Let’s start with the definition of a trap arm. The Florida Building Code defines it as “that portion of a fixture drain between the trap weir and the vent fitting.” Next, you need to know the definition of a trap weir: it is the top level that water settles to in the U-shaped trap under a sink when there is nothing flowing through it. And the “vent fitting” is the connection to the vertical pipe through the roof that allows air into the drain system, which is necessary for free-flowing drains.

    The minimum slope of a trap arm is easy to understand and remember. It is a 1/4” drop per running foot for pipes with a diameter of less than 3”, and 1/8” per foot for pipes larger than 3” diameter. 

    The maximum slope of a trap arm is based on length of the arm and diameter of the pipe. The maximum slope shrinks as the trap arm gets longer and, conversely, increases as the length of the arm shortens. It also grows and shrinks in tandem the with diameter of the pipe. You don’t want a slope of 1/2” or more, though, because the liquid will drain too fast and not float the solids along with it down the pipe.

    There are charts that list the maximum length of a trap arm for each pipe diameter. A trap arm of 2” pipe at a 1/4” slope, for example, is listed at 8 feet maximum length. It is not a coincidence that 8 times 1/4” equals the 2” diameter of the pipe. That’s because there has to be a continuous channel of air between the trap and the vent, or else trapped air in the drain fluid will cause a vacuum that sucks the trap dry and allows sewer gas to enter the home. The same scientific vacuum principle that is used to siphon water out of a fish tank comes into play when the top of the inside of the pipe descends below the level of the trap weir.

    The trap arm length tables always include a note that states that “the vent connection shall not be below the trap weir,” and that is the overriding standard to follow when calculating maximum slope of a trap arm. So the trap arm at the laundry sink shown above definitely exceeds the maximum slope for its length.

    Here’s two examples below of trap arms with negative slope.

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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

• Can galvanized steel pipe still be used for new water lines in a house? 

• How can I tell if I have cast iron pipes in my house? 

• Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection? 

• What is the difference between green and white sewer drain pipes?

• Is a washing machine drain hose required to be secured at the standpipe?

• What are the abandoned pipes sticking out of the wall in my house?  

• What are the code requirements for plumbing vent terminations?

• What are the code requirements for layout of drain piping under sinks?

• What causes a gurgling sound when a bathtub or sink drains? 

• What is a "combination waste and vent" in a plumbing system? 

• What is a building trap?  

• What is a galvanized nipple?

• What are the pipes sticking out near my water valves?

• How do you accurately find a broken water pipe leak under the floor slab?

• What is the difference between water pipe and sewage (waste) pipe? 

• Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water? 

• Is a hot water faucet handle required to be on the left? 

• What is a dielectric union? 

• What's that powdery crust on the pipe connections at the water heater? 

• If all the plumbing drains have water in them and you can still smell sewer gas, what's causing the problem?  

• How can I find out what type of water pipe runs underground from the water meter to the house (service pipe)?

• What is a P-trap?

• Why is old galvanized steel water pipe a problem for homebuyers?

• What does polybutylene pipe look like? Why is it a problem? 

• Which water pipes are an insurance problem and possibly uninsurable?

• Can you connect CPVC pipe directly to a gas water heater?  

     Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

Diagram - courtesy CodeCheck