Rainwater Harvesting: What's Legal, What Works, and Is It Worth It?

I have two water butts in my garden. They live against the back fence, connected to the downpipe from the garage roof. Both fill up in about 45 minutes of heavy rain, which in Somerset happens several times a week from October to April.

we use the collected water almost entirely for the vegetable beds and greenhouse. When it is full, I also use it for the outdoor tap to avoid running the mains for outdoor tasks. In a year, those two butts probably save 3,000–4,000 litres of mains water. Not a huge number in the scheme of things — but it is free, it is available even during hosepipe bans, and it involves almost no effort once set up.

Rainwater harvesting has become more fashionable in the resilience conversation recently, partly because water companies in the UK have had some publicised incidents with supply disruptions, and partly because US droughts have made people think more carefully about water independence.

This guide covers both UK and US contexts, because the questions are very different. In the UK, the law and practicalities are straightforward. In the US, things vary by state in ways that matter.

Is Rainwater Harvesting Legal?

In the UK: Yes. There is no legal restriction on collecting rainwater that falls on your own property in England, Scotland, Wales, or Northern Ireland. The water that lands on your roof is yours to collect. Water companies cannot charge you for it. You can collect as much as you like.

There are nuances for large commercial systems, but for household garden use with a water butt, there is nothing to worry about.

In the US: More complicated, and this is where YouTube gets it wrong.

The popular claim is that rainwater harvesting is illegal in most US states. This is not accurate. The full picture:

No US state has a blanket ban on residential rainwater collection. What varies is the volume you can collect, whether a permit is required, and what you can use the water for.

*Most permissive:* Texas, Oregon, Florida, and most western states now explicitly permit and encourage rainwater harvesting with few or no restrictions.

*Previously restricted, now relaxed:* Colorado was frequently cited as a state where harvesting was banned. This changed in 2016 — residential collection up to 110 gallons (two standard rain barrels) is now legal without a permit.

*More regulated:* Washington State regulates roof runoff as part of its water rights system. Small residential barrel collection is generally tolerated but technically regulated. Large-scale collection requires permits.

The practical bottom line for most American families: collecting 50–200 gallons (one to four standard rain barrels) for garden use is legal and unregulated in most US states. If you want to collect large volumes or use it for anything beyond outdoor irrigation, check your state's Department of Natural Resources.

What You Can Realistically Use Rainwater For

Non-potable uses — no treatment needed: - Watering vegetable gardens and flower beds - Washing tools and outdoor equipment - Topping up water features and ponds - Pressure washing paths and patios - Washing vehicles

Potable use — treated first:

Rainwater can be made safe to drink, but it requires proper treatment. Roof runoff picks up particulates, bird droppings, atmospheric pollutants, and potentially lead from older flashings or paint. The treatment chain for drinking water is: 1. First-flush diverter — discards the most contaminated first flush of each rain event 2. Sediment filtration — removes particles 3. Activated carbon filtration — removes taste, odour, chemicals 4. UV treatment or chlorination — kills biological contaminants

This is achievable but it is a proper installation, not a water butt with a tap. For emergency drinking water, it is worth understanding. For routine drinking water, mains water in the UK and most of the US is safer and substantially cheaper than building and maintaining a full treatment train.

My position: rainwater for the garden is excellent and low-effort. Rainwater as a drinking backup requires taking it seriously. We keep a gravity filter for emergency drinking water; the water butts are for the garden.

The Simple Butt and Barrel System

The standard UK setup: - Water butt (100–200 litres) connected to a downpipe via a diverter fitting - Diverter routes rainwater into the butt; once full, it sends overflow back down the pipe - Stand raises the butt high enough to gravity-feed a watering can underneath - Lid keeps out debris and prevents mosquito breeding

The Strata 100L slimline butt is what we use. The slimline design sits flush against a fence without projecting far. The stand is included. One note: a diverter fitting is not included in this model — buy it separately for about £6, or look at the Fine Garden 100L version which bundles one.

For larger gardens: two butts daisy-chained together are easy to set up and double capacity. A 200-litre butt is an alternative if you have the space, but I find two 100-litre units easier to manage — you can disconnect one for cleaning without losing all your storage.

The standard US setup:

Most American suppliers sell rain barrels rather than water butts — functionally identical. Typical capacity is 50–60 gallons (190–230 litres). Look for: - A food-grade plastic barrel (not a recycled chemical drum) - An overflow outlet at the top - A spigot near the bottom (not at the very bottom — you want standpipe clearance to fill a can) - A sealed lid or screen to prevent mosquito breeding

For those who want more volume: IBC tanks (intermediate bulk containers) hold around 275 gallons (1,040 litres). They are often available secondhand cheaply from food manufacturers. Look for food-grade units — avoid anything that previously held chemicals.

