Portland, ME rainwater: 38,246 gallons off a roof
Rainwater from a roof in Portland, ME
38,246gallons a year
That is a 1,500 sq ft asphalt roof under Portland, ME's normal 48.1 inches of rain, about 795 gallons for every inch that falls.
In Portland, ME 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 Portland Intl Jetport station (5.9 mi away). Portland, ME ranks 5th wettest of the 11 cities covered in Maine.
Month by month in Portland
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 3.50 | 2,782 | |
| Feb | 3.54 | 2,814 | |
| Mar | 4.08 | 3,243 | |
| Apr | 4.41 | 3,505 | |
| May | 3.67 | 2,917 | 85 |
| Jun | 4.15 | 3,298 | 25 |
| Jul | 3.43 | 2,726 | 115 |
| Aug | 3.57 | 2,837 | 97 |
| Sep | 3.77 | 2,996 | 72 |
| Oct | 5.25 | 4,173 | |
| Nov | 4.25 | 3,378 | |
| Dec | 4.50 | 3,577 |
Oct is the wettest month (5.25 in, 4,173 gal off the roof); Jul the driest (3.43 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 Portland?
Best fit: one 55-gallon barrel, which delivers about 315 gallons to the garden in a normal year (80.0% of its need).
If you set one up, a 55-gallon barrel with a downspout diverter is the whole job. Check that the overflow points away from your foundation, because most of Portland's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 315 | 36,716 |
| 110 gal | 315 | 36,716 |
| 165 gal | 315 | 36,716 |
| 275 gal | 315 | 36,716 |
| 550 gal | 315 | 36,716 |
| 1,000 gal | 315 | 36,716 |
| 1,500 gal | 315 | 36,716 |
| 2,500 gal | 315 | 36,716 |
| 5,000 gal | 315 | 36,716 |
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 Maine?
No rainwater harvesting regulations
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