12.1 Individual Vents, Common Vents, and Vents for Stack Offsets
Key Takeaways
- IPC 910.1 permits each trap and trapped fixture to have an individual vent connecting to the fixture drain of the trap or trapped fixture being vented.
- IPC 911.1 permits an individual vent to vent two traps or trapped fixtures as a common vent, provided both are located on the same floor level.
- IPC 911.3 requires the vertical drain between fixture drains connecting at different levels to be sized per Table 911.3, and prohibits the upper fixture from being a water closet.
- IPC Table 911.3 permits 1 dfu from the upper fixture drain on a 1-1/2 inch common vent, 4 dfu on a 2-inch, and 6 dfu on a 2-1/2 to 3-inch.
- IPC 907.1 requires horizontal offsets of drainage stacks to be vented where five or more branch intervals are located above the offset.
Individual vent — IPC 910
Each trap and trapped fixture is permitted to be provided with an individual vent. The individual vent shall connect to the fixture drain of the trap or trapped fixture being vented.
The individual vent is the baseline method. Every other method in Chapter 9 is a controlled relaxation of it, permitted because the code has verified that a particular arrangement still holds trap seals within 1 inch of water column.
Sizing: IPC 906.2 — not less than one-half the required diameter of the drain served, never less than 1-1/4 inches, with the drain size determined from Table 710.1(2).
Common vent — IPC 911
An individual vent is permitted to vent two traps or trapped fixtures as a common vent. The traps or trapped fixtures being common vented shall be located on the same floor level.
Two fixtures, one vent, same floor. Three fixtures on one vent is not a common vent — it is a wet vent, a circuit vent, or a violation.
911.2 Connection at the same level
Where the fixture drains being common vented connect at the same level, the vent connection shall be at the interconnection of the fixture drains or downstream of the interconnection.
This is the classic back-to-back lavatory arrangement on a double sanitary tee or cross, with the vent rising from the top of the fitting.
911.3 Connection at different levels
Where the fixture drains connect at different levels, the vent shall connect as a vertical extension of the vertical drain. The vertical drain pipe connecting the two fixture drains shall be considered to be the vent for the lower fixture drain, and shall be sized in accordance with Table 911.3. The upper fixture shall not be a water closet.
Three requirements, all tested:
- The vent rises as a vertical extension of the vertical drain — the piping is continuous.
- The vertical drain between the two connections is the vent for the lower fixture. It is carrying waste from the upper fixture while venting the lower one, which is why its size is limited.
- The upper fixture shall not be a water closet. A water closet's 3-dfu surge would fill the vertical drain and seal the lower fixture's vent path.
Table 911.3 Common vent sizes:
| Pipe size | Maximum discharge from the upper fixture drain |
|---|---|
| 1-1/2 inches | 1 dfu |
| 2 inches | 4 dfu |
| 2-1/2 to 3 inches | 6 dfu |
The stacked-lavatory arrangement in a back-to-back bathroom on two floors of a hotel is the everyday application: the upper lavatory is 1 dfu, so a 1-1/2-inch vertical drain suffices.
Worked case
An upper-floor kitchen sink (2 dfu) and a lower-floor lavatory (1 dfu) are to be common vented at different levels.
- Upper fixture is not a water closet. Permitted.
- Upper fixture discharge is 2 dfu. Table 911.3: a 1-1/2-inch pipe allows only 1 dfu, so a 2-inch vertical drain is required (it allows 4 dfu).
- The vertical drain between the two fixture drain connections is the vent for the lavatory below.
- The vent above continues as a vertical extension of that 2-inch vertical drain.
Change the upper fixture to a water closet and the arrangement becomes prohibited outright.
Vents for stack offsets — IPC 907
Section 907 is the venting companion to the sizing rules of IPC 711.
907.1 Vent for horizontal offset of drainage stack
Horizontal offsets of drainage stacks shall be vented where five or more branch intervals are located above the offset. The offset shall be vented by venting the upper section of the drainage stack and the lower section of the drainage stack.
Note how this trigger differs from the sizing trigger in IPC 711.2, which is "more than four branch intervals below the top of the stack." Section 907 counts intervals above the offset; Section 711 counts intervals below the top. In a simple stack the two land in the same place, but read each as written.
