6.4 Building Drains, Grade, Gravity Drainage, Backwater Valves, & Washington's Deletion of Building Sewer Rules
Key Takeaways
The UPC defines the building drain as the lowest drainage piping inside the building walls, ending at the building sewer 2 feet outside the building wall.
Horizontal drainage piping slopes at least 1/4 inch per foot (2 percent); piping 4 inches and larger may slope 1/8 inch per foot (1 percent) where approved by the AHJ (UPC 708.1).
Fixtures on a floor lower than the next upstream manhole cover must be protected by a backwater valve, and upper-floor fixtures may not discharge through it (UPC 710.1).
Cleanouts on drains that pass through a backwater valve must be labeled "backwater valve downstream" (UPC 710.1).
Washington deletes UPC Chapter 7 Part II, Sections 713 through 723 and Tables 717.1 and 721.1, so there is no state UPC building sewer size or depth rule.
Where the Building Drain Ends
The UPC Chapter 2 definitions draw the boundaries:
- Building drain: "That part of the lowest piping of a drainage system that receives the discharge from soil, waste, and other drainage pipes inside the walls of the building and conveys it to the building sewer beginning 2 feet outside the building wall."
- Building sewer: The horizontal piping that extends from the end of the building drain to a public sewer, private sewer, private sewage disposal system, or other point of disposal.
- Horizontal branch: A drain extending laterally from a stack or building drain that receives the discharge of one or more fixture drains.
- Stack: The vertical main of soil, waste, or vent piping extending through one or more stories.
Washington Deletes the Building Sewer Rules
Washington's WAC 51-56-003 adopts the UPC "provided further, that those requirements of the Uniform Plumbing Code relating to … building sewers are not adopted." The Chapter 7 amendment makes it explicit:
Chapter 7, Part II Building Sewers. Delete all of Part II (Sections 713 through 723, and Tables 717.1 and 721.1).
What that means for the exam:
- There is no UPC building sewer sizing table (Table 717.1) and no UPC sewer depth, cleanout-spacing, or testing rule (721, 723) in Washington's plumbing code. Local sewer utilities, health jurisdictions, and public works standards set those requirements.
- Washington's definition of plumbing system (218.0) covers drainage piping and building drains within the property lines. It adds that no certification is required to install plumbing systems within the property lines and outside a building.
- Statements such as "the UPC requires a 4-inch minimum building sewer in Washington" are not supported by the Washington code. Size the building drain with Table 703.2 (Section 6.3).
Grade of Horizontal Drainage (UPC 708.1)
| Pipe | Minimum slope | Condition |
|---|---|---|
| Any horizontal drainage pipe | 1/4 inch per foot (2%) toward the point of disposal | Standard |
| 4 inches and larger | 1/8 inch per foot (1%) | Only where the street sewer depth, structural features, or building arrangement make 1/4 inch impractical, and the AHJ approves first |
At 1/8 inch per foot, horizontal fixture unit capacity drops by the 0.8 factor in Table 703.2 note 5. Piping must also be run "in practical alignment." Fittings are made for a 1/4-inch grade (701.3), and hangers must prevent sagging (313.4).
Grade math: 60 feet of drain at 1/4 inch per foot falls 15 inches (60 × 0.25). At 1/8 inch per foot it falls 7.5 inches. Slope percentage = rise ÷ run × 100. For example, 1/4 inch per 12 inches is about 2.08%.
Gravity Drainage and Fixtures Below the Sewer
- 709.1: Where practicable, fixtures drain to the public sewer or private system by gravity.
- 710.1 Backwater valves: Fixtures on a floor level lower than the next upstream manhole cover of the public or private sewer must be protected by an approved backwater valve.
- Fixtures on that floor that are not below the manhole cover do not need one.
- Fixtures on floors above that elevation may not discharge through the backwater valve.
- Cleanouts on drains that pass through a backwater valve must carry a permanent label: "backwater valve downstream."
- 710.2 Below the sewer crown: Fixtures below the crown of the main sewer drain by gravity to a watertight sump or receiving tank, then are lifted into the building drain or sewer (Section 7.2).
- 710.6 Access: Backwater valves, gate and ball valves, unions, motors, and similar devices must be accessible. Unless continuously exposed, they go in a pit with an adequately sized removable cover.
Why a backwater valve matters
If the public sewer surcharges during a storm, sewage rises in the main and backs into the building drain. Any fixture below the next upstream manhole cover would overflow first. The backwater valve closes against reverse flow. Upper-floor fixtures are kept off it so their discharge does not back up behind a closed valve into the lower level.
Existing Building Drains (UPC 102.4.1)
Existing building drains and sewers may serve new work only if examination and testing show they meet the requirements for new work. Otherwise the AHJ orders the needed changes. Openings into a drainage or vent system not connected to fixtures or vent terminals must be permanently plugged or capped (102.4.2).
Testing the Building Drain
The building drain is tested with the rest of the DWV system under 712: a 10-foot water head or 5 psi air for 15 minutes, and no air test on plastic (Section 1.2). A two-way cleanout inside or just outside the wall at the building drain's lower end may substitute for an upper-terminal cleanout (707.4, exception 4; Section 7.1).
In Washington, which UPC sections governing building sewers apply to a new house's sewer line under the state plumbing code?
Only Section 723 for testing
Sections 713 through 723, including Table 717.1
None; Washington deletes Chapter 7 Part II, Sections 713 through 723 and Tables 717.1 and 721.1
Only Table 717.1 for sizing
A 3-inch horizontal building drain cannot reach the street sewer at 1/4 inch per foot because the sewer is shallow. Under UPC 708.1, may it be installed at 1/8 inch per foot?
No slope reduction is ever permitted
Yes, any size may use 1/8 inch per foot with AHJ approval
No; only piping 4 inches or larger may be reduced to 1/8 inch per foot, and only with prior AHJ approval
Yes, if the drain carries fewer than 20 DFU
A basement bathroom sits below the next upstream sewer manhole cover. The first floor above it is higher than the cover. What does UPC 710.1 require?
A sewage ejector for the entire building
Nothing, if the basement has a floor drain
A backwater valve protecting the basement fixtures, with first-floor fixtures connected downstream of it
A backwater valve on the entire building drain, serving all floors
Sections you finish are checked off in the contents.