Chapel Hill, NC rainwater: 34,622 gallons off a roof
Chapel Hill, NC: gallons off the roof each year
34,622gallons a year
That is a 1,500 sq ft asphalt roof under Chapel Hill, NC's normal 43.6 inches of rain, about 795 gallons for every inch that falls.
In Chapel Hill, NC 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 Chapel Hill Williams Ap station (1.3 mi away). Chapel Hill, NC ranks 83rd wettest of the 91 cities covered in North Carolina.
Month by month in Chapel Hill
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 3.39 | 2,694 | |
| Feb | 2.75 | 2,186 | |
| Mar | 3.94 | 3,132 | |
| Apr | 3.56 | 2,830 | 98 |
| May | 3.46 | 2,750 | 111 |
| Jun | 4.36 | 3,465 | |
| Jul | 4.13 | 3,283 | 27 |
| Aug | 3.81 | 3,028 | 67 |
| Sep | 4.41 | 3,505 | |
| Oct | 3.20 | 2,543 | 143 |
| Nov | 3.20 | 2,543 | |
| Dec | 3.35 | 2,663 |
Sep is the wettest month (4.41 in, 3,505 gal off the roof); Feb the driest (2.75 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 Chapel Hill
Best fit: one 55-gallon barrel, which delivers about 358 gallons to the garden in a normal year (80.0% 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 Chapel Hill's roof water will pass straight through it.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 358 | 33,123 |
| 110 gal | 358 | 33,123 |
| 165 gal | 358 | 33,123 |
| 275 gal | 358 | 33,123 |
| 550 gal | 358 | 33,123 |
| 1,000 gal | 358 | 33,123 |
| 1,500 gal | 358 | 33,123 |
| 2,500 gal | 358 | 33,123 |
| 5,000 gal | 358 | 33,123 |
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 North Carolina?
Regulated through the Department of Environment and Natural Resources; allowed through state’s plumbing code; 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