Derby, CT rainwater: 41,354 gallons off a roof

Rainwater from a roof in Derby, CT

41,354gallons a year

That is a 1,500 sq ft asphalt roof under Derby, CT's normal 52.0 inches of rain, about 795 gallons for every inch that falls.

In Derby, CT 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 Ansonia 1 Ne station (1.7 mi away). Derby, CT ranks 6th wettest of the 21 cities covered in Connecticut.

Month by month in Derby

Month	Rain (in)	Off the roof (gal)	Garden shortfall (gal)
Jan	3.95	3,139
Feb	3.62	2,877
Mar	4.72	3,751
Apr	4.72	3,751
May	4.04	3,211	39
Jun	3.93	3,124	52
Jul	4.30	3,418	6
Aug	4.56	3,624
Sep	4.38	3,481
Oct	5.00	3,974
Nov	4.24	3,370
Dec	4.57	3,632

Oct is the wettest month (5.00 in, 3,974 gal off the roof); Feb the driest (3.62 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 Derby?

Best fit: one 55-gallon barrel, which delivers about 74 gallons to the garden in a normal year (76.1% 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 Derby's roof water will pass straight through it.

Tank	Water used (gal/yr)	Spills over (gal/yr)
55 gal	74	40,065
110 gal	74	40,065
165 gal	74	40,065
275 gal	74	40,065
550 gal	74	40,065
1,000 gal	74	40,065
1,500 gal	74	40,065
2,500 gal	74	40,065
5,000 gal	74	40,065

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 Connecticut?

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