Short Answer: Every DWV question starts with drainage fixture units from Table 709.1. Then size branches and stacks with Table 710.1(2), building drains with 710.1(1), and check slope and trap arm limits.
Quick Facts
| Fixture units | IPC Table 709.1 |
|---|---|
| Branches and stacks | Table 710.1(2) |
| Building drains | Table 710.1(1) |
| Slope | 1/4 in/ft up to 2½ in; 1/8 in for 3-6 in |
| Our JP bank | 53 questions; Table 709.1 is the most-cited |
Why DWV Sizing Matters
A drain, waste and vent (DWV) system must carry waste away by gravity without letting sewer gas into the building. That requires pipes large enough to carry the flow without filling completely, sloped enough to keep solids moving, and vented so that air pressure in the pipes stays close to atmospheric. If the pressure drops, water can be siphoned out of the traps. If it rises, it can push sewer gas past them.
Exam questions on DWV sizing follow a predictable pattern. This guide uses the 2021 International Plumbing Code. If your state uses the Uniform Plumbing Code, the method is similar but the numbers differ, so always use your own code book.
Step 1: Count the Drainage Fixture Units
Table 709.1 assigns each fixture a drainage fixture unit (DFU) value and a minimum trap size. Some common values:
| Fixture | DFU | Minimum trap |
|---|---|---|
| Lavatory | 1 | 1¼ in |
| Bathtub (with or without shower) | 2 | 1½ in |
| Shower | 2 | 1½ in |
| Kitchen sink, domestic | 2 | 1½ in |
| Clothes washer standpipe | 3 | 2 in |
| Water closet, 1.6 gpf | 4 | (integral trap) |
Check the table for other fixtures and the footnotes. For example, the IPC also lists values for bathroom groups, and showers with higher flow rates get higher DFU values.
Example. A horizontal branch serves two lavatories, a bathtub and a kitchen sink. DFU = 1 + 1 + 2 + 2 = 6.
Step 2: Size the Drain
The IPC uses two tables:
- Table 710.1(1) sizes the building drain and building sewer by DFU and slope.
- Table 710.1(2) sizes horizontal fixture branches and stacks.
For horizontal fixture branches, Table 710.1(2) allows:
| Pipe size | Maximum DFU on a horizontal branch |
|---|---|
| 1½ in | 3 |
| 2 in | 6 |
| 2½ in | 12 |
| 3 in | 20 |
| 4 in | 160 |
Example. The 6 DFU branch above can be 2 in, because a 2 in horizontal branch carries up to 6 DFU. If you add a clothes washer standpipe (3 DFU), the total becomes 9 DFU, and the branch must be 2½ in or larger.
Two more rules apply to every drain:
- A drain must never be smaller than the trap outlet of the fixture it serves.
- Drains must not get smaller in the direction of flow.
The tables also have footnotes on water closets. Read them before you put a water closet on a small branch.
Step 3: Set the Slope
Table 704.1 sets minimum slopes for horizontal drainage pipe:
- 2½ in or smaller: 1/4 in per foot
- 3 to 6 in: 1/8 in per foot
- 8 in or larger: 1/16 in per foot
Example. A 3 in building drain runs 48 ft. At 1/8 in per foot, it falls 48 × 1/8 = 6 in.
Example. A 2 in branch runs 14 ft. At 1/4 in per foot, it falls 14 × 1/4 = 3.5 in.
Slope questions often hide in word problems: the depth of the sewer at the property line, the elevation of the building drain, or how far a branch can run before hitting a footing.
Step 4: Use the Right Fittings
Table 706.3 lists which fittings are allowed for each change of direction. The most tested rule: a sanitary tee is allowed only for a change from horizontal to vertical. It may not be used for a change from vertical to horizontal, or horizontal to horizontal, because the sharp turn can cause blockages. Use wyes, combination wye and 1/8 bends, or long-sweep bends for those changes.
Cleanouts (708) are required at the base of stacks, at changes of direction greater than 45° in horizontal drains, and at intervals no more than 100 ft apart on horizontal drains. Cleanouts on pipes 3 in and larger need 18 in of clearance in front, and smaller pipes need 12 in.
