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Building Code Reference

Got a CPC 710 correction?

Backwater valve sewer

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CPC 710 is the California Plumbing Code section that requires a backwater valve on your sewer lateral whenever any plumbing fixture's finish floor elevation falls below the nearest upstream manhole cover. The valve acts as a one-way gate: sewage can flow out, but sewer backup can't flow in. Residential and ADU permits get flagged most often when plans don't show finish floor and manhole elevations together, making it impossible for the plan-checker to determine whether the protection is needed.

How to fix it

  1. Find the nearest upstream manhole rim elevation

    Contact your local public works department or use city GIS tools to get the rim elevation of the next upstream manhole cover on the public or private sewer, measured from sea level (same datum as your survey). In Anaheim, Anaheim Public Utilities can provide this. In most OC cities, public works engineering has sewer atlas records. If you can't get it digitally, a licensed civil engineer can field-survey the rim elevation. Write the value down — you'll need it on your plumbing sheet.

  2. Mark both elevations on the plumbing plan

    On your plumbing sheet (or site plan if plumbing is combined), call out: (1) the finish floor elevation of the lowest plumbing fixture, and (2) the upstream manhole rim elevation. Both must use the same vertical datum — typically NAVD88. A simple note block works: 'FFE of lowest fixture: 42.8 ft NAVD88. Nearest upstream manhole rim: 44.1 ft NAVD88. FFE is below manhole — backwater valve required per CPC 710.' This single block satisfies the plan-checker's comparison requirement.

  3. Add a backwater valve to the plumbing plan when required

    If the FFE is below the manhole rim, show a backwater valve on the building drain or branch serving the low fixtures. Install location: where only fixtures below the manhole cover drain through it; fixtures on higher floor levels must not discharge through the valve. Specify a backwater valve meeting ASME A112.14.1 or IAPMO IGC 305. Show it on a plan view and include it in your fixture or equipment schedule with the manufacturer's model or equivalent.

  4. Show the access box or cleanout detail

    CPC 710 requires the backwater valve to be accessible. If the valve is inside the slab, you need a traffic-rated access box set flush with the finished floor. If it's in a crawl space, the crawl space access must allow someone to reach the valve without tearing out flooring. Draw a detail (1/4"=1' or larger) showing the valve, the access box, and the box cover. Note the box dimensions and cover type. Plan-checkers in Orange County cities have rejected otherwise-complete plans solely for missing this detail.

  5. Verify the fixture count and valve sizing

    A single backwater valve on the main building drain works only when every fixture on that drain is in the low zone. If some fixtures are below the manhole rim and others are above, the valve must go on the branch serving only the low fixtures, because higher fixtures may not discharge through it. Check the pipe diameter — the valve must match the pipe size (common residential sizes are 3-inch and 4-inch). Note the pipe diameter and valve size on the plan.

  6. Resubmit with a correction response memo

    Don't just re-upload revised sheets. Include a correction response memo that keys each plan-check comment to your response. For the CPC 710 comment, state: 'Addressed — FFE and manhole rim elevations added to Sheet P-1.0. FFE (42.8 ft NAVD88) is below manhole rim (44.1 ft NAVD88). Backwater valve added at main building drain with access box per CPC 710. See detail on Sheet P-3.0.' Plan-checkers have dozens of projects in queue; a clear response memo cuts the back-and-forth cycle.

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Why you got flagged

Plan-checkers flag CPC 710 when the drainage plans don't show enough information to make the elevation comparison — or when the comparison clearly shows the valve is required but no valve is shown.

The most common reason: the floor plan shows finish floor elevation (FFE) as a note, but the plumbing sheet doesn't indicate the elevation of the nearest upstream manhole. Without both numbers on the same sheet, the plan-checker can't confirm compliance. The correction letter asks you to add the upstream manhole rim elevation and either confirm the slab is above it or add a backwater valve detail. Many contractors assume the plan-checker will look this up themselves, but that's not how it works.

