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Trap arm
The trap arm is the section of drain pipe between the P-trap and the vent connection.
Why it matters
If the trap arm is too long or sloped wrong, the trap can siphon and lose its seal.
Where people get this wrong
A trap arm is not the same as the P-trap. Venting rules limit trap arm length.
Real-world example
A sink smells fine for months, then starts smelling after a remodel. A small layout change can accidentally make the trap arm too long and cause siphoning.
Where this hits your build
This comes up during rough-in and final fixture installation. Accessibility for future repairs matters as much as the initial install.
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