Rainwater harvesting: how to collect it, what to use it for and when it's safe
How to collect rainwater from your roof in barrels or tanks, what you can use it for without treatment (garden, toilet, cleaning), why it isn't safe to drink straight, how to treat it if needed and what to know about local rules and mosquitoes.
In short
- A 1,000 sq ft roof collects roughly 500–600 gallons of water per inch of rain. Even a modest roof fills several barrels in one storm.
- Rainwater is great for the garden, flushing toilets and cleaning. It isn't safe to drink straight: it can contain bacteria from birds, dust and metals from the roof.
- In an emergency it can be made drinkable by filtering and boiling or disinfecting, if it comes off a suitable roof.
- Cover your barrels: otherwise they become mosquito nurseries within days.
Rainwater is free and lands right on your roof. Collected in a few barrels, it waters your garden all summer, trims your water bill and, in a crisis when the taps stop, gives you hundreds of gallons for flushing, washing and cleaning. With the right treatment, it can even become drinking water in a pinch.
How much you can collect
The simple formula: gallons ≈ roof area (sq ft) × rainfall (inches) × 0.5.
(In metric: liters ≈ roof area (m²) × rainfall (mm) × 0.8.) The factor accounts for losses: evaporation, splash and the first water that washes the roof.
| Roof | 0.25 inch of rain | 0.5 inch | 1 inch |
|---|---|---|---|
| 500 sq ft | 60 gal | 125 gal | 250 gal |
| 1,000 sq ft | 125 gal | 250 gal | 500 gal |
| 1,500 sq ft | 190 gal | 375 gal | 750 gal |
How to collect it
- A barrel under a downspout, with a diverter that sends water in and keeps leaves out.
- A sealed, opaque lid to keep out light, debris and mosquitoes.
- A low spigot and an overflow that carries excess away from the foundation.
- Raise the barrel on blocks or a stand so you can fill a bucket easily.
- For larger volumes, link several barrels or install a tank.
A first-flush diverter discards the first gallons, which wash dust and dirt off the roof. Worth it if you want cleaner water.
What to use it for
| Use | Without treatment? |
|---|---|
| Watering the garden and plants | Yes |
| Flushing toilets | Yes |
| Cleaning, washing the car | Yes |
| Laundry | Usually yes (rainwater is soft) |
| Handwashing, dishes | Only after treatment |
| Drinking, cooking | Only after filtering and disinfecting |
If you need to drink it
Rainwater can contain bacteria and parasites from birds and animals, dust, pollen and, depending on the roof, metals. In an emergency:
- Let it settle and strain it through a clean cloth.
- Filter it with a suitable water filter. See our water filter guide.
- Disinfect it: boiling, chlorine or purification tablets.
Filtering and disinfecting don’t remove chemicals or heavy metals from unsuitable roofs. See emergency water purification.
Mosquitoes
An open barrel can breed mosquitoes within 7–10 days. Keep it sealed, put fine mesh over any opening and use the water often. Empty standing water from buckets, pots and toys.
Winter
Drain barrels and hoses before the first freeze: ice cracks plastic and spigots.
For drinking water storage, see storing water in containers.
Recommended equipment
What to look for when buying. Links go to partner shops; if you buy through them we may earn a small commission at no extra cost to you.Details.
Rain barrel with spigot
Sits under a downspout; the simplest system.
- 50–80 gallons (200–300 L)
- Sealed, opaque lid
- Low spigot and overflow
- UV-resistant plastic
Downspout diverter with filter
Diverts downspout water to the barrel and keeps leaves out.
- Fits your downspout size
- Leaf filter
- Automatic overflow when the barrel is full
Frequently asked questions
Is collecting rainwater legal?
In most places, yes, from your own roof for non-drinking use and gardening. A few US states regulate how much you can collect or require permits for larger systems, and rules vary by city and country. Check locally before installing a large or plumbed-in system. Rainwater must never be connected to your drinking water supply.
Which roofs shouldn't I collect from?
Avoid asbestos-cement roofs, roofs treated with biocides, lead-based paints, and old copper or treated wood shake roofs. For vegetable gardens and especially for drinking, metal roofing with a safe coating, clay or concrete tiles, or materials rated for water collection are best.
How long does rainwater keep in a barrel?
A few weeks if the barrel is closed, opaque and shaded. In open or translucent barrels, algae and mosquito larvae appear quickly. For the garden that's not a big deal; for anything else keep it sealed and use it often.
Sources and further reading
Preper is for general education and does not replace instructions from your local authorities or advice from a doctor. In an emergency, call your local emergency number.