When the IAPMO Plumbing Technical Committee reaches Chapter 7 of the plumbing code changes, you know arguments about the minimum pipe size of various drains, vents and storm drains will ensue.

This cycle was no different.

The ASPE Legislative Committee submitted a number of code changes on these three subject areas. You would think code changes coming from the plumbing engineers would pull a lot of weight. That wasn’t the case at the IAPMO Plumbing Technical Committee meeting in Denver.

One of the first changes considered was the introduction of a new category of fixtures called “urinal, nonwater.” The proposal included a fixture unit value of 0.5 dfu (drainage fixture unit) and a minimum trap size of 1 1/2 in. At first glance, you would think this code change is not controversial. Well, that wasn’t the case. After all, these are nonwater urinals.

The biggest argument was over the 1 1/2-in. trap size. That would result in a minimum trap arm of 1 1/2 in. Many thought the trap size must remain 2 in. After all, all urinals have a 2-in. trap. When it was pointed out that such a statement was untrue and the fixture standard allows a 1 1/2-in., trap for urinals, the argument switched to the fixture unit value. If you are allowed to size the drain for 0.5 dfu, it would be too small when you switch the nonwater urinal to a flush urinal. It was emphasized that nonwater urinals eventually will be changed to flush urinals because everyone figures out that a flush urinal is better. Interestingly, there was no data to support this statement, but it apparently sounded good.

The code change to list nonwater urinals was recommended for rejection.


It’s a trap

The very next code change, also submitted by ASPE, proposed to lower the shower drain trap size to 1 1/2 in., from the current 2 in. The plumbing engineers technically supported the change with values on the worst flow rates using the Manning equation. With the roughest piping material used, a 1 1/2-in., trap was shown to still be adequate.

Interestingly, more than half the U.S. has been using the 1 1/2-in., trap value for showers for many years. The BOCA National Plumbing Code changed to a 1 1/2-in., shower trap in 1993 following the federal change in showerhead flow rates. Thus, there is plenty of field experience with the use of a 1 1/2-in., trap for a shower. I have never had any problems taking a shower every morning in my master bath shower with a 1 1/2-in., trap.

After all the arguing, the code change ended in a tie. The chair broke the tie voting to accept the lowering of the shower trap size. However, many of us knew victory would be short lived. Following the meeting, the Technical Committee has to cast a written vote on every change. The written vote requires a two-thirds majority. As anticipated, the shower trap size code change did not receive the neccesary votes for approval on a written vote. The code change failed by two votes out of 29 cast. As a result, the code change will not have a recommendation, which means it will not be accepted. A public comment will be required to have the change approved.

By the time the Technical Committee reached the venting section, it appeared ASPE would lose another code change.

ASPE submitted a proposal to recognize air-admittance valves. If there is one thing worse than a nonwater urinal code change proposal, it is one for air-admittance valves. ASPE pointed out the proposal was consistent with what is published in the ASPE Plumbing Engineering Design Handbook. That didn’t seem to matter. Air-admittance valves don’t belong in the Uniform Plumbing Code. The code change was recommended for rejection.

Another venting change submitted by an individual plumbing engineer would have moved the circuit venting requirement from the appendix to Chapter 9. You can design a circuit vent system under the current code as an engineered design. One of the complaints is you have to identify a long-accepted venting practice as an engineered design. The thought process was to simply identify circuit venting as one of many venting options in Chapter 9. The committee rejected the proposal.

A storm is brewing

ASPE also submitted a change to the storm drainage section that would require a completely different method of designing the storm drainage piping system. The proposed change was based on the result of the ASPE Research Foundation report on roof drainage. As the Research Foundation pointed out, we have been designing storm drainage systems incorrectly for more than 80 years. In the past few years, with an increase in value engineering, there have been a number of roof collapses and storm drainage system failures.

One of the difficulties in presenting storm drainage issues to the IAPMO Plumbing Technical Committee is many committee members are from California. What California calls a heavy rainstorm wouldn’t even qualify as a normal rain event in the Midwest or on the East Coast. It is tough to explain what it is like when rain falls at a rate of three to five inches per hour. California considers two inches per hour to be a monsoon.

The code change might have received leverage until PDI opposed the change. The concern was with some existing numbers in the sizing table. While these values were simply duplicated, it was pointed out a complete change should include appropriate corrections to existing sizing values. The code change was recommended for rejection.

One last attempt by ASPE was to add a section on engineered design to the storm drainage section. Included in the engineered design section was a reference to siphonic roof drainage systems. This code change also appeared to be noncontroversial since there are already sections in water pipe sizing, drainage system design and vent system design that allow for an engineered design. This would simply add storm drainage design to that list.

However, that is not how the Technical Committee viewed the change. They did not see a reason to add engineered design to the chapter. The claim was Chapter 3 already has language that allows any engineered design. That statement was very accurate.

While siphonic roof drainage can be used based on the alternative engineered design, there are no guidelines provided to the authority having jurisdiction. The thought was to include the ASPE standard for siphonic roof drainage design to provide a tool to the inspector. The code change was recommended for rejection.

The full report of the results of the Plumbing Technical Committee meeting will be published in a report on proposals this month. You can download the ROP from www.iapmo.org.


This article was originally titled “What size is correct in the UPC?” in the July 2016 print edition of PM Engineer (pme).