Rainwater Harvesting in Washington
Washington (WA) receives an average of 38.4 inches of rainfall per year, placing it in the moderate-rainfall category among US states. The wettest month is typically Dec, and the driest is Jul. Rainwater harvesting is fully legal in Washington. A standard 1,500-square-foot asphalt shingle roof can collect approximately 27,511 gal per year, saving an estimated $64.20 annually at the state's average municipal water rate of 0.6 cents per gallon.
Monthly Rainfall in Washington
Understanding Washington's monthly rainfall distribution is essential for sizing your storage tank correctly. The table below shows average monthly rainfall and the resulting harvest from a 1,500-square-foot roof. While Washington's 38.4 inches of annual rainfall is moderate, efficient collection from a well-designed system still produces thousands of gallons. Focus on maximizing collection efficiency with a metal roof and capturing every rain event.
| Month | Rainfall (in) | Harvest (1,500 sq ft) |
|---|---|---|
| Jan | 5.4" | 4,037 gal |
| Feb | 3.8" | 2,841 gal |
| Mar | 3.6" | 2,691 gal |
| Apr | 2.6" | 1,944 gal |
| May | 2.0" | 1,495 gal |
| Jun | 1.6" | 1,196 gal |
| Jul | 0.8" | 598 gal |
| Aug | 0.8" | 598 gal |
| Sep | 1.6" | 1,196 gal |
| Oct | 3.2" | 2,392 gal |
| Nov | 5.6" | 4,187 gal |
| Dec | 5.8" | 4,336 gal |
| Total | 38.4" | 27,511 gal |
Harvest by Roof Size in Washington
Your actual harvest depends on how much roof area you connect to your collection system. Not every square foot of your roof needs to drain into your tank — many homeowners start by connecting just one or two downspouts covering a portion of their roof. The table below shows what different collection areas yield annually in Washington using an asphalt shingle roof at 80% efficiency.
| Roof Area | Annual Harvest | Rec. Tank Size | Annual Savings |
|---|---|---|---|
| 1,000 sq ft | 18,341 gal | 5,000 gal | $64.20 |
| 1,500 sq ft | 27,511 gal | 5,000 gal | $64.20 |
| 2,000 sq ft | 36,682 gal | 10,000 gal | $64.20 |
| 2,500 sq ft | 45,855 gal | 10,000 gal | $64.20 |
Upgrading to a metal roof (95% efficiency) boosts collection by about 19% compared to asphalt shingle. For example, a 1,500-square-foot metal roof in Washington captures approximately 32,670 gal per year when used for garden irrigation and toilet flushing for a household of two.
Rainwater Harvesting Laws in Washington
Washington explicitly allows rainwater collection from rooftops. A 2009 law clarified that a water right permit is not needed for rooftop harvesting intended for beneficial use on the same property.
Regulation Summary for Washington
- Legal Status: Legal — no restrictions
- Max Collection: No limit
- Permit Required: No — for standard residential systems
- Municipal Water Rate: 0.6 cents per gallon (state average)
Popular Uses for Harvested Rainwater in Washington
Washington homeowners use harvested rainwater for a variety of purposes. The most common applications are garden irrigation, lawn watering, and toilet flushing. Each use has different water requirements and may require different levels of filtration. Explore detailed guides for the most popular uses in Washington:
How Washington Compares to Similar States
The table below compares Washington with five states that have similar annual rainfall patterns. All calculations use a standard 1,500-square-foot asphalt shingle roof collecting water for garden irrigation.
| State | Annual Rainfall | Legal Status | Annual Harvest | Water Cost | Annual Savings |
|---|---|---|---|---|---|
| Washington | 38.4" | Legal | 27,511 gal | 0.6 c/gal | $64.20 |
| Illinois | 39.2" | Legal | 29,080 gal | 0.6 c/gal | $64.20 |
| Ohio | 39.6" | Legal | 29,155 gal | 0.5 c/gal | $53.50 |
| Oklahoma | 36.5" | Legal | 27,512 gal | 0.4 c/gal | $42.80 |
| District of Columbia | 40.8" | Legal | 29,754 gal | 0.9 c/gal | $96.30 |
| Iowa | 35.9" | Legal | 26,540 gal | 0.5 c/gal | $53.50 |
Tips for Rainwater Harvesting in Washington
With moderate rainfall of 38.4 inches per year, Washington requires a more strategic approach to rainwater harvesting. Maximize your collection efficiency by using the largest possible roof area and choosing a high-efficiency roof material like metal (95% capture rate). Size your tank to store at least one month's harvest to bridge typical dry periods. A 1,000-gallon tank is a good starting point for most Washington homeowners. Connect the tank to a drip irrigation system for maximum efficiency — drip systems use 30-50% less water than sprinklers. In drier months, prioritize high-value uses like vegetable gardens over lawn irrigation.
Calculate Your Specific Harvest in Washington
The estimates above use standardized assumptions. Use our Rainwater Harvesting Calculator to enter your exact roof area, material, household size, and intended uses for a customized analysis tailored to your property in Washington. You can also browse all states to compare potential across the country, or explore tank sizes and water uses for detailed information on system components and water budgeting.