2021 IRC Chapter 30
Lesson 7 covered how drainage pipe is joined, sloped, and fitted with cleanouts. This lesson covers how it gets sized in the first place: the drainage fixture unit (DFU) system that converts each fixture's discharge into a comparable load value, and the rules that turn a total DFU load into a required pipe diameter for a branch, a stack, or a building drain. These are the numbers an estimator uses when a rehab adds a bathroom to a house and has to decide whether the existing branch, stack, or building drain can carry the added load or has to be upsized.
A drainage fixture unit is not a flow rate in gallons per minute; it is a relative load value that accounts for how much a fixture discharges, how quickly, and roughly how often, so that very different fixtures can be added together on one scale. A fixture that dumps a large volume in a few seconds, like a water closet, is weighted far more heavily than one that trickles out a small amount over a longer time, like a lavatory, even though a stopwatch might show the lavatory draining for longer overall. Table P3004.1 assigns these values fixture by fixture and for fixture groups, and P3005.4 uses the running total to size the pipe.
| Fixture | DFU value | Minimum trap size (water closet: drain size) |
|---|---|---|
| Lavatory | 1 | 1-1/4 in. |
| Bathtub / shower | 2 | 1-1/2 in. (shower: by flow rate, Table P3201.7) |
| Kitchen sink | 2 | 1-1/2 in. |
| Clothes washer standpipe | 2 | 2 in. |
| Water closet, 1.6 gpf, private | 3 | 3 in. (minimum drain) |
The table gives 4 DFU for a water closet that uses more than 1.6 gallons per flush, and a fixture not listed takes the highest value of a similar listed fixture. Table P3004.1 also lists fixture groups: a full-bath group with a bathtub and a 1.6 gpf water closet counts as 5 DFU, not the 6 you get by adding the three fixtures one by one, so use the group value when the fixtures are a bath group. Trap sizes come from Table P3201.7.
P3005.4 sizes drain pipes by drainage fixture unit load. Branches and stacks use Table P3005.4.1, which limits the load on any horizontal fixture branch to 3 DFU in 1-1/2 inch pipe, 6 in 2 inch, 12 in 2-1/2 inch, 20 in 3 inch, and 160 in 4 inch; for any one vertical stack the limits are 4, 10, 20, 48, and 240 DFU in the same sizes. The building drain and building sewer use Table P3005.4.2, where the allowed load depends on slope as well as size: for 3-inch pipe, 36 DFU at 1/8 inch per foot, 42 at 1/4 inch, and 50 at 1/2 inch; for 4-inch pipe, 180, 216, and 250 DFU at those slopes. So the same pipe carries a higher load at a steeper slope and a lower load at the code minimum slope. Every fixture branch, every stack, and the building drain itself is checked against these tables for the load actually passing through it, and a local amendment can change them, so confirm the adopted code before quoting a number to a contractor. Two governing rules already introduced in Lesson 7 hold regardless: pipe size is never reduced moving downstream, and a stack is never sized smaller than the largest branch feeding into it (P3005.4.1), no matter how the fixture units happen to work out on paper.
Two minimums apply regardless of the calculated DFU load. A water closet must discharge to a drain of at least 3 inches: Table P3005.4.1 prohibits water closets on 1-1/2, 2, and 2-1/2 inch pipe, so a 3-inch water closet drain is required even on a branch whose total fixture-unit count would otherwise seem to allow something smaller. (A 4-inch by 3-inch closet bend or flange is a listed exception.) And a drain stack must never be sized smaller than the largest horizontal branch that connects to it, which is really the same downstream-reduction rule from Lesson 7 applied specifically to where a branch meets a stack.
In the field
A common rehab request is adding a half bathroom off an existing kitchen, tying the new water closet and lavatory into a branch that already carries the kitchen sink. GCHS first checks whether that existing branch and the stack or building drain it feeds were ever sized to carry a water closet at all; a branch sized only for a kitchen sink and a lavatory, at 3 DFU, is very often not sized, and not even large enough in diameter, to also carry a 3 DFU water closet once it is added, since a water closet has both a fixed 3-inch minimum drain size and a higher DFU value than the fixtures the branch was originally designed around. This is exactly the kind of finding that turns what looked like a simple half-bath addition into a repiping job, and it needs to be priced as one from the estimate stage, not discovered at rough-in.
A rehab project adds a full bathroom containing one 1.6 gpf water closet, one lavatory, and one bathtub, tying into a branch that already carries an existing kitchen sink. Table P3004.1 gives the full-bath group 5 DFU (the individual fixtures would be 3 plus 1 plus 2, or 6 DFU, but the group value applies to a bath group) and the existing kitchen sink 2 DFU, for a combined branch total of 5 plus 2, or 7 DFU. Because the branch now includes a water closet, it must also meet the 3-inch minimum drain size for a water closet regardless of what a 7 DFU load might otherwise allow on a smaller-diameter branch: a 2-1/2 inch branch, for example, is limited to 12 DFU on paper but may not carry a water closet at all. The estimator then checks the combined 7 DFU total against Table P3005.4.1 for the branch and stack and Table P3005.4.2 for the building drain at the slope actually installed, to confirm the branch, the stack it feeds, and the building drain downstream of that connection are all sized for at least that load, upsizing any segment that falls short rather than assuming the existing pipe already has room for the addition.
1. A drainage fixture unit (DFU) is best described as:
2. A 1.6 gallon-per-flush water closet in private residential use is assigned a drainage fixture unit value of:
3. A lavatory is assigned a drainage fixture unit value of:
4. A bathtub, a shower stall, a kitchen sink, and a residential clothes washer standpipe are each commonly assigned a drainage fixture unit value of:
5. A bathroom contains one 1.6 gpf water closet, one lavatory, and one bathtub. Adding the three fixtures' individual Table P3004.1 values, not using the bath-group value, gives a load of:
6. The minimum drain size serving a water closet (Table P3005.4.1, note b) is:
7. A basement bathroom's water closet ties into an existing 2-inch drain line. This is:
8. A soil or waste stack, relative to the horizontal branches connecting to it, must be sized:
9. Moving downstream toward the building sewer, drainage pipe size may:
10. An inspector reviewing a bathroom addition finds the new fixtures tied into an existing branch that previously served only a kitchen sink and a lavatory. The first thing to check is:
11. A rehab estimate calls for tying a new water closet into a branch that previously carried only a lavatory and a kitchen sink. Which outcome is most likely once the branch is checked against the sizing table?
12. Which statement about drainage fixture unit values is accurate?
13. An estimator needs the maximum drainage fixture units a specific pipe diameter and slope may carry on a given project. The correct source for that specific capacity number is:
14. A new full bathroom (1.6 gpf water closet, lavatory, bathtub, counted as a full-bath group of 5 DFU) ties into a branch already carrying a kitchen sink (2 DFU). What is the combined branch total, and what minimum drain size applies for the water closet?