What is the minimum and maximum slope of the trap arm of a plumbing drain?
Sunday, June 24, 2018
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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