907.2 Upper section
The upper section of the drainage stack shall be vented as a separate stack with a vent stack connection installed in accordance with Section 904.4. The offset shall be considered to be the base of the stack.
That last sentence is the key. For venting purposes, the offset is the base of the upper stack, so the vent stack connects there at or below the lowest horizontal branch of the upper section.
907.3 Lower section
The lower section of the drainage stack shall be vented by a yoke vent connecting between the offset and the next lower horizontal branch. The yoke vent connection is permitted to be a vertical extension of the drainage stack. The size of the yoke vent and connection shall be not less than the size required for the vent stack of the drainage stack.
A yoke vent connects upward from a soil or waste stack to a vent stack for the purpose of preventing pressure changes in the stacks. Here it relieves the compression zone just below the offset, where flow has decelerated horizontally and then resumed falling.
Single-stack vent systems — IPC 917
An alternative whole-system method in which the drainage stack and branch piping serve as the vents. 917.1: the drainage stack shall have a stack vent.
917.2 Stack size: stacks are uniformly sized on the total connected dfu load per Table 917.2, and the stack vent is the same size as the drainage stack. A 3-inch stack shall serve not more than two water closets.
Table 917.2 Single stack size (maximum connected dfu):
| Stack size | Less than 75 ft tall | 75 to less than 160 ft | 160 ft and greater |
|---|---|---|---|
| 3 in | 24 | Not permitted | Not permitted |
| 4 in | 225 | 24 | Not permitted |
| 5 in | 480 | 225 | 24 |
| 6 in | 1,015 | 480 | 225 |
| 8 in | 2,320 | 1,015 | 480 |
| 10 in | 4,500 | 2,320 | 1,015 |
| 12 in | 8,100 | 4,500 | 2,320 |
The pattern is unmistakable: each height band shifts the capacity one full pipe size. A 4-inch stack carries 225 dfu under 75 feet and only 24 dfu between 75 and 160 feet.
Other single-stack rules worth knowing:
- 917.3: horizontal branches sized per Table 710.1(2); not more than one water closet may discharge into a 3-inch horizontal branch within 18 inches measured horizontally from the stack; where a water closet is within 18 inches and not more than one fixture with a drain not over 1-1/2 inches connects to a 3-inch branch, the branch connection to the stack shall be made with a sanitary tee.
- 917.4.1: water closet connections not greater than 4 feet in developed length measured horizontally from the stack — 8 feet where the connection is made with a sanitary tee.
- 917.4.2: fixtures other than water closets not greater than 12 feet from the stack.
- 917.5: the vertical portion of piping in a fixture drain to a horizontal branch shall be 2 inches; 3 inches for a water-supplied urinal or standpipe; maximum vertical drop 4 feet. Fixture drains not increased in size, or with a drop over 4 feet, must be individually vented.
- 917.7: fixture connections shall not be made to a stack within 2 feet above or below a horizontal offset.
- 917.8: stacks greater than 2 branch intervals in height shall not receive the discharge of horizontal branches on the lower two floors, and there shall be no connections between the lower two floors and a distance of not less than 10 pipe diameters downstream from the base.
- 917.9: the building drain and sewer are sized per Table 710.1(1).
Choosing among the individual and common methods
| Situation | Method |
|---|---|
| One trap, one vent | Individual vent (910) |
| Two fixtures, same floor, drains connecting at the same level | Common vent, vent at or downstream of the interconnection (911.2) |
| Two fixtures, same floor, drains connecting at different levels | Common vent as a vertical extension; vertical drain sized per Table 911.3; upper fixture not a water closet (911.3) |
| Two fixtures on different floors | Not a common vent — each needs its own method |
| Three or more fixtures on one branch | Wet vent, circuit vent, waste stack vent or combination waste and vent (Chapter 13 of this guide) |
| Horizontal stack offset with 5 or more branch intervals above | Vent upper section as a separate stack per 904.4, lower section by yoke vent (907) |
Two fixtures are common vented with their fixture drains connecting at different levels. What does IPC 911.3 prohibit?
Under Table 911.3, what is the maximum discharge from the upper fixture drain on a 2-inch common vent?
A horizontal offset in a drainage stack has six branch intervals above it. How is the lower section vented under IPC 907.3?
In a single-stack vent system, how far horizontally from the stack may a water closet connection be made where the connection is NOT a sanitary tee?