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Written by Deb Kingsbury
Living on a 36-acre off-grid property in northern Arizona, Deb and her husband Jeremy embrace sustainable living through solar power, rainwater harvesting, and a passive solar home built with recycled polystyrene block. A freelance writer and editor, Deb grows food in raised beds and a greenhouse and is an avid hiker and backpacker. A long-time search & rescue volunteer, she has published two novels and previously homesteaded on the East Coast.
When it comes to rainwater harvesting, ensuring the water you collect is as clean as possible is crucial, especially if you want to use it for potable purposes.
One of the most effective ways to improve the quality of your harvested rainwater is by installing a first flush diverter, also known as a roof washer. In this guide, we'll walk you through making your first flush diverter using simple materials. This is similar to the ones we use on our potable rainwater collection system here at 36-Acre Wood.
What Is a First Flush Diverter?
A first flush diverter is a simple yet essential component of a rainwater harvesting system. Its primary function is to capture and discard the initial flow of rainwater that washes off your roof. This first "flush" of water is typically the most contaminated, as it collects dirt, debris, bird droppings, and other pollutants that can accumulate on the roof during dry periods.
By diverting this dirty water from your storage tank, a first flush diverter can ensure that only cleaner water enters your rainwater collection system. This step is particularly important if you plan to use the water for drinking, cooking, or other potable uses, as it can significantly reduce the risk of contamination.
Materials You'll Need
Before we dive into the construction process, let's gather the materials you'll need for your DIY first flush diverter:
PVC pipe (3-inch diameter)
PVC end cap
PVC T-junction
Ball valve
90-degree PVC elbow joint
Small mesh screen
Rubber washer
Silicone sealant
Pipe cleaner and PVC cement
Zip ties or hose clamps
Gravel or small stones (optional)
These materials are generally easy to find at your local hardware store, sometimes at a discount or used material store (i.e., if you have a Habitat for Humanity ReStore near you), or even free, and the entire project should cost you less than purchasing a first flush diverter kit.
DIY Roof Washers: Step-by-Step Instructions
1. Assemble the PVC Pipe
Start by cutting the PVC pipe to the desired length. The length will determine the volume of water that gets diverted before clean water enters your storage tank. A good rule of thumb is diverting the first 5-10 gallons of rainwater, depending on your roof size and how much debris accumulates.
2. Attach the End Cap
Once you've cut the pipe, attach the PVC end cap to one end. Before securing it, drill a small hole in the center of the end cap to allow for slow drainage. This hole will ensure the diverter empties itself between rain events, preparing it for the next rainfall. You can fill the pipe with gravel or small stones to regulate the drainage speed, but this is optional. We have a drip nozzle on our first flush, and instead of gravel, a first flush filter that helps keep the nozzle from clogging. If you want to simplify this part for yourself, you can buy either a 3-inch first flush downspout kit or a 4-inch downspout kit and then customize it by adding a section of PVC if needed.
3. Install the T-Junction
Next, attach the T-junction to the other end of the PVC pipe. The T-junction will serve as the connection point between your downspout, the first flush diverter, and your main collection system. Be sure to use PVC cement to secure all connections tightly.
4. Add the Ball Valve
Now, install the ball valve on the side opening of the T-junction. This valve will allow you to manually flush out any remaining debris and empty the diverter if needed. Make sure the valve is easily accessible for regular maintenance.
5. Attach the Elbow Joint
At the top of the T-junction, attach the 90-degree PVC elbow joint. This joint will connect to your downspout, guiding the initial flow of rainwater into the diverter. Ensure the joint is securely fitted to prevent any leaks.
6. Install the Mesh Screen
To prevent larger debris, such as leaves and twigs, from entering the diverter, install a small mesh screen or leaf filter at the entrance of the downspout, or you can use these simple gutter guards (just remember to get up on a ladder now and then clear them off). Secure the screen with a rubber washer and silicone sealant to keep it in place. This step will help maintain the effectiveness of your diverter and extend its lifespan.
