2021 IPC Chapter 7
A water closet dumps several gallons in seconds; a lavatory trickles. Chapter 7 rates that difference in drainage fixture units (dfu), adds them up, and lets you look up the smallest pipe that can carry the total. This lesson is the arithmetic: Table 709.1 for the values, Tables 710.1(1) and 710.1(2) for pipe sizes, and a full example. Houston enforces the Uniform Plumbing Code (Lesson 1), which has its own fixture unit and sizing tables, so never carry these IPC numbers into a Houston permit unchecked.
A drainage fixture unit is a relative load weight, not a flow rate. It reflects how much a fixture discharges, for how long, and how often. Table 709.1 gives the value and minimum trap size for each fixture. You add the values of everything draining to a pipe and compare the total with the limits in Table 710.1(1) or 710.1(2).
| Fixture | dfu |
|---|---|
| Water closet, public, 1.6 gpf | 4 |
| Water closet, private, 1.6 gpf | 3 |
| Urinal (over 1 gpf); 1 gpf or less; nonwater | 4; 2; 1/2 |
| Lavatory | 1 |
| Bathtub; shower of 5.7 gpm or less | 2; 2 |
| Kitchen sink, service sink, floor drain | 2 each |
| Drinking fountain | 1/2 |
| Clothes washer: commercial; residential | 3; 2 |
| Bathroom group (1.6 gpf closet) | 5 |
A shower rises with flow: 3 dfu up to 12.3 gpm, 5 up to 25.8 gpm, and 6 up to 55.6 gpm. A showerhead over a tub adds nothing (footnote b), and fixtures added to a bathroom group add their own values (footnote f). For building drain and sewer loads, do not rate water closets and urinals lower than the table shows unless testing confirms it (footnote e).
Table 709.2 rates a fixture that is not listed by its drain or trap size: 1 1/4 in is 1 dfu, 1 1/2 is 2, 2 is 3, 2 1/2 is 4, 3 is 5, and 4 is 6. Where discharge is known only in gallons per minute, 1 gpm equals 2 dfu (709.3). An indirect waste receptor carries the sum of the fixtures draining to it, not less than its own table value. A receptor taking only clear-water waste from display cases, coolers, ice bins or freezers counts as half a dfu (709.4.1).
A horizontal fixture branch is sized by the total dfu it carries. A branch of the building drain itself is sized with Table 710.1(1) instead (footnote a to Table 710.1(2)). A stack is sized by its accumulated load and by the load entering at any one branch interval, a vertical distance of 8 feet or more between branch connections. Stacks of three intervals or less have a lower total limit than taller ones. As the load thins toward the top, a stack may step down in size, but never below half the diameter of the largest size required.
The building drain and sewer are sized by the total dfu connected to any portion, including branches of the building drain, and by slope: a steeper pipe carries more. The smallest building drain serving a water closet is 3 inches, and no drain shrinks in the direction of flow (704.2).
| Pipe | Building drain at 1/8 in per ft | Building drain at 1/4 in per ft | Horizontal branch | Stack, one interval | Stack, 3 intervals or less |
|---|---|---|---|---|---|
| 2 in | not listed | 21 | 6 | 6 | 10 |
| 3 in | 36 | 42 | 20 | 20 | 48 |
| 4 in | 180 | 216 | 160 | 90 | 240 |
| 6 in | 700 | 840 | 620 | 350 | 960 |
Worked example. The figure shows a one-story community building. Women's room: 3 public closets x 4 = 12, plus 2 lavatories = 14. Men's room: 2 closets x 4 = 8, 2 urinals (over 1 gpf) x 4 = 8, 2 lavatories = 2, total 18. Break room: a sink at 2 and two fountains at 1/2 each = 3. Janitor closet: service sink 2 and floor drain 2 = 4. Building total: 14 + 18 + 3 + 4 = 39 dfu. The toilet room branch is a fixture branch above the drain, so Table 710.1(2) applies: it carries 14 + 18 = 32 dfu, which is over the 20 dfu a 3 inch branch may carry, so it is 4 inch (limit 160). At 1/8 inch per foot a 3 inch drain holds only 36, so it fails at 39, while 4 inch holds 180. A 3 inch drain at 1/4 inch per foot would hold 42 on paper, but it leaves almost no margin. Use 4 inch at 1/8 inch per foot.
