6.1 Vent Purpose, Vent Stacks, Connections, Terminals & Vent Sizing
Key Takeaways
The vent system must keep any trap seal from a pressure differential of more than 1 inch of water column (IPC 901.2), and every building drain needs at least one dry vent extending outdoors (IPC 904.1).
A vent stack is required for every drainage stack with five or more branch intervals, and it connects at or below the lowest horizontal branch or to the building drain within 10 stack diameters downstream (IPC 904.2, 904.4).
Every dry vent rises vertically to at least 6 inches above the flood level rim of the highest trap it serves before offsetting horizontally (IPC 905.4), and it connects to a horizontal drain above the drain's centerline (IPC 905.3).
Stack vents and vent stacks are sized from Table 906.1 by dfu and developed length, never smaller than 1-1/4 inches or half the drain served. Other vents are at least half the required drain size, and vents over 40 feet long are increased one size (IPC 906.2).
A vent terminal may not be directly beneath a door, openable window or air intake, nor within 10 feet horizontally of one unless it is at least 3 feet above the top of the opening (IPC 903.5).
Why this section matters
Venting questions fill much of Drain Waste and Vents and Sizing and Design. Chapter 9 of the 2021 IPC is long. Tab Sections 903 (terminals), 904 (vent stacks), 905 (connections) and 906 (sizing) first.
What vents do (901)
- 901.2: The vent system must let air in and out so that no trap seal is subjected to a pressure differential of more than 1 inch of water column. With seals of 2 to 4 inches (1002.4), that leaves a margin.
- Vents relieve the vacuum behind falling water, which would siphon traps, and the pressure ahead of it, which would blow seals out.
- 901.4: Vent piping may be used only for venting.
- 901.3: Chemical waste vents are independent and terminate outdoors or at an ASSE 1049 air admittance valve.
Required outdoor vent and vent stacks (904)
- 904.1: The system serving each building drain must have at least one vent pipe extending outdoors. It is a dry vent connected to the building drain or an extension of a drain connected to it. It may not be an island fixture vent, and it is sized per 906.2 from the required size of the building drain.
- 904.2: A vent stack is required for every drainage stack with five or more branch intervals. The exception is a waste stack vent system under 913.
- 904.3: Vent stacks and stack vents terminate outdoors or at a stack-type air admittance valve (918).
- 904.4: A vent stack connects at the base of the drainage stack, at or below the lowest horizontal branch. If it connects to the building drain, the connection must be downstream of the stack and within 10 stack diameters.
- 904.5: Stack vents and vent stacks joined in a common vent header are sized by 906.1 for the sum of their dfu, using the longest developed length.
Stack vent vs. vent stack. A stack vent is the extension of a soil or waste stack above its highest horizontal drain. A vent stack is a separate vertical vent pipe whose main job is to circulate air to and from the drainage system.
Relief vents for tall stacks (908)
- 908.1: Soil and waste stacks in buildings with more than 10 branch intervals need a relief vent at each tenth interval, counting from the top floor.
- 908.2: The relief vent is the same size as the vent stack. Its lower end connects to the soil or waste stack through a wye below the horizontal branch serving that floor. Its upper end connects to the vent stack through a wye at least 3 feet above the floor.
Connections and grades (905)
| Section | Rule |
|---|---|
| 905.1 | Individual, branch and circuit vents connect to a vent stack, stack vent or air admittance valve, or extend outdoors |
| 905.2 | Vents must be graded so they drain back to the drain by gravity. The code does not set a numeric slope |
| 905.3 | A dry vent connecting to a horizontal drain must connect above the drain's centerline |
| 905.4 | Every dry vent rises vertically at least 6 inches above the flood level rim of the highest trap it serves before going horizontal (outdoor interceptor vents excepted) |
| 905.5 | Connections to a vent stack or stack vent, and horizontal branch, relief or loop vents, must be at least 6 inches above the flood level rim of the highest fixture served |
| 905.6 | Roughed-in future fixtures get a vent rough-in at least half the drain size, labeled as a vent |
If a drain stops up, the wastewater rises and shows at the fixture before it can enter a vent that turns horizontal 6 inches above the rim. That keeps solids out of horizontal vents.
