7/28/2019 Designers Guide 200707
1/2
Designer's GuideIPC 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
moreaccurately, the CPC, which is the UPC with California
amendments. The differences between UPC and the code of my former
stompingground, New York City, are significant with respect to
stack sizing. Recently, I began wondering about NYC's new code, the
IPC (withamendments) 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 neverworked
in a jurisdiction under its governance, so I never had cause to
familiarize myself with IPC. But UPC is so different than the
oldNYC 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 leastwith
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 inappearance. 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 anabbreviated 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 isa Reader's Digest version of four separate tables
in the IPC. So, the first and mostobvious difference is that the
UPC sizing criteria is much more concise. Is this agood thing? It's
too early to tell. Brevity is not necessarily superior to
verbosity. Inorder to judge the differences between these two codes
we'll have to look at someexamples.
Take, for example, a 30-story, 300-ft. tall building with a
stack (one of many) servinga typical toilet room on each floor --
one water closet, one lavatory, and onebath/shower. Regardless of
which Code you reference, the sum on these fixturesyield 6 drainage
fixture units. Since the stack serves 30 floors that totals 180
DFUsfor the stack.Since both Codes allow the stack to telescope in
size, and since water closetsrequire a 3-in. minimum waste, the top
of the stack will be a 3-in. minimum. The IPCallows three water
closets on a 3-in. stack; the UPC allows four, so the
transitionfrom 3-in. to 4-in. can occur one floor lower using the
UPC -- but that's a subtledifference.
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. Ifyou are reading the table correctly you
will see that a 4-in. stack with up to 320DFUs 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 as256 DFUs and cannot exceed 300 ft. in height. A
4-in. vent can serve up to 256fixture 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,
acomparison of their numbers reveals great differences. For
example, as noted, a 4-in. stack in the IPC can serve 500 fixture
units andhas 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 thesubject 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 UPCwould 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 drainis 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 areidentical, 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
storiesor 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
areoften 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 inthe UPC it is in the appendix. The IPC assigns 5 DFUs to
the typical bathroom group in our example, while the UPC assigns
only 3DFUs. If one applies the bathroom group sizing technique,
then the sizing difference between the two codes narrows. However,
the
7/28/2019 Designers Guide 200707
2/2
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
thesequestions. There are surely others out there with greater Code
history knowledge than myself who might be able to answer
theserhetorical 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
sizingmimics 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, butI 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.
