Rainwater Harvesting in Illinois
Illinois (IL) receives an average of 39.2 inches of rainfall per year, placing it in the moderate-rainfall category among US states. The wettest month is typically May, and the driest is Jan. Rainwater harvesting is fully legal in Illinois. A standard 1,500-square-foot asphalt shingle roof can collect approximately 29,080 gal per year, saving an estimated $64.20 annually at the state's average municipal water rate of 0.6 cents per gallon.
Monthly Rainfall in Illinois
Understanding Illinois's monthly rainfall distribution is essential for sizing your storage tank correctly. The table below shows average monthly rainfall and the resulting harvest from a 1,500-square-foot roof. While Illinois's 39.2 inches of annual rainfall is moderate, efficient collection from a well-designed system still produces thousands of gallons. Focus on maximizing collection efficiency with a metal roof and capturing every rain event.
| Month | Rainfall (in) | Harvest (1,500 sq ft) |
|---|---|---|
| Jan | 2.0" | 1,495 gal |
| Feb | 2.0" | 1,495 gal |
| Mar | 2.8" | 2,093 gal |
| Apr | 3.6" | 2,691 gal |
| May | 4.5" | 3,364 gal |
| Jun | 4.2" | 3,140 gal |
| Jul | 3.8" | 2,841 gal |
| Aug | 3.6" | 2,691 gal |
| Sep | 3.2" | 2,392 gal |
| Oct | 3.0" | 2,243 gal |
| Nov | 3.4" | 2,542 gal |
| Dec | 2.8" | 2,093 gal |
| Total | 39.2" | 29,080 gal |
Harvest by Roof Size in Illinois
Your actual harvest depends on how much roof area you connect to your collection system. Not every square foot of your roof needs to drain into your tank — many homeowners start by connecting just one or two downspouts covering a portion of their roof. The table below shows what different collection areas yield annually in Illinois using an asphalt shingle roof at 80% efficiency.
| Roof Area | Annual Harvest | Rec. Tank Size | Annual Savings |
|---|---|---|---|
| 1,000 sq ft | 19,389 gal | 2,500 gal | $64.20 |
| 1,500 sq ft | 29,080 gal | 5,000 gal | $64.20 |
| 2,000 sq ft | 38,776 gal | 5,000 gal | $64.20 |
| 2,500 sq ft | 48,470 gal | 5,000 gal | $64.20 |
Upgrading to a metal roof (95% efficiency) boosts collection by about 19% compared to asphalt shingle. For example, a 1,500-square-foot metal roof in Illinois captures approximately 34,536 gal per year when used for garden irrigation and toilet flushing for a household of two.
Rainwater Harvesting Laws in Illinois
Illinois passed the Rainwater Harvesting Act in 2017, explicitly legalizing collection and encouraging its use. The state offers tax incentives for commercial rainwater harvesting installations.
Regulation Summary for Illinois
- Legal Status: Legal — no restrictions
- Max Collection: No limit
- Permit Required: No — for standard residential systems
- Municipal Water Rate: 0.6 cents per gallon (state average)
Popular Uses for Harvested Rainwater in Illinois
Illinois homeowners use harvested rainwater for a variety of purposes. The most common applications are garden irrigation, lawn watering, and toilet flushing. Each use has different water requirements and may require different levels of filtration. Explore detailed guides for the most popular uses in Illinois:
How Illinois Compares to Similar States
The table below compares Illinois with five states that have similar annual rainfall patterns. All calculations use a standard 1,500-square-foot asphalt shingle roof collecting water for garden irrigation.
| State | Annual Rainfall | Legal Status | Annual Harvest | Water Cost | Annual Savings |
|---|---|---|---|---|---|
| Illinois | 39.2" | Legal | 29,080 gal | 0.6 c/gal | $64.20 |
| Ohio | 39.6" | Legal | 29,155 gal | 0.5 c/gal | $53.50 |
| Washington | 38.4" | Legal | 27,511 gal | 0.6 c/gal | $64.20 |
| District of Columbia | 40.8" | Legal | 29,754 gal | 0.9 c/gal | $96.30 |
| Oklahoma | 36.5" | Legal | 27,512 gal | 0.4 c/gal | $42.80 |
| Missouri | 42.2" | Legal | 30,651 gal | 0.5 c/gal | $53.50 |
Tips for Rainwater Harvesting in Illinois
With moderate rainfall of 39.2 inches per year, Illinois requires a more strategic approach to rainwater harvesting. Maximize your collection efficiency by using the largest possible roof area and choosing a high-efficiency roof material like metal (95% capture rate). Size your tank to store at least one month's harvest to bridge typical dry periods. A 1,000-gallon tank is a good starting point for most Illinois homeowners. Connect the tank to a drip irrigation system for maximum efficiency — drip systems use 30-50% less water than sprinklers. In drier months, prioritize high-value uses like vegetable gardens over lawn irrigation.
Calculate Your Specific Harvest in Illinois
The estimates above use standardized assumptions. Use our Rainwater Harvesting Calculator to enter your exact roof area, material, household size, and intended uses for a customized analysis tailored to your property in Illinois. You can also browse all states to compare potential across the country, or explore tank sizes and water uses for detailed information on system components and water budgeting.