IPC vs. UPC Stack Sizing

By Timothy Allinson, P.E.

Murray Company, Long Beach, Calif.

As a Southern California design-build contractor, my projects are all locally based, and as such, they are governed by UPC-or more accurately, the CPC, which is the UPC with California amendments. The differences between UPC and the code of my former stomping ground, New York City, are significant with respect to stack sizing. Recently, I began wondering about NYC's new code, the IPC (with amendments) and how it compares to UPC.

I am somewhat embarrassed to admit that I am shamefully ignorant about IPC. As that code developed its presence I have never worked in a jurisdiction under its governance, so I never had cause to familiarize myself with IPC. But UPC is so different than the old NYC Code that I decided to explore how it compared with NYC's new Code.

It turns out that the IPC lies somewhere between the NYC Code of old and the UPC, leaning more toward the old NYC Code, at least with respect to stack sizing. In fact, the stack sizing table in the IPC are nearly identical to those in the old NYC Code.

So how do the IPC and UPC compare? To start, they are very different in appearance. Figure 1 is an abbreviated version of the IPC waste stack-sizing table. There is a separate table for sizing building drains not shown here. Figure 2 is an abbreviated version of the IPC vent-sizing table. In contrast, Figure 3 is an abbreviated version of the UPC waste and vent sizing table. This one small table is a Reader's Digest version of four separate tables in the IPC. So, the first and most obvious difference is that the UPC sizing criteria is much more concise. Is this a good thing? It's too early to tell. Brevity is not necessarily superior to verbosity. In order to judge the differences between these two codes we'll have to look at some examples.

Take, for example, a 30-story, 300-ft. tall building with a stack (one of many) serving a typical toilet room on each floor -- one water closet, one lavatory, and one bath/shower. Regardless of which Code you reference, the sum on these fixtures yield 6 drainage fixture units. Since the stack serves 30 floors that totals 180 DFUs for the stack.

Since both Codes allow the stack to telescope in size, and since water closets require a 3-in. minimum waste, the top of the stack will be a 3-in. minimum. The IPC allows three water closets on a 3-in. stack; the UPC allows four, so the transition from 3-in. to 4-in. can occur one floor lower using the UPC -- but that's a subtle difference.

Once the stack transitions to 4 in. both codes yield the same stack sizing result: a 4-in. stack is required sown to the first floor. Looking at IPC, Figure 1 reveals that a 4-in. stack can serve as many as 500 fixture units. Figure 2 further reveals that a 4-in. vent stack is required, since the stack is 4-in. with 180 DFUs and 300 ft. in height. If you are reading the table correctly you will see that a 4-in. stack with up to 320 DFUs requires a parallel 4-in. vent when it is greater than 170 ft. and less than 640-ft. long.

Looking at UPC, Figure 3 reveals that a 4-in. vertical stack can carry as many as 256 DFUs and cannot exceed 300 ft. in height. A 4-in. vent can serve up to 256 fixture units with a maximum length of 300 ft. Based on UPC sizing, we are on the cusp of the 4-in. stack criteria. @body:Although the sizing in this random example produces the same results with these two codes, a comparison of their numbers reveals great differences. For example, as noted, a 4-in. stack in the IPC can serve 500 fixture units and has no length limitations, whereas a 4-in. stack in the UPC can only serve 256 DFUs and is limited to 300 ft. in length. Therefore, if the subject building were a few stories taller, the stack sized by the IPC would remain 4 in. in size, while the waste stack sized by the UPC would grow to 5 in. for the lower few floors and the entire vent would increase to 5 in.

Oddly, while the sizing criteria for vertical stacks are dramatically different in these two codes, the sizing for the horizontal building drain is very much the same. In both Codes a 4-in. drain with 2% slope can service 216 DFUs. Not all of the building drain sizing criteria are identical, but it is close -- much closer than the vertical stack sizing criteria.

Also noteworthy is the fact that both codes require relief vents interconnecting the waste and vent stacks when the stacks are 10 stories or more in height; however, the IPC requires relief vents every 10 floors while the UPC requires them every 5 floors.

So the UPC is much more conservative than the IPC when it comes to waste and vent sizing, since the pipes in the UPC system are often larger than the IPC system. Or are they?

We should consider that both codes have an allowance for "bathroom group" sizing. In the IPC it is in the body of the code whereas in the UPC it is in the appendix. The IPC assigns 5 DFUs to the typical bathroom group in our example, while the UPC assigns only 3 DFUs. If one applies the bathroom group sizing technique, then the sizing difference between the two codes narrows. However, the length limitations of the UPC will still generate larger pips sizes, and as noted above, the UPC requires twice as many relief vents.

So why do these differences exist between these two codes? And for that matter, why the similarities? I wish I knew the answer to these questions. There are surely others out there with greater Code history knowledge than myself who might be able to answer these rhetorical questions.

It is my personal opinion that UPC produces stack sizing that is larger than necessary. I base this statement on the fact that IPC sizing mimics the NYC criteria of old, and clearly NYC has many high-rise buildings sized with this criteria that have functioned well for many, many years. I think it would be prudent for IAPMO to consider embracing these sizing criteria for the sake of economy and uniformity, but I won't hold my breath in anticipation of such a code change.

 

Timothy Allinson is a Senior Professional Engineer with Murray Company, Mechanical Contractors, in Long Beach, Calif. Prior to entering the design-build industry he worked for Popov Engineers, Inc. in Irvine, Calif, and JB&B in New York City. Tim holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed Accredited Professional. Tim is a past-president of ASPE, both the New York and Orange County Chapters, and sits on the board of the Society of American Military Engineers, Orange County Post.