Washington, DC rainwater: 36,235 gallons off a roof
Rainwater from a roof in Washington, DC
36,235gallons a year
That is a 1,500 sq ft asphalt roof under Washington, DC's normal 45.6 inches of rain, about 795 gallons for every inch that falls.
In Washington, DC 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 Natl Arboretum Dc station (2.6 mi away).
Month by month in Washington
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 3.03 | 2,408 | |
| Feb | 2.77 | 2,202 | |
| Mar | 3.96 | 3,147 | |
| Apr | 3.44 | 2,734 | 113 |
| May | 4.32 | 3,434 | 4 |
| Jun | 4.56 | 3,624 | |
| Jul | 4.62 | 3,672 | |
| Aug | 3.67 | 2,917 | 85 |
| Sep | 4.12 | 3,275 | 29 |
| Oct | 4.23 | 3,362 | 15 |
| Nov | 3.20 | 2,543 | |
| Dec | 3.67 | 2,917 |
Jul is the wettest month (4.62 in, 3,672 gal off the roof); Feb the driest (2.77 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
Barrel or cistern in Washington?
Best fit: one 55-gallon barrel, which delivers about 191 gallons to the garden in a normal year (77.8% 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 Washington's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 191 | 34,874 |
| 110 gal | 191 | 34,874 |
| 165 gal | 191 | 34,874 |
| 275 gal | 191 | 34,874 |
| 550 gal | 191 | 34,874 |
| 1,000 gal | 191 | 34,874 |
| 1,500 gal | 191 | 34,874 |
| 2,500 gal | 191 | 34,874 |
| 5,000 gal | 191 | 34,874 |
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 District of Columbia?
Legal and subsidised. DC's Department of Energy and Environment RiverSmart Homes installs rain barrels for a $100 copayment per barrel (limit two per property) after a free stormwater audit, and RiverSmart Rebates pay residents who install their own rain barrels. No storage cap stated beyond the program limit.
Checked against the current source on 2026-11-06. Source