12.3 Sizing Stack Vents, Vent Stacks, Branch Vents, and Headers
Key Takeaways
- IPC 906.1 requires stack vents and vent stacks to be sized from Table 906.1 on developed length and total dfu, but never less than one-half the diameter of the drain served or less than 1-1/4 inches.
- IPC 906.2 sizes individual, branch, circuit and relief vents at not less than one-half the required diameter of the drain served, determined from Table 710.1(2), with a 1-1/4 inch floor.
- IPC 906.2 requires vents exceeding 40 feet in developed length to be increased one nominal pipe size for the entire developed length.
- IPC 906.3 measures developed length from the farthest point of vent connection to the drainage system to the point of connection to the vent stack, stack vent or outside termination.
- IPC 904.5 sizes vent headers on the sum of all fixture units on all stacks connected and the longest vent length as a direct extension of one stack.
Two sizing sections, two scopes
| Section | Applies to | Method |
|---|---|---|
| 906.1 | Stack vents and vent stacks | Table 906.1, three-way lookup |
| 906.2 | Individual, branch, circuit and relief vents | One-half the required diameter of the drain served |
Both carry the same two floors: never less than one-half the diameter of the drain served, and never less than 1-1/4 inches.
Table 906.1 — stack vents and vent stacks
IPC 906.1:
The minimum required diameter of stack vents and vent stacks shall be determined from the developed length and the total of drainage fixture units connected thereto in accordance with Table 906.1, but in no case shall the diameter be less than one-half the diameter of the drain served or less than 1-1/4 inches.
Table 906.1 is read across three axes:
- Diameter of the soil or waste stack being vented (left column).
- Total fixture units being vented (second column).
- Maximum developed length of vent in feet, tabulated under each candidate vent diameter from 1-1/4 inches to 12 inches.
The procedure: find the row matching your stack diameter and dfu load, read across to find the smallest vent diameter whose tabulated maximum developed length equals or exceeds your actual developed length. Then check both floors.
Because the exam is open book, the table is in front of you. What you must bring is the method and the two floor rules — and the discipline to verify that the answer you read out of the table is not smaller than half the drain diameter.
The half-diameter floor in practice
| Drain served | Half diameter | Minimum vent |
|---|---|---|
| 1-1/2 in | 0.75 in | 1-1/4 in (absolute floor governs) |
| 2 in | 1 in | 1-1/4 in (absolute floor governs) |
| 3 in | 1-1/2 in | 1-1/2 in |
| 4 in | 2 in | 2 in |
| 6 in | 3 in | 3 in |
| 8 in | 4 in | 4 in |
Below a 3-inch drain, the 1-1/4-inch absolute minimum always governs. At 3 inches and above, the half-diameter rule governs. That single observation resolves most sizing items quickly.
Sizing other vents — IPC 906.2
The diameter of individual vents, branch vents, circuit vents and relief vents shall be not less than one-half the required diameter of the drain served. The required size of the drain shall be determined in accordance with Table 710.1(2). Vent pipes shall be not less than 1-1/4 inches in diameter. Vents exceeding 40 feet in developed length shall be increased by one nominal pipe size for the entire developed length of the vent pipe. Relief vents for soil and waste stacks in buildings having more than 10 branch intervals shall be sized in accordance with Section 908.2.
Note the phrase "required diameter of the drain served." You size the vent against the drain the code requires, not the drain someone happened to install. If a 2-inch branch was installed where Table 710.1(2) required only 1-1/2 inches, the vent is sized against 1-1/2 inches.
The 40-foot rule
A vent whose developed length exceeds 40 feet is increased one nominal pipe size for the entire developed length. Not for the excess — for the whole run. This trips up candidates who try to size the first 40 feet at one diameter and the remainder at another.
IPC 906.3 defines the measurement: from the farthest point of vent connection to the drainage system to the point of connection to the vent stack, stack vent or termination outside of the building.
Multiple branch vents — IPC 906.4
Where multiple branch vents are connected to a common branch vent, the common branch vent shall be sized based on the size of the common horizontal drainage branch that is or would be required to serve the total drainage fixture unit load being vented.
