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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 term shows up during construction and affects decisions that are hard to reverse once the work moves forward. Understanding it now saves time, money, and frustration later.
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