Joliet, IL rainwater: 31,967 gallons off a roof

How much rain a roof in Joliet, IL collects

31,967gallons a year

That is a 1,500 sq ft asphalt roof under Joliet, IL's normal 40.2 inches of rain, about 795 gallons for every inch that falls.

In Joliet, IL 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 Joliet 2.9 Wnw station (2.2 mi away). Joliet, IL ranks 103rd wettest of the 225 cities covered in Illinois.

Month by month in Joliet

Month	Rain (in)	Off the roof (gal)	Garden shortfall (gal)
Jan	2.17	1,725
Feb	1.90	1,510
Mar	2.50	1,987
Apr	3.95	3,139	50
May	4.67	3,712
Jun	4.77	3,791
Jul	4.40	3,497
Aug	4.28	3,402	9
Sep	3.28	2,607	133
Oct	3.39	2,694	120
Nov	2.74	2,178
Dec	2.17	1,725

Jun is the wettest month (4.77 in, 3,791 gal off the roof); Feb the driest (1.90 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 Joliet?

Best fit: one 55-gallon barrel, which delivers about 253 gallons to the garden in a normal year (81.2% 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 Joliet's roof water will pass straight through it.

Tank	Water used (gal/yr)	Spills over (gal/yr)
55 gal	253	30,664
110 gal	253	30,664
165 gal	253	30,664
275 gal	253	30,664
550 gal	253	30,664
1,000 gal	253	30,664
1,500 gal	253	30,664
2,500 gal	253	30,664
5,000 gal	253	30,664

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 Illinois?

Local government can regulate rainwater harvesting per state plumbing law

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