What is a rainwater harvesting system?
A basic system collects rain from a roof, carries it through gutters and downspouts, filters out debris and stores it in a tank or barrel. From there it is used by gravity or a pump. Systems range from a single barrel to large tanks with filtration.
The size and design depend on roof area, rainfall, storage space and what you plan to use the water for.
Even a small roof collects a surprising volume in a heavy rain, so plan where overflow will go before the first storm fills the barrel.
What are the main parts?
Typical parts include the catchment surface (the roof), gutters and screens, a way to divert the first rush of runoff that carries dust and debris, a storage tank with a cover and screened inlets and outlets, an overflow and a distribution method.
Each part affects water quality and reliability. Ask a supplier or your extension office to explain how the parts fit together in your climate.
A first-flush device sends the dirtiest early runoff away from the tank. Ask your supplier how it works and how often it needs cleaning.
- Roof and gutters
- Screens and first-flush diversion
- Covered storage with screened openings
- Overflow to a safe place
- Pump or gravity distribution
What can I use the water for?
Many people use harvested rain for garden watering, which is the simplest use. Other uses, such as livestock drinking water, washing or household use, raise water quality and legal questions, and may require treatment, testing and approvals.
Do not drink or cook with harvested rainwater unless the system was designed for it, treated and tested according to your local health authority. Roofs, gutters and tanks can add contaminants.
For livestock, ask your veterinarian and extension office whether harvested water is suitable for your animals, since quality needs differ by species and by roof and tank materials.
What about water quality and roof materials?
Roofing, gutters and flashing can add metals, chemicals, bird droppings and debris. Some materials are better suited than others, and treated or old roofing may not be appropriate for certain uses. Ask your local health department or extension office about water testing and safe uses.
If you use water on edible crops, apply it to the soil rather than to leaves where possible, and ask your extension office about food-safety guidance.
Some water tests are inexpensive and available through your county health department or state lab. Ask which tests suit your intended use.
How do I control mosquitoes and keep the tank safe?
Standing water breeds mosquitoes. Keep tanks and barrels covered, with fine mesh on every opening, and keep the overflow screened. Clean screens and gutters regularly and check for cracks and leaks.
Tanks should be opaque to limit algae and placed on a stable, level base, since full tanks are very heavy. Keep children and animals from falling in, and never enter a tank. Confined spaces are dangerous.
Compare notes with neighbors who already collect rain, since local experience with roofs, tanks and winters is hard to find in books.
- Fine mesh on every opening
- Cover and opaque walls
- Level, stable base
- Regular cleaning of gutters and screens
- No entry into tanks
What rules apply?
Rules on collecting rainwater vary by state and sometimes by county and town. Some places encourage it, some limit the amount or the use, and building, plumbing and health codes may apply to larger systems or to any connection to a house. Water rights vary in many regions.
Call your county building office, your state water resources office and your local health department before you buy or install anything. Plumbing and electrical connections should be done by licensed professionals where rules require.
Freezing weather can crack tanks, pipes and pumps. Ask your supplier how to drain or insulate a system for winter in your climate.
How do I size a system and plan costs?
Estimate supply from roof area and local rainfall, and demand from what you will water or feed. Ask your extension office for local rainfall data. Plan storage for the dry periods you want to bridge.
Cost categories include tanks, gutters, screens, first-flush parts, base preparation, pumps, plumbing, permits and maintenance. Prices vary widely, so compare local suppliers and start small. A single barrel for the garden teaches a lot before you invest in more.
Keep a log of rainfall, tank levels and maintenance. After a season you will know how much the system really supplies.
Key takeaways
- A harvesting system has a roof, gutters, filtering, storage, overflow and distribution.
- Garden use is simplest; other uses need testing, treatment and approvals.
- Cover tanks, screen openings and keep them stable and safe.
- Check local rules and health requirements before installing.
Sources and further reading
Last reviewed: November 12, 2026. Written by the Homestead & Small Farm Editorial Team. See our editorial policy.