Gaithersburg, MD rainwater: 37,117 gallons off a roof
How much rain a roof in Gaithersburg, MD collects
37,117gallons a year
That is a 1,500 sq ft asphalt roof under Gaithersburg, MD's normal 46.7 inches of rain, about 795 gallons for every inch that falls.
In Gaithersburg, 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 Gaithersburg 2 Wnw station (2.2 mi away). Gaithersburg, MD ranks 11th wettest of the 23 cities covered in Maryland.
Month by month in Gaithersburg
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 3.27 | 2,599 | |
| Feb | 3.20 | 2,543 | |
| Mar | 4.05 | 3,219 | |
| Apr | 3.76 | 2,988 | 74 |
| May | 4.55 | 3,616 | |
| Jun | 4.77 | 3,791 | |
| Jul | 4.28 | 3,402 | 9 |
| Aug | 3.81 | 3,028 | 67 |
| Sep | 4.40 | 3,497 | |
| Oct | 3.72 | 2,957 | 79 |
| Nov | 3.36 | 2,671 | |
| Dec | 3.53 | 2,806 |
Jun is the wettest month (4.77 in, 3,791 gal off the roof); Feb the driest (3.20 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 Gaithersburg
Best fit: one 55-gallon barrel, which delivers about 179 gallons to the garden in a normal year (78.5% 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 Gaithersburg's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 179 | 35,814 |
| 110 gal | 179 | 35,814 |
| 165 gal | 179 | 35,814 |
| 275 gal | 179 | 35,814 |
| 550 gal | 179 | 35,814 |
| 1,000 gal | 179 | 35,814 |
| 1,500 gal | 179 | 35,814 |
| 2,500 gal | 179 | 35,814 |
| 5,000 gal | 179 | 35,814 |
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