Rain Barrel Setup: Downspout Diverters, Overflow, and Using Collected Water

A rain barrel collects runoff from your roof via the gutter downspout and stores it for garden use. A typical barrel holds 50 to 65 gallons. One inch of rain on 500 square feet of roof produces about 300 gallons of runoff, enough to fill multiple barrels. Setup takes an afternoon and the components cost $50 to $150 depending on whether you buy a ready-made barrel or build one from a food-grade drum.

Choosing and Placing the Barrel

You have two main options: buy a purpose-built rain barrel with an integrated screen, spigot, and overflow port, or convert a food-grade drum into a rain barrel yourself. Purpose-built barrels run $80 to $200 and come ready to install. Food-grade drums from soft drink bottlers or food processing plants cost $15 to $40 and work just as well after you add a spigot, screen, and overflow fitting.

Whatever barrel you choose, it must be opaque. Translucent or clear containers allow sunlight in, which promotes algae growth that clogs spigots and fouls the water. Dark-colored plastic barrels are the standard choice. If you are reusing a drum, make sure it previously held food-grade material, not industrial chemicals.

Place the barrel directly under a downspout on a level, stable surface. Elevate it 12 to 24 inches on cinder blocks, a dedicated timber platform, or a purpose-built rain barrel stand. This elevation is important. Gravity-fed pressure from a barrel sitting at ground level is too weak to push water through a hose with any useful flow. Every foot of elevation adds approximately 0.43 PSI of pressure at the spigot.

A full 55-gallon barrel weighs about 460 pounds. Whatever platform you use must support that weight without sinking or shifting. A pad of concrete pavers on compacted gravel works well. Cinder blocks on firm soil work. Cinder blocks on soft garden soil will sink after the first heavy rain fills the barrel.

Check local regulations before installing. Most municipalities allow rainwater collection for garden irrigation without restriction. A few states, primarily in the arid West, have complex water rights laws that historically restricted collection. Most of these have been updated in recent years to allow residential barrels of reasonable capacity, but the specifics vary. A quick search for your state's rainwater harvesting rules takes five minutes and avoids potential issues.

Downspout Diverter Installation

A downspout diverter connects the barrel to your existing gutter downspout. The simplest version cuts the downspout at barrel-inlet height and redirects the flow into the barrel. A better version includes an automatic bypass that routes water back to the original downspout path once the barrel is full, so the overflow follows the same route it did before the barrel was installed.

To install, mark the downspout at the height of the barrel inlet. Cut the downspout at that point with a hacksaw or tin snips. Attach the diverter fitting according to the manufacturer's instructions. Most diverters connect with sheet metal screws and include adapters for standard rectangular downspouts (2x3-inch and 3x4-inch) as well as 3-inch and 4-inch round downspouts. The connection should be snug but does not need to be watertight, as the diverter works by redirecting the flow path, not by pressurizing the system.

Install a fine mesh screen over the barrel inlet to keep leaves, shingle grit, and insects out of the barrel. Many purpose-built barrels include a built-in screen. If yours does not, a piece of aluminum window screen secured over the opening with a hose clamp or bungee cord works well. Check and clean the screen after storms that drop heavy leaf debris, as a clogged screen causes water to bypass the barrel entirely.

Overflow Management

An overflow port near the top of the barrel routes excess water away from the foundation when the barrel is full. This is the single most important detail that people skip during installation. Without proper overflow routing, water pours over the top of the barrel and pools against the house foundation. This is worse than not having a barrel at all, because the barrel concentrates all the roof runoff from that downspout into one spot instead of letting it flow down the original downspout and away from the house.

Connect a garden hose or PVC pipe to the overflow port and direct the discharge to a garden bed, a rain garden, a dry well, or back to the downspout below the diverter cut point. The overflow discharge should be at least 4 feet from the foundation, farther if your soil drains slowly. Grade the overflow path so water flows away from the house by gravity.

You can chain multiple barrels together by connecting the overflow port of one barrel to the inlet of the next with a short hose. This increases total storage capacity and delays the point at which overflow begins. Three 55-gallon barrels connected in series provide 165 gallons of storage. Only the last barrel in the chain needs an overflow route to a safe discharge point.

