Catchment First Flush Diverter

First-Flush Diverter, Rainwater Harvesting, Rainwater Catchment, , Renewables News DFW, Dallas Fort Worth North Texas North Central Texas

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Although larger rooftop debris is collected by the leaf screens, smaller debris such as dust, insect bodies, animal feces, pesticides, etc., is not collected here. A diverter takes care of the smaller debris. 

The first flush diverter routes the initial flow of water from the harvest surface away from the storage tank. 

A hose bibb fixture left slightly open (shown left as option ) or a pinhole drilled at the bottom of the pipe, allows water to gradually leak out. Water from the first-flush diverter be routed to a planted area if desired. 

Pipe Diameter Needed to Catch Gallon of Water

Diameter PVC Pipe Length Per Gallon
3" 33"
4" 18"
6" 8"

Although not shown here, equipping the standpipe with ball valve is better. As the chamber fills, the ball floats up, traps the first-flush water, and routes the balance of the water to the tank.

How much water should you divert?

A rule of thumb is to divert  a minimum of 10 gallons for every 1,000 square feet of collection surface. However, opinions vary, and the correct answer will also depend upon the number of dry days dust has been allowed to accumulate, the amount and type of debris, the intensity of the rain (see below), the roof slope, and any tree overhang. Even the season of the year can affect the amount of water to divert.  

The rain intensity needed to wash your roof is a function of both the roof slope and the intensity of the rain. 

  • Sloped Roof: Needs 0.10 inches of rain per hour. 

  • Flat or Nearly-Flat Roof: Needs 0.18 inches of rain per hour. 

A roof washer may also be needed for potable water or if you use the water for drip irrigation.

Once more: Particularly if you intend to use the harvesting system as drinking water, it is very important to consult a professional about your system.

Do you need an energy consultant, energy auditor, 
green builder, or green remodeler
 in North Central Texas 
or the Dallas-Fort Worth area? 
Are you purchasing or building a home?
Ready to add renewable energy?
Interested in living more sustainably?
Contact Terry Jensen 
Frugal Energy, LLC
972 251-1532 or 817 443-2553 Just have a question or two?
Please call. If I can answer without
seeing your home or business, 
I will do so at no charge.


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