Rainwater harvesting

Short definition

Rainwater harvesting captures roof runoff for landscape irrigation, garden use, or non-potable supply. WA allows it without a water right under the Department of Ecology’s 2009 interpretive policy. Seattle’s RainWise rebate program subsidizes rain gardens and cisterns in eligible Combined Sewer Overflow basins. A simple rain barrel costs under $100; a cistern install is several thousand.

What it is

Rainwater harvesting routes water from your roof — through gutters and downspouts, sometimes through a first-flush diverter and leaf filter — into a storage container instead of letting it run off to the storm system or sewer. The captured water is then used for landscape irrigation, garden watering, vehicle washing, or (in some jurisdictions, with treatment) non-potable indoor uses.

System components:

1. Catchment — the roof. Different roof materials have different collection profiles; asphalt shingles and metal both work for non-potable use.
2. Conveyance — gutters and downspouts carrying water to the storage.
3. Pre-filter — first-flush diverter and a leaf filter at the downspout. Keeps debris out of the storage.
4. Storage — sized to your catchment area and intended use:
   – Rain barrel (50–100 gallons) — entry-level; one to four per downspout; manually tapped for use.
   – Cistern (300–10,000+ gallons) — much higher capacity; often pumped or gravity-fed.
5. Distribution — gravity feed for hand watering, drip irrigation, or pumped delivery.
6. Overflow — every system needs an overflow path to a drywell, rain garden, or natural-ground discharge for storms that exceed storage.
7. Insect screen at the inlet — mosquito control is essential.

Common storage materials are polyethylene (most common), galvanized steel, and food-grade FRP for potable applications.

A quick sizing calculation: Seattle gets about 37 inches of rain per year. A 1,000-square-foot roof captures roughly 21,000 gallons annually after typical runoff losses (37 inches × 1,000 sq ft × 0.85 runoff coefficient × 0.62 conversion). A standard 50-gallon barrel fills in the first heavy rain — most of that 21,000 gallons goes to overflow unless you have meaningful storage.

Why it matters to a homeowner

In WA, rainwater harvesting is one of the few permanent water-saving installs that’s both legal everywhere and rebate-eligible in the right places. The combination of rain barrels and a smart sprinkler controller plus drip irrigation produces a meaningful reduction in summer water bills while supporting landscape during the dry season.

For Seattle homeowners specifically, the RainWise rebate program (run jointly by King County Wastewater Treatment Division and Seattle Public Utilities) offers significant incentives for rain gardens and cisterns that disconnect downspouts from the combined sewer. Eligibility is restricted to specific CSO basins shown on the program’s eligibility map. The program has cited different per-square-foot rebate rates in different sources — verify the current 2026 rate directly at 700milliongallons.org before relying on a specific dollar figure for budgeting.

When a contractor quotes a cistern install, the cost spread is wide: $2,000–$10,000+ for installed systems, depending on storage size, above- vs. below-ground, and whether pumped distribution is included. RainWise rebates can cover a meaningful share of the cost in eligible basins.

Common failure modes

• Algal growth in transparent or sun-exposed barrels — use opaque material.
• Mosquito breeding — insect screen mandatory; check seasonally.
• Overflow during heavy storms causing foundation seepage — undersized storage or no overflow path. Always design overflow first.
• Freeze damage in winter (Eastern WA, exposed installations) — drain in fall, seal inlet, or use freeze-rated storage.
• Roof material contamination — zinc strips, asphalt off-gas, copper flashings; relevant for potable use but mostly not for landscape.

Common variants

• Rain barrel (small, simple, gravity) vs. cistern (large, often pumped, RainWise-eligible at scale).
• Above-ground (cheaper, freeze-vulnerable, easy access) vs. below-ground (more expensive, freeze-protected, requires excavation).
• Direct distribution (pumped on demand) vs. indirect (header tank plus gravity).
• Non-potable use (irrigation, vehicle wash) vs. potable use (advanced filtration plus UV; county-specific approval).

Washington note

WA is one of the most rainwater-friendly states by regulation. Three references matter:

WA Department of Ecology Interpretive Policy POL-1017 (2009) clarified that on-site storage and beneficial use of rainwater collected from a fixed structure (roof or guzzler) is not subject to RCW 90.03 water-right permitting. Counties may set local restrictions if rainwater collection negatively affects existing water rights, but no current WA county is known to trigger this.

Potable rainwater is handled by individual counties — some allow drinking-water use with appropriate treatment; many do not. Verify with your county health department before designing for potable use. Olympic Peninsula and rural Methow homesteads often use rainwater as a supplemental potable source with UV disinfection and filtration.

Seattle RainWise (a joint King County WTD and SPU program) subsidizes rain gardens and cisterns that disconnect downspouts in eligible CSO basins. Verify the current per-square-foot rebate amount directly at 700milliongallons.org before committing to a specific budget number — the figure cited has differed across sources.

For a typical Seattle CSO-basin home, a properly sized RainWise cistern install combined with rain garden often qualifies for thousands of dollars of rebate, covering a substantial share of the install cost.

FAQ

Do I need a permit to install a rain barrel in WA?

For a simple residential rain barrel under typical residential use, no Department of Ecology water-right permit is required. Local jurisdictions may have building or stormwater permit requirements for larger cisterns or for downspout disconnection — check with your city’s permit center. RainWise installations in Seattle CSO basins are coordinated through the program.

Can I drink rainwater I collect in WA?

Some WA counties allow potable rainwater use with appropriate treatment; many do not. Verify with your county health department. Even where allowed, potable use requires substantial treatment — typically pre-filtration, multi-stage filtration, and UV disinfection at minimum.

How much water can I actually collect from my roof?

The rough math: every 1 inch of rain on 1 square foot of roof produces about 0.6 gallons after typical runoff losses. A 1,000-square-foot roof in Seattle (37 inches of annual rainfall) captures roughly 21,000 gallons per year — but most of that overflows unless storage is sized to match seasonal use patterns.