8.2 Vent Grades and Connections, Vent Stacks, Relief and Yoke Vents, & Suds Relief (UPC 905, 907, 711)
Key Takeaways
Where a vent connects to a horizontal drain, its invert must be taken off above the drain's centerline, downstream of the trap it serves (UPC 905.2).
Each vent must rise at least 6 inches above the flood-level rim of the fixture served before offsetting horizontally or joining another vent (UPC 905.3).
Except for water closets and similar fixtures, the vent opening may not be below the weir of the trap (UPC 905.5).
A drainage stack of 10 or more stories needs a parallel vent stack with a yoke vent at every fifth floor, connected at least 42 inches above the floor (UPC 907.1, 907.2).
No drain may connect within 8 feet of the base of a stack carrying suds-producing fixtures, and parallel vent stacks connect 8 feet above the stack's lowest point (UPC 711.1).
Vent Grades and Connections (UPC 905)
| Section | Rule | Why |
|---|---|---|
| 905.1 Grade | Vents have no drops or sags. They are level, or graded to drip back by gravity to the drain they serve | Condensation and rain drain back out of the vent |
| 905.2 Horizontal drains | Where a vent connects to a horizontal drain, take the vent's invert off above the drain's centerline, downstream of the trap served | Waste does not flow into and clog the vent |
| 905.3 Vent rise | Unless structure prevents it, each vent rises vertically at least 6 inches above the flood-level rim of the fixture served before offsetting horizontally. Where vents converge, each rises 6 inches above its own fixture's rim before joining. Vent piping lower than that must use drainage fittings, material, and grade | If a drain stops, the fixture overflows before waste can back into the horizontal vent |
| 905.4 Roof termination | Vents extend undiminished in size above the roof, or reconnect to a soil or waste vent of proper size | Full air flow |
| 905.5 Opening location | Except for water closets and similar fixtures, the vent opening from a soil or waste pipe may not be below the trap weir | A vent below the weir acts as a siphon (a "crown vent" effect) |
| 905.6 Common vertical pipe | Two fixtures may share a common vertical pipe when each wastes separately into an approved double fitting with inlets at the same level | Back-to-back lavatories or sinks |
Kinds of Vents in Practice
- Individual vent: one trap, connecting to the vent system above the fixture. It is at least 1-1/4 inches and at least half the drain size (904.1).
- Common vent: two fixtures on one vent through a double fitting (905.6).
- Branch vent: collects individual vents and runs to a vent stack or stack vent. It is sized by total DFU and length (Table 703.2).
- Stack vent: the extension of a soil or waste stack above its highest horizontal drain.
- Vent stack: a separate vertical vent run alongside a drainage stack. It connects at or below the lowest fixture drain and extends undiminished to the roof or back to the stack vent above the highest fixture.
- Relief vent: relieves pressure on a stack or horizontal branch. Examples include the relief vent required for circuit venting (911.4, Section 8.3) and the yoke vent below.
Vent Stacks and Yoke Vents for Tall Buildings (UPC 907)
- 907.1: Each drainage stack extending 10 or more stories needs a parallel vent stack. It extends undiminished from its upper terminal and connects to the drainage stack at or immediately below the lowest fixture drain.
- The vent stack also connects to the drainage stack at each fifth floor, counting down from the uppermost fixture drain, through a yoke vent. The yoke vent may not be smaller than the drainage stack or the vent stack, whichever is smaller.
- 907.2: The yoke vent connects to the vent stack at least 42 inches above the floor. It connects to the drainage stack through a wye branch below the lowest drainage branch connection on that floor.
Why every fifth floor? Water falling down a tall stack reaches a terminal velocity and pushes an air column ahead of it. The yoke vent gives that air a path so it does not blow trap seals on lower floors.
Suds Relief (UPC 711.1)
Suds from washers, tubs, and sinks stack up at the base of a drainage stack where it turns horizontal. There they create positive pressure that can push suds out of lower fixtures.
- No drainage connection may be made within 8 feet of a vertical-to-horizontal change of direction of a stack containing suds-producing fixtures. Those fixtures are bathtubs, laundries, washing machine standpipes, kitchen sinks, and dishwashers.
- Where parallel vent stacks are required, they connect to the drainage stack 8 feet above its lowest point.
- Exceptions: single-family residences, and stacks receiving discharge from fewer than three stories of fixtures.
Worked Example: Venting a Two-Story Commercial Restroom Stack
Each floor has 2 water closets, 2 lavatories, and 1 integral-trap urinal, all public. The stack carries 4 water closets and 24 DFU in total. That is within the 3-inch vertical limits (48 DFU, four water closets), so the stack may be 3 inches. The horizontal building drain below it carries four water closets, and note 4 allows only three on a 3-inch horizontal, so the building drain must be 4 inches.
- Individual vents: Water closets on 3-inch fixture drains need at least 1-1/2-inch vents (half of 3 inches). Lavatories on 1-1/4-inch trap arms need at least 1-1/4 inches. Urinals on 2-inch trap arms need at least 1-1/4 inches, because half of 2 is 1, which is below the 1-1/4 floor.
- Branch vent per floor: total DFU per floor is 2 × 4 + 2 × 1 + 2 = 12 DFU. A 1-1/2-inch vent allows only 8 DFU, so use 2 inches (24 DFU), assuming the developed length is under 120 feet.
- Rise: each vent rises 6 inches above its fixture's flood-level rim before turning horizontal (905.3).
- Stack vent: it carries 24 DFU, and Table 703.2 allows a 2-inch vent up to 24 DFU within 120 feet. It must extend undiminished through the roof (905.4).
- Aggregate venting (904.1): the required building drain is 4 inches (12.57 square inches). If the stack vent is the only roof terminal, a 2-inch stack vent (3.14 square inches) fails, so upsize it to 4 inches, or add vents until the total area reaches at least 12.57 square inches.
- Yoke vents: not required, because the stack is under 10 stories.
A vent connects to a horizontal waste pipe serving a lavatory. Under UPC 905.2, where must the vent connect?
With its invert taken off above the centerline of the horizontal drain, downstream of the trap
Anywhere on the side of the pipe
Upstream of the trap weir
At the bottom of the pipe so it can drain
A 14-story building's drainage stack is served by a parallel vent stack. Under UPC 907.1 and 907.2, how are yoke vents installed?
Every tenth floor, connected at the floor line
Every fifth floor counting down from the uppermost fixture drain, connected to the vent stack at least 42 inches above the floor and to the drainage stack through a wye below that floor's lowest branch
Only at the base of the stack
At every floor, using sanitary tees
In a four-story apartment building, a stack receives bathtubs and kitchen sinks from all floors. Under UPC 711.1, what is prohibited?
Connecting any fixture above the third floor
Using a 4-inch stack
Any vent on the stack
A drainage connection within 8 feet of the stack's vertical-to-horizontal change of direction
Sections you finish are checked off in the contents.