UPC Nuggets: The code says what to do; it’s just sometimes sketchy about how to do it
Last month we began digging a little deeper into the Uniform Plumbing Code to discover not just what the UPC says but why some sections seem so cryptic. My intent is to continue digging for the next several months. This month let’s begin by looking at Section 704.0.
704.0 Fixture Connections (Drainage)
704.1 Inlet Fittings. Drainage piping shall be provided with approved inlet fittings for fixture connections, correctly located according to the size and type of fixture proposed to be connected.
At this point it is important to pause and make an observation regarding the Uniform Plumbing Code.
One of the quirks and perhaps frustrations of those who use the UPC is, many times you’re not told specifically how to do something.
There are requirements — sometimes minimums or maximums, not to exceed, not more than, not less than, approved fitting and a host of other parameters that establish boundaries. As long as your design or installations are within the boundaries of the code, it should be approved.
The positive of this is the freedom of and innovation to do something different. The downside is you have to know and understand the wherefores and the whys and figure it out. You can do it like you’ve always done it, like you were taught, or look for a new ways that may save material or labor. Last month I shared that plumbing designs are geometric problems looking for a solution.
That is especially true of DWV systems and section 704. A novice reading Section 704.1 would likely ask, “What the blazes does that mean,” because the language of this section doesn’t tell you much. But when you dig deeper, it tells you everything you need to know, if you know where to look.
1. It says the fitting has to be an “approved inlet fitting.”
What is so special about this that it has to have approval? Look back at the title of this section again and let it begin to dawn on you this is not a connection from a waste pipe to a waste pipe. This is where a fixture trap connects to the DWV system. As such, the fitting will serve three functions: a connection for the fixture trap, a discharge into the drainage system, and a vent connection to protect the trap seal from siphonage and backpressure.
Because there is a trap and vent, much of what determines the approved fitting is in chapters 9, Traps and 10, Vents. Beginning in:
905.5 Location of Opening. The vent pipe opening from a soil or waste pipe, except for water closets and similar fixtures, shall not be below the weir of the trap.
The vent pipe opening shall not be below the weir of the trap. Or you could say, the vent pipe opening must be above the weir of the trap. In either case, the reason is if the elevation of the weir of the trap (the elevation at which water begins to spill out of the trap and down the trap arm) is equal to or higher than the vent pipe opening it can effectively block off the vent opening. If that occurs the fixture is no longer vented (see image).
Once you know where the vent pipe opening and the weir of the trap are, this begins to makes sense. If you know your drainage fittings, you know the only drainage fitting which meets the criteria is a sanitary tee. There is an exception (with good reason) for water closets and similar fixtures but we’ll hold off for now on the wherefores and whys.
2. And be “correctly located according to the size and type of fixture proposed to be connected.” Referring to:
1002.2 Fixture Traps. Each fixture trap shall have a protecting vent so located that the developed length of the trap arm from the trap weir to the inner edge of the vent shall be within the distance given in Table 1002.2, but in no case less than two times the diameter of the trap arm.
So, now we have to locate our sanitary tee at the proper height (based on the elevation of a fixtures drain outlet) and a distance not less than and not to exceed that given in Table 1002.2. For example if the fixture requires a 1 1/2-in. trap, the distance from the weir of the trap to the vent pipe opening shall not exceed 42 in. The reason for the length limitations is, as the distance from the vent pipe opening to the weir increases, the weir of the trap rises. (It rises because of the required slope on the trap arm).
704.2 Single Vertical Drainage Pipe. Two fixtures set back-to-back, or side-by-side, within the distance allowed between a trap and its vent shall be permitted to be served by a single vertical drainage pipe provided that each fixture wastes separately into an approved double-fixture fitting having inlet openings at the same level.
This section allows the use of a single vertical drainline to serve multiple fixtures. It makes financial and structural sense to locate plumbing fixtures close enough that a single pipe may serve them. Once again, this is for “fixture connections,” not some random drain line connecting to another. As such, the “approved double-fixture fitting” has to be constructed so the vent pipe opening is above the weir of the fixture trap (just like in Section 704.1) and the discharge from one trap arm does not enter the trap arm of the opposing inlet.
A double sanitary tee is not approved for this application. You may have seen a double sanitary tee used in drainage but they are not approved for fixture connections.
One problem with the “approved double fixture fitting” is that while the Schedule 40 ABS and PVC fitting is called a Double-Fixture Fitting, the equivalent made of cast iron is called a figure No. 1 or No. 5 depending on size and configuration.