How to connect a rain barrel to a downspout

Last Updated: October 23, 2026

How a rain barrel connects to a downspout

A rain barrel takes water out of a downspout, stores it for the garden, and hands the rest back to the downspout or to the ground. Gutters of Kent installs downspouts and downspout extensions across Kent and South King County, with the work done by licensed and insured contractors. The barrel itself is a homeowner add-on; what affects the house is the downspout it taps and the route its overflow takes. There are two common ways to make the rain barrel downspout connection.

A diverter in the downspout. A short fitting replaces a section of the downspout. A hose runs from it to an inlet near the top of the barrel. While the barrel has room, water flows in; once the barrel is full, water backs up in the hose to the diverter and continues down the downspout to its usual discharge. The overflow never leaves the downspout’s route, which keeps things simple.

The downspout into the barrel. The downspout is cut short and empties through a screen into the top of the barrel. Everything the downspout carries goes into the barrel first, and the excess leaves through an overflow port near the top. That overflow is now a new discharge point at the base of the wall and needs its own hose or extension.

Either way, the barrel should sit on a level, solid stand high enough for a watering can or hose to fit under the spigot, with a screened inlet so debris and insects stay out.

How fast a barrel fills in Kent

Roof water is simple arithmetic. One inch of rain on one square foot of roof is 144 cubic inches, and a gallon is 231 cubic inches, so:

Gallons = roof area (sq ft) × rainfall (in) × 0.623

On 1,000 square feet of roof, one inch of rain is about 623 gallons. The roof area that matters is the part that drains to the downspout feeding the barrel, measured in plan. Using 500 square feet as an example drainage area for one downspout:

The rainfall figures come from NOAA’s 1991 to 2020 normals for the Kent station: 6.23 inches in an average November, 0.67 inches in July and 0.92 in August, with 96.7 days a year carrying at least a tenth of an inch and 23.8 days with half an inch or more. The water amounts are the formula above applied to those inputs; your own drainage area will change them in proportion.

Two conclusions follow. In the wet season, a single barrel on a downspout draining 500 square feet fills in less than a fifth of an inch of rain, so it overflows on any day wetter than that, and on every one of the 23.8 days a year with half an inch or more. And in the dry months when the garden wants water, the whole of July sends only a few barrels’ worth to that downspout. A barrel is useful storage for the start of summer, not a way to handle a Kent winter.

Overflow is the part that matters for the house

Because the barrel stays full through much of the wet season, its overflow carries most of the water that downspout delivers from October through January. Wherever the overflow goes is effectively where the downspout drains.

That brings the barrel under the same rules as any downspout. Kent City Code 7.04.180 bars roof runoff from the sanitary sewer unless the director authorizes it. Under the King County manual Kent adopts, a splash block serves no more than 700 square feet of roof and needs a vegetated flowpath of at least 50 feet, and dispersion is not used near steep slopes or landslide hazard areas, or on slopes over 15%, without geotechnical approval. Where the barrel sits on a downspout that was tied to a buried line, the overflow should go back into that downspout route through a diverter, not onto the ground beside the foundation.

An overflow hose that dribbles out at the base of the barrel, beside the wall, puts a winter’s worth of roof water right where gutters are meant to keep it away. The guide to where downspouts should drain covers the discharge options, and the guide to gutters and basement water explains why that patch of soil matters.

Debris, mosquitoes and winter

Debris. Needles, leaves and roof grit travel down the downspout with the water. A screen at the barrel inlet or a strainer at the gutter outlet keeps the barrel from turning into a sludge tank, and gutter guards reduce what enters in the first place. Mean tree canopy from USFS/MRLC national canopy data runs about 26% to 30% on Kent’s East Hill and West Hill against about 7% on the valley floor, so where a hillside house has trees over the roof, the screen needs more attention.

Mosquitoes. Standing water with an open surface is where mosquitoes breed. A sealed lid, fine screen over every opening including the overflow, and a diverter hose that seals at the barrel keep them out. Check screens after storms, since debris can push them loose.

Winter. Snow is light and infrequent here, with NOAA normals for SeaTac showing about 6.3 inches a year, but an occasional hard freeze can crack a full barrel or a diverter fitting. Switching the diverter to bypass and draining the barrel ahead of a freeze avoids it.

Rebates and programs

No City of Kent rebate for rain barrels was found. The RainWise program, run by King County and Seattle Public Utilities, helps pay for rain gardens and cisterns only on private properties in eligible combined sewer basins in Seattle, so it does not apply in Kent. If you want to check for any current Kent stormwater program, ask City of Kent Public Works.

Barrels and the rest of the gutter system

A barrel works best on a downspout that already carries water reliably. If the gutter above overflows, the downspout is undersized, or the discharge is already causing trouble, fix that first: the how many downspouts guide covers layout, the downspout sizes guide covers capacity, and the rain chains guide covers another way to handle a small roof. When you want the downspouts and the overflow route set up properly, Get an Estimate.