• Welcome to The Building Code Forum

    Your premier resource for building code knowledge.

    This forum remains free to the public thanks to the generous support of our Sawhorse Members and Corporate Sponsors. Their contributions help keep this community thriving and accessible.

    Want enhanced access to expert discussions and exclusive features? Learn more about the benefits here.

    Ready to upgrade? Log in and upgrade now.

Water closet fixture drain

Sifu

Sifu

SAWHORSE
Joined
Sep 3, 2011
Messages
3,318
Knowledge check (mine): What is the minimum size of a water closet fixture drain in the IPC? Please include code reference.

Not a loaded question, I am trying to validate what I thought I knew but am finding difficult to find.
 
2024 IPC 425.4 Water closet connections. A 4-inch by 3-inch (102 mm by 76 mm) closet bend shall be acceptable. Where a 3-inch (76 mm)
bend is utilized on water closets, a 4-inch by 3-inch (102 mm by 76 mm) flange shall be installed to receive the fixture horn.
 
The water closet is a "branch of the building drain", not a fixture branch. Fixture branches serve at least two fixtures.

See footnote a for Table 710.1(2), then check out the definition of fixture branch.

I didn't know that either until you made me look it up again, lol.
 
I have always thought it to be 3" min., but I have a submittal showing a 2" from the w/c connection to the point it picks up another fixture. This is an engineered design, but it is unusual and didn't ring true so I went looking. And, FWIW, everything I have seen regarding this also says it must be 3", except the IPC as far as I can find. The closest I can find in the IPC is that it would be 4 dfu's, which would use t709.2 but only if the fixture isn't listed in t709.1, which doesn't seem to cover fixture drain sizing. To my knowledge, the fixture drain is based on the trap size...except (I thought) for water closets).

"The water closet is a "branch of the building drain". Where is this stated, and how would the section of pipe in the drawing fit the definition of building drain in all circumstances?
 
wwhitney said:
Certainly not when the water closet connects to a stack rather than directly to the building drain.

Cheers, Wayne
Click to expand...
Fair enough. I was looking at the water closet labeled "fixture drain", did not notice the one that was on the same branch as the LAV and the VTR.

Sifu said:
I have a submittal showing a 2" from the w/c connection to the point it picks up another fixture.
Click to expand...
That is crazy... why would someone do that?
 
Maybe it is not a standard toilet.

2018 IPC
405.5 Plumbing fixtures with a pumped waste.
Plumbing fixtures with a pumped waste shall comply with ASME A112.3.4/CSA B45.9. The plumbing fixture with a pumped waste shall be installed in accordance with the manufacturer’s instructions.

712.4.1 Macerating toilet systems.
Macerating toilet systems shall comply with ASME A112.3.4/CSA B45.9 and shall be installed in accordance with the manufacturer’s instructions.

712.4.2 Capacity.
A sewage pump or sewage ejector shall have the capacity and head for the application requirements. Pumps or ejectors that receive the discharge of water closets shall be capable of handling spherical solids with a diameter of up to and including 2 inches (51 mm). Other pumps or ejectors shall be capable of handling spherical solids with a diameter of up to and including 1/2 inch (13 mm). The capacity of a pump or ejector based on the diameter of the discharge pipe shall be not less than that indicated in Table 712.4.2.

Exceptions:

1. Grinder pumps or grinder ejectors that receive the discharge of water closets shall have a discharge opening of not less than 11/4 inches (32 mm).

2. Macerating toilet assemblies that serve single water closets shall have a discharge opening of not less than 3/4 inch (19.1 mm).
 
But no matter what it is, I assumed it was in the code that a toilet had to have a 3" min. fixture drain, but I now assume I was conflating the notes for building drain with fixture drain. I found many references to the fact that it had to be 3" "on the line", (including the all knowing wizard AI) but every one of the references I found were for building drains, not the fixture drain. I did find a specific city code that has it in their code, but nowhere else. Could it be there is no minimum, other than it must match the drain outlet of the fixture and not reduce in size from the trap? Has the good practice of 3" min. just been accepted and morphed into a min. requirement?

In this case, the fixture specification provided by the DP says a 4" waste connection is required, so I can comment on that, but it really makes me wonder about the 3" min.
 
1736810452947.png

Water closet, private (1.6 gpf) 3e Note d

d. Trap size shall be consistent with the fixture outlet size.

