editions checked
- 2018 · 2021
- Used by
- Courses
- Amendments
No state or local amendment to this rule is on file. Not on file means the model code as written — check your jurisdiction.
A model code the authority having jurisdiction adopts.
The trap arm is the drain from a fixture's trap to its vent. Too long, and the drain can run full and siphon the trap when the fixture empties, or when a water closet flushes into the same pipe. The code caps the distance by trap size. The course's lavatory has a 1-1/2 inch arm of four feet, inside the six the table allows.
On a bid a long arm is a change: a fixture the plan moves away from its wall needs a vent of its own, which is pipe, fittings and often another roof penetration.
What the app does with it
Nothing yet: the app does not apply this rule. It is on two course cards (the plumbing course, Chapter 5, "Trace the stack", and Chapter 9, "What the rows mean"). Bid Check has a manual row, Trap arm lengths within the table, that you tick when you have read the arms on the riser against this table; it measures nothing itself.
Verify against your edition
All five rows matched the 2021 text of Table 909.1; the table had another number before the 2018 edition. The course cards and the sample riser's note 2 cited it as Table 1002.2 until 2026-09-27, which is the Uniform Plumbing Code's trap arm table, and they now say 909.1. The UPC table is shorter: 3 feet 6 inches for a 1-1/2 inch arm, so in a UPC jurisdiction the course's four-foot lavatory arm would fail. Settled 2026-09-27 by Claude on the owner's delegation, from the dossier's research and the model code text; not checked against a printed book or a local amendment.
Cited by section, not reprinted: the code text belongs to its publisher. Company practice is not a rule here — it lives with your pricing.