Using Collected Rainwater

Rain barrel water is suitable for watering lawns, flower beds, ornamental landscaping, and garden plants. For vegetable gardens, apply the water to the soil around the base of plants rather than to the leaves or edible parts. This minimizes any risk from the trace contaminants that roof runoff can contain, including bird droppings, shingle compound residues, pollen, and atmospheric dust.

Do not use rain barrel water for drinking, cooking, bathing, or filling swimming pools and hot tubs. Roof runoff is not potable water and should not be treated as such, regardless of filtration. The collection surface (your roof) and storage container are not designed or maintained for drinking water standards.

Connect a watering can or a soaker hose to the spigot for irrigation. The gravity-fed pressure from an elevated barrel is low, typically about 1 to 2 PSI depending on the height of the barrel above the spigot outlet. This is not enough pressure to run a sprinkler effectively. A soaker hose works well because it is designed for low-pressure drip delivery and does not need high flow rates to function.

In cold climates, drain the barrel completely before the first hard freeze of the season. Water expands as it freezes, and ice formation can crack plastic barrels, split fittings, and damage spigots. Store the empty barrel upside down or inside a garage or shed over winter. Disconnect the diverter and reconnect the original downspout section so winter runoff continues to flow away from the foundation normally.

Mosquito Prevention

Standing water breeds mosquitoes. A rain barrel with an open top, a loose-fitting screen, or uncovered overflow and spigot ports becomes a productive mosquito nursery within days during warm weather. Prevention is straightforward but requires attention to detail.

Keep all openings screened with fine mesh. The inlet, overflow port, and spigot opening (when no hose is attached) should all be covered with screen material having openings smaller than 1/16 inch. Standard aluminum window screen meets this requirement. Check screens periodically for tears or gaps, especially after cleaning debris.

If mosquito larvae appear in the water despite screening, add a mosquito dunk to the barrel. Mosquito dunks contain Bti (Bacillus thuringiensis subspecies israelensis), a biological larvicide that kills mosquito larvae but is non-toxic to humans, pets, fish, birds, and plants. One dunk treats up to 100 square feet of water surface and lasts approximately 30 days. Break the dunk into quarters for standard barrel-sized openings.

Use the water regularly. A barrel that sits full and undisturbed for weeks provides ideal mosquito breeding conditions, as the still water and warm temperatures accelerate larval development. Regular draw-down and refilling from rain events disrupts the mosquito breeding cycle. If you are not using the water fast enough, drain the barrel partially every week or two and allow it to refill naturally.

Maintenance and Seasonal Care

Clean the barrel at the end of each season, or more frequently if you notice sediment buildup, odor, or reduced flow from the spigot. Disconnect the barrel, drain it completely, and scrub the interior with a stiff brush and a solution of one part white vinegar to ten parts water. Rinse thoroughly before returning to service.

Check the spigot washer and overflow fitting for leaks at the start of each season. Plastic fittings can become brittle after sun exposure and freeze-thaw cycling. Replacement spigots and bulkhead fittings cost a few dollars and take minutes to swap.

Inspect the gutter screen or downspout diverter screen each spring and after heavy storms. Accumulated leaf debris reduces the flow rate into the barrel and can cause water to bypass the inlet entirely during heavy rain. A clean screen ensures you capture the maximum amount of water from each storm.

Frequently Asked Questions

How much money does a rain barrel save?

The savings are modest in most areas, typically $10 to $30 per season in water costs for a single barrel. The value is more about reducing stormwater runoff, having non-chlorinated water for gardens, and having a water supply available during municipal watering restrictions.

Can I use a rain barrel with a first-flush diverter?

Yes, and this improves water quality. A first-flush diverter captures the initial burst of roof runoff, which contains the highest concentration of debris, pollen, and contaminants, and diverts it away from the barrel. Only the cleaner water that follows fills the barrel. First-flush diverters are available as add-on kits for about $30 to $50.

Do rain barrels attract pests?

Only if they are not properly sealed. A barrel with tight-fitting screens and a sealed lid does not attract pests any more than a closed trash can does. The overflow and spigot should also be screened or capped when not in use.

Related Reading

• Gutter Downspout Guide
• Irrigation Repair Guide
• French Drain Installation