RainReserve is a complete downspout-to-barrel rain diverter system designed to make collecting roof runoff simple, efficient, and low-maintenance. Built with DIYers in mind, RainReserve channels rainwater from your downspout into a storage barrel—helping you save on irrigation, reduce stormwater runoff, and support a more sustainable landscape. Its sleek design can be printed in single or multi-color to match the color scheme of your home, so it doesn’t just work well—it looks good doing it.

UPDATE 2025-05-26: RainReserve can be used with 4"x3", 3"x2", and 3" (3.125") square downspouts. For 3"x2" and 3" downspouts, you'll need to print top and bottom adapters which can be glue in place.

IMPORTANT: For 3" square downspouts, there's also a different top. So if you have 3" downspouts, make sure to print the appropriate top (it's on a plate labelled “Top 3.125x ONLY".

There are supports for each downspout size. The spacers are universal to all size supports.

One more thing, I improved the design of the diverter so it prints better.

UPDATE 2025-05-11: You can now use the RainReserve System for either 4"x3" or 3"x2" downspouts. For 3"x2" downspouts, you'll just need to print top and bottom adapters and glue them in place with either silicone glue or CA glue. There are also 3"x2" supports in the new profile.

The RainReserve Modular System

I designed RainReserve as a modular system with the following components:

1. Top Section (same for all downspout sizes but 3.125" square): Slides freely up and down your downspout, allowing easy access to clean out debris from the basket. IMPORTANT: If you have downspouts that are 3.125(ish)" square, do not print the plate labelled “Top Section”. Instead print the Top Section labelled “Top - 3.125” ONLY". 
2. Basket + Diverter (same for all size downspouts): The basket is engineered to let leaves and larger debris pass through while still capturing rainwater. The diverter enhances collection by splashing water outward toward the edges of the bottom section, increasing flow into the barrel.
3. Bottom Section (same for all size downspouts): The basket nests inside this piece, which features an outlet that can be oriented to either the left or right, depending on where you want to position your rain barrel. The outlet is sized to fit the inner diameter of a ¾" PVC ball valve—readily available at most hardware stores.
   Two versions are included in the print profile: a single-color and a two-color version. The second color is limited to the top of the part, so no color-change tower is needed—saving time and filament.
4. Downspout Supports (specific to your downspout size): These sleeves slide over both the top and bottom sections of your downspout. Each includes: One front-facing hole for securing to the downspout with a sheet metal screw; Four mounting holes to attach to your house. You'll only need to print the supports which match your downspout size – either 4"x3", 3"x2" or 3.125" square.
5. Spacers (same for all downspout sizes): If your downspout sits flush against your house, you may not need these. But if there's a gap or offset, spacers can help. I used 15mm spacers on my install to perfectly align the diverter.
6. Adapters (specific to your downspout size): If you have 3"X2" or 3.125" square downspouts, I've got you covered! Simply print the top and bottom adapters and glue them to to the top and bottom sections. I recommend using either exterior grade silicone adhesive or CA glue (aka Super Glue).

Filaments Used

Bambulab PETG HF (Gray), Bambulab PETG Translucent (Teal)

Why Use PETG HF?

PETG HF maintains the inherent strengths of regular PETG, providing superior resistance to water, UV, and high temperatures. PETG HF excels in printability among PETG filaments with its higher adaptability for printers and printing settings, achieving ideal print results without fine-tuning parameters. Tougher and more durable than PLA, with higher temperature resistance, it's the perfect choice for printing outdoor items such as planter pots, bird cages, watering cans, and automotive parts. Also, if you're going to add the RainReserve System to your house, you'll want color choices. This is the current offering from Bambulab:

Installation Notes

• To install RainReserve, you’ll need to cut a 220mm (8-11/16") section out of your existing 3"x4" downspout—positioned above the height of your rain barrel. I used an angle grinder with a cutoff disc, then switched to a hacksaw near the wall. A flap disc helped smooth any sharp edges.
• The system is currently built for 3"x4" downspouts. If there’s interest, I can expand to other sizes.
• The outlet OD on the bottom section fits the ID of a ¾" PVC ball valve.
• Some parts require supports for printing—there’s no way around it, but they’re easy to remove and won’t be visible once installed.
• My rain barrel is a 30-gallon model (about 32" H x 17" W).
• Make sure the outlet of your barrel sits above the end of your hose to allow proper water flow. Don't expect high water pressure– this is a gravity system.

Non-Printed Parts Used (for reference)

Your setup might differ, but here’s what I used to connect my RainReserve system to a barrel:

• 30 gallon barrel
• Blocks to raise the barrel height
• (2) ¾" x ¾" Threaded Female Adapter Union Fittings
  – One for the input on the barrel lid, one for the output on the bottom of the barrel (also called a “bulkhead adapter” at some stores)
• (2) Schedule 40 ¾" PVC Ball Valves
  – One for the diverter outlet, one for the bottom of the barrel
• Short leader hose (or washing machine supply hose)
  – Must have ¾" threaded ends to match PVC fittings
•  (5) ¾" PVC Male Adapters
• 2' of ¾" Schedule 40 PVC Pipe
• Quick-connect adapters (optional, see images for details)

How Much Rain Water Can Be Collected?

To estimate how much rain can be collected from a roof, you can use this formula:

Gallons = (Square footage of roof) × (Rainfall in inches) × 0.623

Assuming a 30'X50' roof:

• Roof area = 30 ft × 50 ft = 1,500 sq ft
• Rainfall (1 inch) = Standard example
• Conversion factor = 0.623 gallons per square foot per inch of rain

Calculation: 1,500 sq ft × 1 inch rain × 0.623 = 934.5 gallons

Here’s the table showing how much rainwater you can collect from a 30' × 50' roof at various rainfall amounts:

Of course, you can only collect a percentage of the total amount because you're limited to a single gutter. So let's assume you have 6 gutters on your 30'x50' roof, and there's 1" of rain:

Calculation: 934.5 gallons / 6 gutters = 155.9 gallons

I estimate that approximately 20% of the water is diverted using the RainReserve System:

Calculation: .20 x 155.9 gallons = 31.18 gallons

So, assuming 1" of rain on a 30'X50' roof, you should expect to completely fill a 30 gallon rain barrel. This is an estimate because I haven't had the opportunity to do a proper test. We did have a very short rain storm last week (<.25") and my rain barrel collected approximately 10 gallons. Please feel free to submit your findings in the comments below.