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Branched Drain Greywater System


Branched Drain Greywater System

In a branched drain greywater system the greywater from the house drains through a series of branching pipes and is dispersed into the landscape into mulch basin outlets. Showers, sinks and laundry can individually drain into the landscape, or be collected together into a single branched drain system. Typically branched drain systems are installed on showers, and a laundry to landscape system is installed on the washing machine. The greywater irrigation zone must be downhill relative to the shower, sinks and laundry inside the home.

The information below provides an overview of the specific components in a branched drain system, but there are many details not included. We recommend you read the chapter in Create an Oasis with Greywater by Art Ludwig on branched drain systems, or hire an installer with experience before installing this type of system.

System Components

3-way Diverter valve:

Jandy Neverlube 3-way greywater diverter valveThe 3-way diverter valve allows the homeowner to send greywater to the landscape or to the sewer.  Greywater is sent to the landscape during the irrigation season and usually sent to the sewer during the rainy season when irrigation is not required. The most common valves are the Pentair 1.5"/2" 3-way valve and the Jandy Neverlube 1.5"/2" 3-way valve. The 3-way valve can be automated by mounting an automatic actuator on top of the valve to allow remote switching of the greywater system. The 3-way valve accepts both 2" or 1.5" drain pipe and the 3-way valve can be altered so any port can be the inlet. The valve is manufactured with the middle of the tee as the inlet.

The 3-way valve is installed in the drainline of the desired greywater fixture, upstream of any connections with any  blackwater fixtures. Use proper transition coupling to connect to the existing plumbing.

3-way Valve actuator:

Goldline 3-way Valve Actuator GVA-24The 3-way diverter valve is manually controlled by turning the handle on top of the valve. In some situations the 3-way valve may need to be installed in an inaccessible location, or the user may not want to have to go outside/under the house to switch the valve. A Goldline 3-way Valve Actuator GVA-24 can be mounted on top of the 3-way valve and connected to a switch installed inside the building to control the valve. The actuator is wired to a plug in 24 volt transformer.

Backwater valve:

A backwater valve is a type of swing check valve, that allows water to flow in one direction only. Backwater valves are installed on greywater system piping to prevent sewer water from backing up into the greywater system. If your jurisdiction does not require one, we do not recommend using them as they have had clogging and longevity issues.

Backwater valves are installed on the sewer side of the 3-way valve and prevents a sewer back up from reaching the 3-way valve. Since the 3-valve itself shuts off the sewer pipe connection, sewer would not be able to enter the greywater pipe during a back up. The only time the backwater valve would prevent sewer water from backing up into the greywater system is if the 3-way valve wasn't turned properly and the sewer connection was left partially open.

Collection plumbing:

After installing the 3-way valve the greywater pipe needs to be plumbed outside. For a single shower, 1.5" drain pipe is typically used, though a permitted system will most likely require 2" drain pipe (2" is the current code size for shower drains, most older homes are plumbed with 1.5"). The pipe should be routed outside following standard plumbing guidelines; use clean-outs, proper sweeps, constant downwards slope of 2% to avoid clogging. Refer to the drainage chapter of your state's plumbing code, a good plumbing book, or hire a plumber to ensure proper plumbing techniques are implemented.

Double ell (Twin 90) flow splitters:

ABS 1.5" double ell- branched drain greywater systemThe double ell, or twin 90, flow splitter divides the flow of greywater in half. For this to work properly the flow splitter must be installed exactly level, and a 2 foot run of straight pipe must enter the splitter. If the pipe enters on a curve, or the splitter is crooked, more water will flow to one side. Place a small brick or flat stone under the splitter for support. Since the splitter divides flow in half, your options for distributing greywater are to send 1/2, 1/4, 1/8, 1/16th the flow of water. Draw an accurate sketch of the system to help you divide the flow in the desirable amounts to your plants. Double ells come in two sizes, ABS 1.5" Double ell and ABS 2" Double ell. If the greywater line is 2" coming from the house, use a 2" double ell for the first split, then reduce down to 1.5" double ells for the rest of the system.

You can also install double ells tapped with a clean-out plug. This feature allows you to easily clean out a future clog, and this point would be the most likely place that a clog would occur. You can purchase tapped double ells, or make them yourself. Some jurisdictions may not allow the tapped double ell, so if you're getting a permit make sure you show the inspector this part ahead of time.

Distribution plumbing:

In the landscape, use 1.5" drain pipe, maintaining a minimum 2% downhill slope. Use long sweep 90 degree fittings, and incorporate clean-outs (either in the double ells, or regular clean-outs).


Greywater will flow out the final pipe through the 1.5" ABS pipe into a "mulch shield" located inside a mulch basin in the drip line of the tree or plant. Features desired in an outlet are:

  • Greywater flows through the air by 4-6" before landing onto mulch.
  • There is a thick (4-6") layer of mulch below the outlet.
  • The mulch shield is  large enough to accommodate surges in the system, typically 5 gallons is a sufficient size. You can drill holes in the sides of the shield to allow more infiltration.

Mulch basin or mulch trench:

Distribute greywater into a mulch basin or mulch trench, located in the drip line of the tree or large plant you want to irrigate. You can size the area of the basin based on the soil type and quantity of greywater produced, see Chapter 16A of the California plumbing code for an example soil chart. For clay soil you will you need roughly 1 square foot of soil per 1 gallon of greywater discharged each day. If an estimated 10 gallon will be discharged from an outlet daily, you'd need at least 10 square feet of mulched area to accommodate that water. Sandy or loamy soil needs much less area.

PDF icon Branched Drain System Diagram150.74 KB