Total fall in a fixture drain

[Robert Ellenberg]

Section P3105, Fixture Vents, under P3105.2 Fixture drains, states: The total fall in a fuxture drain resulting from pipe slope shall not exceed one pipe diameter, nor shall the vent pipe connection to a fixture drain, except for water closet, be below the weir of the trap.

I think this pipe slope not exceeding one pipe diameter refers to the distance from the fixture to the vent, including the weir but it doesn't say that--it refers to the fixture drain. You could have the vent within the prescribed distance to the weir as spelled out in table P3105.1 with proper fall, but the drain could continue for quite some distance until it joins into the main horizontal building sewer line. I would think that additional distance would be considered part of the fixture drain and it would certainly exceed a pipe diameter if it was much distance.

Am I reading this incorrectly (I think it means the distance to the vent) or is it just not well worded and it does mean the drain slope between the fixture and the vent.

 

[peach]

I think it's the total fall to where it enters the building drain. If the drain from a fixture (think toilet) is too steep, you can have solids stuck in the pipe.. the water will drain away without taking the solids along for the ride.

 

[Uncle Bob]

Robert,

Note that the requirement is under Fixture Vents; the slope must not exceed the pipe diameter from the trap wier to the vent fitting. Table P3105.1 gives examples of maximum distances: 1/4" X 6 =

1 1/2" which equals the pipe diameter of the 1 1/2" trap.

The fixture drain is the drain section between the trap and the vent fitting.

Hope this helps,

Uncle Bob

 

[Glennman CBO]

Its the slope of the trap arm to the vent. It cannot be the fall of the entire system, since it would already be violated at the first vertical drop.

The issue is the amount drop prior to the vent. After the vent, the fall is unlimited because the "falling" water has air from the vent that keeps traps from being sucked out. Prior to the vent, the trap is basically unprotected during that particular length of trap arm, hence, the limited lengths and drop as Uncle Bob mentioned.