Sammamish, WA rainwater: 37,706 gallons off a roof
Rainwater from a roof in Sammamish, WA
37,706gallons a year
That is a 1,500 sq ft asphalt roof under Sammamish, WA's normal 47.4 inches of rain, about 795 gallons for every inch that falls.
In Sammamish, WA a single barrel does most of the work. Storms come often enough that a small tank refills before it runs dry, and a bigger one would mostly sit full.
Rainfall: NOAA 1991-2020 climate normals, the Issaquah 3.6 Nw station (3.0 mi away). Sammamish, WA ranks 22nd wettest of the 91 cities covered in Washington.
Month by month in Sammamish
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 5.92 | 4,705 | |
| Feb | 4.61 | 3,664 | |
| Mar | 5.52 | 4,387 | |
| Apr | 4.06 | 3,227 | 36 |
| May | 2.46 | 1,955 | 236 |
| Jun | 2.07 | 1,645 | 284 |
| Jul | 0.99 | 787 | 419 |
| Aug | 1.12 | 890 | 403 |
| Sep | 2.01 | 1,598 | 292 |
| Oct | 4.41 | 3,505 | |
| Nov | 7.40 | 5,882 | |
| Dec | 6.87 | 5,460 |
Nov is the wettest month (7.40 in, 5,882 gal off the roof); Jul the driest (0.99 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
What size rain barrel for Sammamish
Best fit: 2 linked 55-gallon barrels, which delivers about 1,296 gallons to the garden in a normal year (77.6% of its need).
If you buy one, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Sammamish's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 970 | 34,846 |
| 110 gal | 1,296 | 34,520 |
| 165 gal | 1,356 | 34,460 |
| 275 gal | 1,356 | 34,460 |
| 550 gal | 1,356 | 34,460 |
| 1,000 gal | 1,356 | 34,460 |
| 1,500 gal | 1,356 | 34,460 |
| 2,500 gal | 1,356 | 34,460 |
| 5,000 gal | 1,356 | 34,460 |
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