So if the trap size is 3 inch shouldn't the outlet be 3" also? A water closet is a trap
 
Yes, but the trap in a water closet is usually 2 or 2 1/4" I think, and this specific one is 2 1/8". That note is where I think the only sizing for a fixture drain from a water closet is located, just not the way I have ever seen it or understood it. It also explains why some other codes have specified it.

I think the code is leading me to believe that you could have a <3" fixture drain to a 3" building drain, not that it is a good idea, or something I have ever seen, or something I think the engineer did intentionally, and maybe something the code didn't do intentionally. I also can't find a <3" closet flange on the line so it would need to be reduced which is a problem too.

So maybe 3" is not directly required, but I'll never tell.
 
How many fixture units?
How far to from branch line to main line?


But
according to the International Plumbing Code (IPC), and the International Residential Code (IRC) , the minimum size of a water closet fixture drain is 3 inches
 
IPC 740.2 says "The size of the drainage piping shall not be reduced in the direction of the flow." Since you can't have a 4" fixture drain going into a 3" building drain, 3" is OK.

Also, Table 709.2 allows 5 FU on a 3" fixture drain, and a WC is 3 or 4 FU, except for a public toilet greater than 1.6 gal per flush, and Table 710.1(2) allows 20 FU on a 3" fixture branch.
 
mark handler said:
But
according to the International Plumbing Code (IPC), and the International Residential Code (IRC) , the minimum size of a water closet fixture drain is 3 inches
Click to expand...
Code reference? All I see is for a building drain.
 
Paul Sweet said:
IPC 740.2 says "The size of the drainage piping shall not be reduced in the direction of the flow." Since you can't have a 4" fixture drain going into a 3" building drain, 3" is OK.

Also, Table 709.2 allows 5 FU on a 3" fixture drain, and a WC is 3 or 4 FU, except for a public toilet greater than 1.6 gal per flush, and Table 710.1(2) allows 20 FU on a 3" fixture branch.
Click to expand...
T709.2 is for fixtures not listed in t709.1, and a 1.6 w/c is listed so we use t709.1. T709.1 shows 4 DFU's for 1.6 w/c, with two notes that say you can't reduce the DFU load for a building drain without jumping through a hoop and that the trap size is dependent on the fixture outlet, but that doesn't make a lot of sense to me since the trap is integral and is upstream of the fixture outlet (when are they not for a w/c?). Even if we used t709.2 and the DFU's from t709.1 the fixture drain would be 2 1/2".

Believe me when I say I would love to validate what I thought I knew, I just can't find any direct support for it.

The more I read this the more I realize how messy it is.
 
As stated on this forum before ICC does not mean Intelligent Clear or Concise but I believe the intent of footnote (a) of Table 710.1(1) is a minimum 3" for a water closet

TABLE 710.1(1)
BUILDING DRAINS AND SEWERS

MAXIMUM NUMBER OF DRAINAGE FIXTURE UNITS CONNECTED TO ANY PORTION OF THE BUILDING DRAIN OR THE BUILDING SEWER, INCLUDING BRANCHES OF THE BUILDING DRAIN a
a. The minimum size of any building drain serving a water closet shall be 3 inches.

BRANCH. Any part of the piping system except a riser, main or stack.
 
mtlogcabin said:
As stated on this forum before ICC does not mean Intelligent Clear or Concise but I believe the intent of footnote (a) of Table 710.1(1) is a minimum 3" for a water closet

TABLE 710.1(1)
BUILDING DRAINS AND SEWERS

MAXIMUM NUMBER OF DRAINAGE FIXTURE UNITS CONNECTED TO ANY PORTION OF THE BUILDING DRAIN OR THE BUILDING SEWER, INCLUDING BRANCHES OF THE BUILDING DRAIN a
a. The minimum size of any building drain serving a water closet shall be 3 inches.

BRANCH. Any part of the piping system except a riser, main or stack.
Click to expand...
I went down the branch path, but it just doesn't seem to hold water (sorry). I agree the intent may be there, but for this it would seem to be too easy to have just stated that a w/c fixture drain is 3" min. The commentary for the definition of branch even says a fixture drain doesn't become a branch until it merges with another fixture drain. To me it seems it either got lost somewhere along the way or assumed. FWIW the 2009 UPC (most recent version I have) explicitly requires a 3" "trap arm" for a water closet. However it also says a 3" trap is required. Is there anything different in the more recent UPC's?
 
You must log in or register to reply here.
Back
Top