Portland, OR rainwater: 35,655 gallons off a roof
How much rain a roof in Portland, OR collects
35,655gallons a year
That is a 1,500 sq ft asphalt roof under Portland, OR's normal 44.9 inches of rain, about 795 gallons for every inch that falls.
Portland, OR 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 Portland 0.8 Ene station (0.6 mi away). Portland, OR ranks 24th wettest of the 57 cities covered in Oregon.
Month by month in Portland
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 6.50 | 5,166 | |
| Feb | 4.39 | 3,489 | |
| Mar | 4.90 | 3,895 | |
| Apr | 3.71 | 2,949 | 80 |
| May | 3.01 | 2,392 | 167 |
| Jun | 1.72 | 1,367 | 328 |
| Jul | 0.55 | 437 | 474 |
| Aug | 0.57 | 453 | 471 |
| Sep | 1.62 | 1,288 | 340 |
| Oct | 3.87 | 3,076 | 60 |
| Nov | 6.74 | 5,357 | |
| Dec | 7.28 | 5,786 |
Dec is the wettest month (7.28 in, 5,786 gal off the roof); Jul the driest (0.55 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 Portland
Best fit: a 275-gallon tote or tank, which delivers about 1,599 gallons to the garden in a normal year (83.3% 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 | 887 | 33,283 |
| 110 gal | 1,169 | 33,001 |
| 165 gal | 1,353 | 32,817 |
| 275 gal | 1,599 | 32,571 |
| 550 gal | 1,684 | 32,486 |
| 1,000 gal | 1,684 | 32,486 |
| 1,500 gal | 1,684 | 32,486 |
| 2,500 gal | 1,684 | 32,486 |
| 5,000 gal | 1,684 | 32,486 |
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 Oregon?
Collecting and using rain from an artificial impervious surface (e.g. a rooftop) is an exempt use: no water right application, permit or certificate needed. Statute sets no storage cap in this section.
Checked against the current source on 2026-11-06. Source