Calculating How Much You Can Realistically Collect

The formula: roof area (square metres or square feet) multiplied by rainfall (mm or inches) equals collection volume in litres or gallons, minus losses to evaporation and first-flush discard.

A UK example:

Average UK annual rainfall: around 885mm. Typical garage or outbuilding roof: 30 square metres. Collection efficiency (accounting for loss): around 75%.

30 × 885mm × 75% = 19,913 litres per year from a single outbuilding roof.

At that rate, a 100-litre butt fills roughly once a week on average through a UK year, with much higher frequency in winter and spring. You will overflow regularly in wet months. The question is not whether you can collect enough — it is what you do with it.

For US readers: 1 inch of rain on 100 square feet of roof = approximately 62 gallons.

Example: 500 square foot garage roof, 30 inches of rain per year: 500 × 30 × 0.623 = 9,345 gallons per year.

Enough for substantial garden irrigation from a single roof.

Water Quality: Can You Drink Rainwater?

The short answer is no without treatment — but the risks are more nuanced than "definitely unsafe."

In the UK, roof runoff from modern roofs (post-1970s) is generally free of lead. The main contaminants are bird and bat droppings, atmospheric particulates from traffic, and organic matter if the butt is not maintained.

For garden irrigation, these are not meaningful concerns. Plants are not harmed by trace E. coli on their leaves; soil bacteria process organic matter.

For drinking water, contamination levels are low but not zero, and the first flush is significantly more contaminated than mid-event water. If you wanted to use rainwater as a drinking backup in a genuine emergency, the minimum treatment is: first-flush diversion, sediment filter, ceramic or activated carbon filter (see Best Gravity Water Filter 2026 for options), and boiling or UV treatment. That treatment chain removes the practical risk.

For US Readers: State-by-State Quick Reference

Verify current rules at your state's DNR or Department of Environmental Quality website if planning a larger installation.

Setting Up Your System: Practical Steps

1. Choose your collection point — a downpipe from a roof with good area. Garage roofs are ideal: no household plumbing to affect and typically less shaded by trees.

2. Install a diverter fitting — connects to the downpipe and routes water into your butt. They fit most standard UK or US downpipe diameters. Installation takes about 10 minutes with a hacksaw.

3. Position the butt on a stand — you need at least 15–20cm clearance between the tap and the ground to get a watering can underneath.

4. Add an overflow pipe — critical. Without it, your butt fills up and water runs wherever it likes. Route the overflow back to the downpipe or to a second butt.

5. Maintain it — once or twice a year: empty the butt, rinse it out, check the lid seal, clear the diverter of leaf debris. We do this in March and October.

Frequently Asked Questions

How much does a basic system cost?

In the UK: a 100-litre water butt with stand and lid costs around £30–50. A diverter fitting is around £5–10 separately. Total setup: £35–60. Two butts and all fittings: under £120.

In the US: a 50-gallon rain barrel typically costs $50–120. A complete setup with stand and diverter is $80–150.

Does water go stagnant in a butt?

If you have a sealed lid and are using the water regularly (the butt empties and refills), no. If water sits untouched for months, algae can develop. This is not a safety issue for garden use, but it can affect quality if you were treating it for drinking. Keep the lid on and use the water regularly.

Can I harvest rainwater from a green or living roof?

Not for anything other than general garden irrigation. Green roof runoff contains organic matter and fertiliser residue from the substrate. Fine for irrigating beds, not suitable for any use requiring clean water.

How long does a water butt last?

A quality plastic water butt lasts 10–15 years with normal use. UV degradation is the main issue — a butt in full sun will not last as long as one in partial shade. The Strata butt referenced above has a 5-year guarantee.

Can we use collected rainwater on vegetables?

Yes, including edible crops. The trace contamination in UK and US roof runoff is not a meaningful risk for vegetable growing. Wash vegetables before eating as you normally would. we has been using collected water on her vegetable beds for several years without any concern.

Related Guides

Filtering collected water for emergencies: Best Gravity Water Filter 2026 How much water to store: How Much Water Should Your Family Store? Long-term storage containers: How to Store Drinking Water Long-Term Water purification methods: Water Purification Methods Compared

For the containers to store what you collect, the water storage containers guide covers the best options.