The common branch vent is sized against a hypothetical drain: the horizontal drainage branch that would be required to carry the whole vented load, whether or not such a branch exists.
Worked example
Four individual vents serve fixtures totaling 14 dfu, joining a common branch vent that runs 55 feet to the stack vent.
- What drain would serve 14 dfu? Table 710.1(2), horizontal branch column: 2-1/2 inch carries 12 (too small), 3 inch carries 20 — so the hypothetical common branch is 3 inches.
- Half of 3 inches = 1-1/2 inches. Above the 1-1/4-inch floor, so 1-1/2 inches.
- Developed length 55 feet exceeds 40 feet — increase one nominal size for the entire length: 2 inches.
Vent headers — IPC 904.5
Stack vents and vent stacks connected into a common vent header at the top of the stacks and extending to the open air at one point shall be sized in accordance with Section 906.1. The number of fixture units shall be the sum of all fixture units on all stacks connected thereto, and the developed length shall be the longest vent length from the intersection at the base of the most distant stack to the vent terminal in the open air, as a direct extension of one stack.
Three instructions:
- Size by 906.1 (the stack vent and vent stack table).
- Sum all fixture units on all connected stacks.
- Developed length is the longest path — measured from the intersection at the base of the most distant stack to the outdoor terminal — treated as if it were a direct extension of one stack.
The header is a common design in commercial buildings where combining four roof penetrations into one is worth the extra pipe.
Relief vents in tall buildings — IPC 908.2
The size of the relief vent shall be equal to the size of the vent stack to which it connects. The lower end of each relief vent shall connect to the soil or waste stack through a wye below the horizontal branch serving the floor, and the upper end shall connect to the vent stack through a wye not less than 3 feet above the floor.
Both connections are wyes, and the upper connection is at least 3 feet above the floor so that the vent stack is entered above any possible surcharge.
Sump vents and pneumatic ejectors — IPC 906.5
906.5.1: drainage piping below sewer level is vented in the same manner as a gravity system. Sump vents for sewage pumps and ejectors other than pneumatic are sized from Table 906.5.1 on the pump discharge capacity in gpm against the vent diameter, yielding a maximum developed length. Footnote a directs you to add an allowance for entrance losses and friction — 50 percent of the developed length if a more precise value is not available.
906.5.2: a pneumatic sewage ejector's air pressure relief pipe connects to an independent vent stack terminating as required for roof extensions, sized to relieve air pressure inside the ejector to atmospheric pressure, but not less than 1-1/4 inches.
A complete sizing pass
A four-story building has a 4-inch soil stack with six branch intervals, carrying 160 dfu. A vent stack is required (904.2, five or more branch intervals). The vent stack runs 62 feet from the base connection to the roof terminal.
| Step | Result | Basis |
|---|---|---|
| Vent stack required? | Yes, six branch intervals | 904.2 |
| Connection point | Base of the drainage stack, at or below the lowest horizontal branch; if to the building drain, within 40 inches (10 x 4 in) | 904.4 |
| Table 906.1 lookup | 4-inch stack, 160 dfu, 62 feet developed length — read the smallest vent diameter whose tabulated length is at least 62 feet | 906.1 |
| Half-diameter floor | Half of the 4-inch drain = 2 inches minimum, whatever the table says | 906.1 |
| Absolute floor | 1-1/4 inches | 906.1 |
| Branch vents on each floor | Half the required branch drain size, never below 1-1/4 inches; one size up if over 40 feet | 906.2 |
| Header, if this stack is combined with others at the roof | Sum all dfu on all stacks; developed length is the longest path as a direct extension of one stack | 904.5 |
| Terminal | Through the roof at the jurisdiction's height, clear of openings by 903.5 | 903 |
An individual vent serves a fixture on a 3-inch required drain. What is the minimum vent size?
Four branch vents join a common branch vent. On what basis is the common branch vent sized under IPC 906.4?
How is the developed length of a vent header determined under IPC 904.5?
What size is a relief vent in a building having more than 10 branch intervals, and how does it connect?