Flint, MI rainwater: 26,133 gallons off a roof

Rainwater from a roof in Flint, MI

26,133gallons a year

That is a 1,500 sq ft asphalt roof under Flint, MI's normal 32.9 inches of rain, about 795 gallons for every inch that falls.

In Flint, MI 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 Flint 7w station (4.0 mi away). Flint, MI ranks 79th wettest of the 90 cities covered in Michigan.

Month by month in Flint

Month	Rain (in)	Off the roof (gal)	Garden shortfall (gal)
Jan	2.15	1,709
Feb	1.67	1,327
Mar	1.87	1,486
Apr	3.41	2,710	117
May	3.68	2,925	84
Jun	3.06	2,432	161
Jul	3.35	2,663	125
Aug	3.34	2,655	126
Sep	2.88	2,289	183
Oct	2.97	2,361	172
Nov	2.37	1,884
Dec	2.13	1,693

May is the wettest month (3.68 in, 2,925 gal off the roof); Feb the driest (1.67 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.

What size rain barrel for Flint

Best fit: one 55-gallon barrel, which delivers about 765 gallons to the garden in a normal year (79.1% 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 Flint's roof water will pass straight through it.

Tank	Water used (gal/yr)	Spills over (gal/yr)
55 gal	765	24,288
110 gal	765	24,288
165 gal	765	24,288
275 gal	765	24,288
550 gal	765	24,288
1,000 gal	765	24,288
1,500 gal	765	24,288
2,500 gal	765	24,288
5,000 gal	765	24,288

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 Michigan?

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