Mercer Island, WA rainwater: 30,560 gallons off a roof
Mercer Island, WA: gallons off the roof each year
30,560gallons a year
That is a 1,500 sq ft asphalt roof under Mercer Island, WA's normal 38.5 inches of rain, about 795 gallons for every inch that falls.
Mercer Island, WA has gaps between storms long enough to drain a single barrel. Around 275 gallons of storage catches most of what is worth catching.
Rainfall: NOAA 1991-2020 climate normals, the Mercer Island 1.5 Nw station (1.8 mi away). Mercer Island, WA ranks 62nd wettest of the 91 cities covered in Washington.
Month by month in Mercer Island
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 5.57 | 4,427 | |
| Feb | 3.67 | 2,917 | |
| Mar | 3.79 | 3,012 | |
| Apr | 3.04 | 2,416 | 163 |
| May | 2.17 | 1,725 | 272 |
| Jun | 1.65 | 1,311 | 337 |
| Jul | 0.76 | 604 | 448 |
| Aug | 1.05 | 835 | 411 |
| Sep | 1.76 | 1,399 | 323 |
| Oct | 3.60 | 2,861 | 94 |
| Nov | 5.92 | 4,705 | |
| Dec | 5.47 | 4,348 |
Nov is the wettest month (5.92 in, 4,705 gal off the roof); Jul the driest (0.76 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
Which tank size is worth it in Mercer Island
Best fit: a 275-gallon tote or tank, which delivers about 1,763 gallons to the garden in a normal year (86.1% of its need).
If you buy one, link barrels with a short hose at the base so they fill and drain together, and check the overflow points away from the house.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 1,024 | 28,231 |
| 110 gal | 1,354 | 27,901 |
| 165 gal | 1,534 | 27,721 |
| 275 gal | 1,763 | 27,492 |
| 550 gal | 1,798 | 27,457 |
| 1,000 gal | 1,798 | 27,457 |
| 1,500 gal | 1,798 | 27,457 |
| 2,500 gal | 1,798 | 27,457 |
| 5,000 gal | 1,798 | 27,457 |
Simulated storm by storm through a normal year using the station's count of days with 0.1 in or more of rain each month.
Is rainwater collection legal in Washington?
Collecting and storing rooftop rainwater for use on site needs no water right permit, provided the roof is part of a fixed above-ground structure whose primary purpose is not rain collection. Ecology may set local restrictions on new systems if they harm instream values or existing water rights. No state storage cap in the policy.
Checked against the current source on 2026-11-06. Source