Stack example. A three-story office has an 18 dfu restroom group on each floor, all on one stack. Each branch interval receives 18, and the accumulated load at the bottom is 54. A 3 inch stack passes the interval test (20) but fails the three-interval total (48 is less than 54). A 4 inch stack (90 and 240) passes.
In the field
A parish hall with a 3 inch building drain at 1/8 inch per foot already serves 30 dfu, and the owner wants a service sink and a new 14 dfu restroom group. The new total is 44. Even at 1/4 inch per foot, a 3 inch drain tops out at 42, so the drain must be replaced. Ask for the count fixture by fixture before accepting that the old pipe is big enough, and remember Section 703.4 calls for internal examination of any old drain you reuse.
Section 711 covers stack offsets in buildings of five stories or more: a horizontal offset more than four branch intervals below the top of the stack must be vented and sized as the section directs (711.2), while below the lowest branch a vertical offset needs no change in diameter and a horizontal offset is sized as a building drain (711.3). Section 712 covers building subdrains that cannot drain by gravity. They go to a tightly covered, vented sump with a pit at least 18 inches across and 24 inches deep, a gastight cover not more than 2 inches below the floor, and a check valve and full-open valve on the discharge. The pump control keeps effluent at least 2 inches below the gravity inlet invert (712.3.4). A pump that receives water closet discharge must pass 2 inch spherical solids, and a 2 inch discharge needs at least 21 gpm (Table 712.4.2).
Section 714 covers backwater valves (not 715, which is vacuum drainage). Fixtures on a floor whose finished floor is below the manhole cover of the next upstream public sewer manhole must be protected by a backwater valve, and fixtures above that elevation must not drain through one, although an existing building may keep them on the valve. The valve needs access to its working parts (714.3).
1. What drainage fixture unit value does Table 709.1 give a public water closet flushing 1.6 gallons per flush in a Group B office restroom?
2. A school restroom (Group E) has 6 public water closets (1.6 gpf) and 4 lavatories. What is the total drainage fixture unit load from Table 709.1?
3. A break room in a small office has 4 drinking fountains and no other fixtures. What dfu total do they add under Table 709.1?
4. An equipment pump in a mechanical room discharges 15 gpm into a floor sink, and no fixture unit value is otherwise available. Under Section 709.3, its load is:
5. A fixture not listed in Table 709.1 has a 2 inch trap. Under Table 709.2, what dfu load is assigned?
6. An inspector totals 40 dfu on a 3 inch building drain sloped 1/8 inch per foot in a Group B building. Under Table 710.1(1), this drain:
7. A building drain carries 200 dfu at a slope of 1/8 inch per foot. What is the smallest pipe size Table 710.1(1) allows?
8. What is the smallest size allowed for a building drain that serves a water closet, according to the note to Table 710.1(1)?
9. A horizontal branch in a small clinic carries 24 dfu. Under Table 710.1(2), what is the smallest pipe size that can serve it?
10. A three-story building has restroom groups that put an accumulated 60 dfu on one drainage stack (three branch intervals). What is the smallest stack size Table 710.1(2) allows?
11. Design requires a 6 inch drainage stack at its base. Under the note to Table 710.1(2), the stack may step down toward the top, but not below:
12. A basement restroom in a community hall has a finished floor below the manhole cover elevation of the next upstream public sewer manhole. Under Section 714.1, the fixtures:
13. A sump pit is being installed for a building subdrain in a rehab. Section 712.3.2 sets the minimum pit size, unless otherwise approved, at:
14. A caretaker apartment in a small community building has a bathroom group (1.6 gpf water closet), a domestic kitchen sink, and a residential automatic clothes washer. What is the total dfu load under Table 709.1?