A second scenario: the FFE is shown below the manhole elevation and the backwater valve is simply missing from the plumbing plan. This happens frequently on hillside lots or when an addition or ADU is built at a lower grade than the street. The plan-checker has no choice but to red-line it.

Third: the valve is shown on the plans but there's no access detail. CPC 710 requires the valve to be accessible — a concrete slab poured directly over the valve without a box or cleanout access point doesn't satisfy the code. The valve has to be serviceable, and your plans need to show how.

A fourth situation is remodels or conversions where the original structure had no backwater valve requirement, but the proposed work lowers a bathroom or laundry into a basement or garage below the manhole rim. The plan-checker sees the new finish floor elevation and immediately checks whether a valve is now required.

The official code

California Plumbing Code Chapter 7 — Sanitary Drainage, Section 710 (Backwater Valves)

Read the source →

Common questions

Does CPC 710 apply to ADUs?
Yes. An ADU with any bathroom, laundry, or kitchen fixture whose finish floor is below the upstream manhole rim needs a backwater valve on the sewer lateral, just like a primary dwelling. Detached ADUs are especially common triggers because they're often built at the rear of a sloped lot, putting them physically lower than the street-level manhole. Plan-checkers reviewing ADU permits routinely check the site survey for grade information before approving the plumbing set.
How do I find the upstream manhole elevation for my lot?
Start with your city's public works or sewer division. In Anaheim, Anaheim Public Utilities maintains sewer atlas records and can provide manhole rim elevations for permit purposes. For unincorporated OC areas, Orange County Public Works (ocgov.com) has similar records. A licensed civil or soils engineer can also field-survey the manhole if city records aren't available or are outdated. The elevation needs to match your site survey datum (usually NAVD88) so the comparison is apples-to-apples.
My finish floor is only a few inches below the manhole — do I still need the valve?
Yes. CPC 710 doesn't have a de minimis threshold. If the finish floor elevation of any plumbing fixture is below the upstream manhole rim elevation by even an inch, the valve is required. The concern is hydraulic: a sewer main blockage can cause backpressure that pushes sewage up any drain below the manhole. The cost of installing a backwater valve ($300–$800 typical) is far less than cleanup after a sewage backup.
Can I put the backwater valve outside the building?
Yes, with conditions. Many contractors install the valve in a cleanout box just outside the building's foundation. This works as long as the box is accessible (not buried under concrete or landscaping), the valve is protected from freezing and traffic loads, and the installation is clearly shown on the plumbing plan with a detail. Inside-slab installation is also common with a flush-mount access box. Either location satisfies CPC 710 as long as access is maintained and the valve is serviceable without demolishing finished work.
Does a basement bathroom always require a backwater valve?
Almost always. Basement bathrooms are typically below grade, which puts them below any street manhole. Unless your site survey shows the basement FFE is somehow above the upstream manhole rim — unusual but possible on a reverse-slope lot — CPC 710 will require the valve. Don't assume it's not needed; always show both elevations on the plans and let the math make the call. Missing the valve on a basement bathroom is one of the most common first-round plumbing corrections on residential additions.
What type of backwater valve does CPC 710 require?
The California Plumbing Code requires a normally open backwater valve: one that stays open during normal drainage flow and closes only when backpressure pushes sewage back toward the building. The valve must meet ASME A112.14.1 or an equivalent referenced standard. Plastic gate-type valves are widely accepted. Normally closed (spring-loaded) valves are generally not allowed for new construction under the current code because they require maintenance and can fail in the closed position, restricting normal drainage.
Will the city inspect the backwater valve before I pour the slab?
Yes. Backwater valves installed under a slab are a rough plumbing inspection item — the inspector needs to see the valve and the access box in place before you pour concrete. Call for rough plumbing inspection after the valve is set and the access box frame is installed, before any slab work. If you pour first and the valve isn't accessible or documented, you may face a costly cut-and-patch. Some inspectors will also want to see a function test, hand-operating the valve to confirm it opens and closes freely.

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