Vent terminals (902–903)
| Section | Terminal rule |
|---|---|
| 903.1.1 | Open vent pipes through a roof terminate at least [NUMBER] inches above the roof. The model code leaves the number to the adopting jurisdiction. Alabama's Board adopts the 2021 IPC without filling in a number, so use the local building department's value (many jurisdictions use 6 to 12 inches) |
| 903.1.2 | On roofs used as a promenade, restaurant, bar, sunbathing or observation deck: at least 7 feet above the roof |
| 903.1.3 | Under a roof-mounted panel (such as solar PV) or a decorative shroud on a sloped roof: at least 2 inches above the roof, with open area at least equal to the pipe's area and protection from birds and rodents |
| 903.1.4 | Sidewall terminals: at least 10 feet from the lot line and 10 feet above the highest grade within 10 feet horizontally. Not under an overhang with soffit vents. Protected from birds and rodents |
| 903.2 | Frost closure: where the 97.5% outdoor design temperature is 0°F or lower, vent extensions must be at least 3 inches, with the increase made at least 1 foot inside the thermal envelope |
| 903.5 | Not directly beneath any door, openable window or air intake. Not within 10 feet horizontally of one unless at least 3 feet above the top of the opening |
| 903.6 | Exterior vent pipes are protected from freezing where the design temperature is below 0°F |
| 903.3, 902.2–902.3 | Flashing is watertight. Sheet copper at least 8 oz per square foot; sheet lead at least 3 lb per square foot field-formed or 2-1/2 lb prefabricated |
Alabama's 97.5% winter design temperatures are well above 0°F. IPC Appendix D lists Huntsville at 16°F, Birmingham at 21°F, Montgomery at 25°F and Mobile at 29°F. So the 903.2 frost-closure increase does not normally apply here. It is still fair game on the exam.
Sizing stack vents and vent stacks (906.1, Table 906.1)
Find the row for the drainage stack diameter and a dfu value at least equal to the load. Read across to the first vent diameter whose maximum developed length is at least the actual length. The result may never be smaller than 1-1/4 inches or half the drain served.
| Stack (in) | dfu | 1-1/4 in | 1-1/2 in | 2 in | 2-1/2 in | 3 in | 4 in | 5 in |
|---|---|---|---|---|---|---|---|---|
| 1-1/2 | 8 | 50 | 150 | — | — | — | — | — |
| 2 | 12 | 30 | 75 | 200 | — | — | — | — |
| 2 | 20 | 26 | 50 | 150 | — | — | — | — |
| 3 | 21 | — | 32 | 110 | 270 | 810 | — | — |
| 3 | 53 | — | 27 | 94 | 230 | 680 | — | — |
| 3 | 102 | — | 25 | 86 | 210 | 620 | — | — |
| 4 | 43 | — | — | 35 | 85 | 250 | 980 | — |
| 4 | 140 | — | — | 27 | 65 | 200 | 750 | — |
| 4 | 320 | — | — | 23 | 55 | 170 | 640 | — |
| 4 | 540 | — | — | 21 | 50 | 150 | 580 | — |
| 5 | 490 | — | — | — | 21 | 63 | 250 | 760 |
All lengths are in feet. Excerpt from Table 906.1; use the full table in your code book for other rows.
Example. A 4-inch soil stack carries 240 dfu and needs a vent stack with 160 feet of developed length. Use the 4-inch, 320-dfu row, because 140 dfu is too small. A 2-1/2-inch vent allows only 55 feet, but a 3-inch vent allows 170 feet. The minimum vent stack is 3 inches, which is also more than half the 4-inch drain.
Sizing other vents (906.2–906.4)
- Individual, branch, circuit and relief vents are at least half the required diameter of the drain served, using the drain size required by Table 710.1(2), and never smaller than 1-1/4 inches.
- A vent longer than 40 feet in developed length must be increased one nominal size for its entire length.
- Developed length runs from the farthest drainage connection to the vent stack, stack vent or outdoor terminal (906.3).
- A common branch vent is sized from the drain size that would serve its total dfu (906.4).
- Sump vents follow Table 906.5.1 (section 4.4).
Exam traps
- 1 inch of water column is the code's pressure limit at any trap.
- Five branch intervals call for a vent stack. More than 10 call for relief vents every tenth interval.
- 6 inches above the flood rim before a vent turns horizontal.
- Roof terminal height is a local number in the 2021 IPC. The 7-foot rule applies only to roofs used as decks or gathering places.
Under IPC 904.2, when is a separate vent stack required for a drainage stack?
When the drainage stack has five or more branch intervals
When the stack serves more than two water closets
Only when the stack is 4 inches or larger
When the drainage stack has three or more branch intervals
A vent terminal must be 6 feet horizontally from an openable window. Under IPC 903.5, where may it terminate?
At least 3 feet above the top of the window
At the same height as the window head
At least 1 foot above the top of the window
Anywhere, if it is fitted with a screen
A 4-inch soil stack carries 240 dfu, and its vent stack has a developed length of 160 feet. Using IPC Table 906.1, what is the minimum vent stack size?
4 inches
2-1/2 inches
2 inches
3 inches
Sections you finish are checked off in the contents.