Rockville, MD rainwater: 32,841 gallons off a roof
How much rain a roof in Rockville, MD collects
32,841gallons a year
That is a 1,500 sq ft asphalt roof under Rockville, MD's normal 41.3 inches of rain, about 795 gallons for every inch that falls.
In Rockville, MD 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 Rockville 1 Ne station (1.2 mi away). Rockville, MD ranks 22nd wettest of the 23 cities covered in Maryland.
Month by month in Rockville
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 3.11 | 2,472 | |
| Feb | 2.60 | 2,066 | |
| Mar | 3.66 | 2,909 | |
| Apr | 3.23 | 2,567 | 140 |
| May | 3.55 | 2,822 | 100 |
| Jun | 3.96 | 3,147 | 49 |
| Jul | 3.98 | 3,163 | 46 |
| Aug | 3.05 | 2,424 | 162 |
| Sep | 4.01 | 3,187 | 42 |
| Oct | 3.52 | 2,798 | 103 |
| Nov | 3.27 | 2,599 | |
| Dec | 3.38 | 2,686 |
Sep is the wettest month (4.01 in, 3,187 gal off the roof); Feb the driest (2.60 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 Rockville?
Best fit: one 55-gallon barrel, which delivers about 522 gallons to the garden in a normal year (81.3% of its need).
Before you buy, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Rockville's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 522 | 31,270 |
| 110 gal | 522 | 31,270 |
| 165 gal | 522 | 31,270 |
| 275 gal | 522 | 31,270 |
| 550 gal | 522 | 31,270 |
| 1,000 gal | 522 | 31,270 |
| 1,500 gal | 522 | 31,270 |
| 2,500 gal | 522 | 31,270 |
| 5,000 gal | 522 | 31,270 |
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 Maryland?
No statewide rainwater harvesting regulations; check county for local incentives
From the 2015 (Pacific Northwest National Laboratory report 24347); not yet re-checked against current law, so check with your state water agency before building anything large. Source