7. Secure the Diverter to Your Downspout
With the diverter assembled, it's time to attach it to your downspout. Use zip ties or hose clamps to securely fasten the PVC elbow joint to the downspout. Make sure the diverter is positioned vertically so gravity can do its work. The water should flow easily from the downspout, through the elbow joint, into the T-junction, and down into the PVC pipe.
8. Test Your First Flush Diverter
After installing your first flush diverter, testing it during the next rainfall is essential. Check for any leaks or blockages, and ensure that the first flow of rainwater is being diverted properly. If everything works correctly, you should see the dirty water being directed away from your main collection system.
First Flush Maintenance Tips
While a first flush diverter is a low-maintenance addition to your rainwater harvesting system, it does require some occasional upkeep:
Regularly Check the Drainage Hole: Over time, the drainage hole in the end cap can become clogged with sediment or debris. Make sure to clear it out regularly to ensure the diverter empties between rainfalls.
Clean the Mesh Screen: The mesh screen, leaf filter, or downspout gutter guard should be inspected and cleaned periodically to prevent blockages that could reduce the diverter's efficiency.
Inspect the Ball Valve: The valve should be opened and closed periodically to ensure it remains functional. This also helps flush out any accumulated debris.
Why Use a First Flush Diverter?
Using a first flush diverter in your rainwater harvesting system offers several benefits, especially for those aiming to use their collected rainwater for potable purposes. Removing the most contaminated water from your system significantly reduces the risk of introducing harmful bacteria, chemicals, or debris into your storage tank. This simple addition can really improve the quality of your water, making it safer for drinking and other household uses.
Also, building your own first flush diverter is a cost-effective way to enhance your rainwater harvesting system. It also aligns with sustainable living practices, allowing you to make the most out of natural resources while minimizing waste.
Maximizing the Benefits of a DIY First Flush Diverter for Clean Rainwater Harvesting
A first flush diverter is invaluable for anyone serious about rainwater harvesting, especially if the water is intended for potable use. While you can always buy a customizable pre-made first flush or kit, if you prefer to do things the DIY way, you can definitely make an effective, affordable diverter that helps ensure the water you collect is as clean and safe as possible and can also pass your local building code, if applicable.
Not only does this project promote sustainable living, but it also empowers you to take control of your water quality, one rainstorm at a time.
Looking for more ways to filter and clean the rainwater you collect? Here are 5 DIY methods for home use.
First Flush FAQs
1. What is the purpose of a first flush diverter in a rainwater harvesting system?
A first flush diverter is designed to capture and discard the initial flow of rainwater from your roof, which is often the most contaminated with dirt, debris, and other pollutants. By diverting this water, the diverter ensures that cleaner water enters your storage tank, improving the quality of your harvested rainwater.
2. How much water should a first flush diverter divert?
The amount of water diverted by a first flush diverter typically depends on the size of your roof and the amount of debris that tends to accumulate. A general guideline is to divert the first 10-20 gallons of water, but this can vary based on your specific situation. Adjusting the length of the PVC pipe in your DIY diverter can help control the volume of water diverted.
3. Can I use a first flush diverter to harvest rainwater for non-potable uses?
Yes, a first flush diverter can still be beneficial even if you’re using the water for non-potable purposes like irrigation or flushing toilets. Diverting the first flush of contaminated water helps reduce the amount of debris and pollutants that could clog pipes or cause other issues in your system, ultimately prolonging the life of your rainwater harvesting setup.
4. How often should I maintain my first flush diverter?
Regular maintenance of a first flush diverter is essential to ensure its effectiveness. You should check the drainage hole, clean the mesh screen, and inspect the ball valve at least every few months, especially during the rainy season. This will help prevent blockages and ensure that the diverter functions properly.
5. Is it difficult to install a DIY first flush diverter?
Installing a DIY first flush diverter is a relatively simple project that can be completed with basic tools and materials. By following step-by-step instructions, most people can successfully build and install their own diverter in a few hours. It’s a cost-effective way to enhance your rainwater harvesting system and improve the quality of the water you collect.
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