11.3 Part 2 Worked Example: Sizing Individual, Branch, and Main Vents and Checking Aggregate Venting
Key Takeaways
UPC Table 703.2 limits a 1-1/4-inch vent to 1 DFU and 45 feet, a 1-1/2-inch vent to 8 DFU and 60 feet, and a 2-inch vent to 24 DFU and 120 feet.
A fixture with more than 1 DFU, such as a 2-DFU urinal or floor drain, needs at least a 1-1/2-inch vent even though the half-drain rule alone would allow 1-1/4 inches.
A water closet on a 3-inch drain needs at least a 1-1/2-inch individual vent under the half-drain rule of UPC 904.1.
Aggregate venting requires the roof vents' total area to be at least that of the largest required building sewer; a single 3-inch vent (7.07 square inches) fails when a 4-inch sewer (12.57 square inches) is required.
A 3-inch vent carries up to 84 DFU and may be 212 feet long; no more than one-third of a minimum-sized vent may run horizontally (UPC 904.2).
The Three Tests for Every Vent
- Load and length (Table 703.2 vent columns):
| Vent size | 1-1/4 in | 1-1/2 in | 2 in | 3 in | 4 in |
|---|---|---|---|---|---|
| Maximum DFU | 1 | 8 | 24 | 84 | 256 |
| Maximum developed length | 45 ft | 60 ft | 120 ft | 212 ft | 300 ft |
- Half-drain rule (904.1): An individual vent is at least 1-1/4 inches and at least one-half the diameter of the drain it serves.
- Horizontal portion (904.2): No more than one-third of a minimum-sized vent's permitted length may be horizontal. Upsizing a vent one pipe size for its entire length removes the length limits.
Take the largest size that any of the tests requires.
Individual Vents in the Example (Public Fixtures)
| Fixture | DFU | Drain served | Half-drain minimum | Table 703.2 load minimum | Individual vent |
|---|---|---|---|---|---|
| Lavatory | 1.0 | 1-1/4 in trap arm | 1-1/4 in | 1-1/4 in (1 DFU) | 1-1/4 in |
| Urinal (integral trap) | 2.0 | 2 in | 1-1/4 in (half of 2 is 1) | 1-1/2 in (1-1/4 carries only 1) | 1-1/2 in |
| Floor drain | 2.0 | 2 in | 1-1/4 in | 1-1/2 in | 1-1/2 in |
| Water closet | 4.0 | 3 in | 1-1/2 in (half of 3) | 1-1/2 in (8 DFU) | 1-1/2 in |
| Mop sink | 3.0 | 2 in | 1-1/4 in | 1-1/2 in | 1-1/2 in |
| Drinking fountain | 0.5 | 1-1/4 in | 1-1/4 in | 1-1/4 in | 1-1/4 in |
Check the lengths: each 1-1/4-inch vent is 45 feet or less, and each 1-1/2-inch vent is 60 feet or less. Each vent rises 6 inches above the fixture's flood-level rim before turning horizontal (905.3) and connects above the drain's centerline (905.2).
Branch Vents and the Main Vent
| Vent | DFU served | Minimum by Table 703.2 | Notes |
|---|---|---|---|
| Second-floor branch vent (all 6 fixtures) | 14.0 | 2 in (24 DFU) | 1-1/2 in allows only 8 |
| First-floor branch vent (8 fixtures) | 17.5 | 2 in (24 DFU) | Developed length 120 feet or less |
| Vent stack or main vent to the roof (whole building) | 31.5 | 3 in (84 DFU) | 2 in allows only 24 |
Use the total developed length of the vent path from the farthest fixture connection to the open air when checking the length limit. A 2-inch branch vent must also keep its horizontal portion to 40 feet or less (one-third of 120) unless it is upsized one size for its full length.
The Aggregate Venting Check (904.1)
L&I tells candidates to maintain aggregate venting. Section 904.1 requires the vents serving the building drainage to have an aggregate cross-sectional area not less than that of the largest required building sewer, as determined from Table 703.2.
In the example, the required building drain is 4 inches (Section 11.2), so the required area is 12.57 square inches.
| Vents through the roof | Total area | Result |
|---|---|---|
| One 3-inch main vent | 7.07 sq in | Fails |
| One 3-inch plus one 2-inch | 7.07 + 3.14 = 10.21 sq in | Fails |
| One 3-inch plus two 2-inch | 7.07 + 3.14 + 3.14 = 13.35 sq in | Passes |
| One 4-inch main vent | 12.57 sq in | Passes |
On the exam, if the drawing shows one vent through the roof and asks for its minimum size, the Table 703.2 answer (3 inches) is not the final answer. Aggregate venting forces it to 4 inches when it is the only terminal serving a 4-inch building sewer. Vents from fixtures upstream of ejectors or backwater valves do not count toward the aggregate.
Cross-sectional areas to memorize: 1-1/4 in = 1.23; 1-1/2 in = 1.77; 2 in = 3.14; 2-1/2 in = 4.91; 3 in = 7.07; 4 in = 12.57 square inches.
Wet-Vent and Special-Vent Shortcuts
- A horizontal wet vent for a bathroom group is 2 inches for 4 DFU or less and 3 inches for 5 or more (908.2.2).
- A circuit vent is at least 2 inches. Add a 2-inch relief vent if four or more water closets are on the circuit and the stack receives upper branches (911.2, 911.4). Washington bars restaurant kitchen equipment from circuit-vented branches.
- Frost closure: in qualifying cold locations, increase each terminal to at least 2 inches, 1 foot below the roof inside insulation (906.7). A 1-1/2-inch vent becomes 2 inches at the terminal.
Vent Answers for the Example
| Question | Answer |
|---|---|
| "Minimum vent size for a water closet?" | 1-1/2 inches |
| "Minimum vent size for the urinal?" | 1-1/2 inches |
| "Minimum second-floor branch vent?" | 2 inches |
| "Minimum main vent by DFU?" | 3 inches |
| "Minimum main vent if it is the only roof terminal?" | 4 inches (aggregate venting) |
A floor drain on a 2-inch trap arm (2 DFU) is individually vented. What is the minimum vent size?
1-1/2 inches
2 inches
1 inch
1-1/4 inches
A building's required building sewer is 4 inches. Its only vent through the roof is sized by Table 703.2 at 3 inches for 31.5 DFU. What size must that vent be to meet UPC 904.1?
3 inches, because Table 703.2 controls
4 inches, because the aggregate vent area must be at least the area of the 4-inch building sewer
2 inches, because of the half-drain rule
5 inches, one size larger than the sewer
A branch vent collects 14 DFU of fixtures, and its developed length is 70 feet. What is its minimum size under Table 703.2?
2 inches
3 inches
1-1/4 inches
1-1/2 inches
Sections you finish are checked off in the contents.