Step 5: Check the Trap Arm
The trap arm is the part of the fixture drain between the trap weir and the vent. If it is too long, the pipe can run full and siphon the trap, or the fall can put the vent opening below the weir. The IPC’s trap-to-vent table (Table 909.1 in the 2021 edition) limits the developed length:
| Trap size | Slope | Maximum distance to vent |
|---|---|---|
| 1¼ in | 1/4 in per ft | 5 ft |
| 1½ in | 1/4 in per ft | 6 ft |
| 2 in | 1/4 in per ft | 8 ft |
| 3 in | 1/8 in per ft | 12 ft |
| 4 in | 1/8 in per ft | 16 ft |
Example. A lavatory has a 1¼ in trap and a 1½ in trap arm. The limit follows the trap size in the table, so the vent must be within 5 ft. Check your edition’s table and its notes, because the measurement rules are specific.
The trap seal itself must be between 2 and 4 in deep (1002.4). S-traps, bell traps and crown-vented traps are prohibited (1002.3), and no fixture may be double trapped (1002.1).
Step 6: Size the Vents
Vents are sized from the drain they serve and the developed length of the vent. General rules in the IPC include:
- A vent must be at least half the diameter of the drain it serves, and never smaller than 1¼ in.
- Vent stacks and stack vents are sized from the DFU load and developed length using the vent sizing table in Chapter 9.
- Vent pipes must be graded so moisture drains back to the drainage system.
- Dry vents must connect above the flood level rim of the fixture they serve before running horizontally, except where the code allows otherwise.
Vent terminals must be protected and properly placed. Under 903.5, a vent terminal within 10 ft horizontally of a door, openable window or air intake must be at least 3 ft above the top of that opening.
Venting Methods You Should Recognise
The IPC allows several venting methods. You do not need to design each one on the exam, but you should recognise them:
- Individual vent: one vent for one fixture trap.
- Common vent: one vent shared by two fixtures connected at the same level or at different levels within limits.
- Wet vent: a pipe that serves as both a drain and a vent, allowed for certain fixture groups.
- Waste stack vent: a stack sized to vent fixtures that drain into it.
- Circuit vent: used for batteries of floor-outlet fixtures on a horizontal branch.
- Air admittance valve (918): a one-way valve that lets air in but not sewer gas out, allowed in some cases. Individual and branch AAVs must be at least 4 in above the horizontal branch drain or fixture drain they serve.
A Complete Example
A bathroom group has a lavatory (1 DFU), a bathtub (2 DFU) and a 1.6 gpf water closet (4 DFU), for 7 DFU in total. A 2 in branch carries only 6 DFU, so 2 in is too small. The water closet’s 3 in outlet also sets a minimum, because a drain cannot be smaller than the fixture outlet it serves, so the branch is 3 in. At 3 in, the slope is 1/8 in per foot. The lavatory has a 1¼ in trap, so its vent must be within 5 ft. The bathtub has a 1½ in trap, so its vent must be within 6 ft.
Practise the Method
Work several DFU totals and slope problems each day until they are automatic. Then take our Journeyman Plumber Practice Test (IPC 2021), which includes sizing, slope and venting questions with an IPC reference for every answer. For the rest of the code, see our Journeyman Plumber Exam Study Guide.
Common Mistakes
- Reading the wrong table. Building drains and horizontal branches are sized in different tables.
- Forgetting the trap outlet rule. The drain cannot be smaller than the trap.
- Applying 1/4 in slope to 3 in pipe. Under the IPC, 3 in pipe needs only 1/8 in per foot.
- Using a sanitary tee on its back. It is not allowed for vertical-to-horizontal changes.
- Measuring the trap arm from the wrong point. It runs from the trap weir to the vent connection.
Where to Go Next
Steps to Take
- Add up the DFU for every fixture.
- Pick the right sizing table.
- Find the smallest pipe that carries the load.
- Check minimum sizes for water closets.
- Check slope, trap arm length and vent connection.
Frequently Asked Questions
What is a drainage fixture unit?
A DFU is a measure of how much load a fixture puts on the drainage system, based on its flow rate, duration and frequency of use. It is not a flow rate in gallons per minute.
Can a drain pipe be smaller than the trap it serves?
No. The drain must not be smaller than the fixture trap outlet, and drains do not get smaller in the direction of flow.
Why are trap arm lengths limited?
If a trap arm is too long, the pipe can fill and siphon the trap seal, and the fall can drop the weir below the vent. The IPC table keeps the vent close enough to protect the seal.
Are air admittance valves allowed?
The IPC allows them in many cases under Section 918, but they must be accessible and installed above the branch they serve. Some local codes restrict them, so check your jurisdiction.