Rainwater in an emergency: can you drink it?

The short answer from GOV.UK is clear: if you have outside space, a water butt is worth having, but do not drink the water, because it could contain bacteria or disease. It is fine for flushing toilets and watering plants. For drinking, GOV.UK says bottled water is the safest way to prepare, and recommends 3 litres per person per day for 3 days. Think of rainwater as your backup for everything else, and as a drinking source only if there is truly nothing else.

Why rainwater isn't safe straight from the butt

Rain itself starts fairly clean, but it picks things up on the way. The CDC says that even when rainwater looks clean, it can contain germs and chemicals that could make you sick:

• From the air. Dust, smoke and other particles can get into rain before it reaches your roof. Our note: expect more of this near busy roads, industry or lots of wood burners.
• Bird droppings. The CDC notes that bird poop on your roof can end up in your rain barrel, and the Drinking Water Inspectorate (DWI) says droppings will contain bacteria. This is especially likely when it rains after several dry days (CDC).
• Roofs, gutters and pipes. Chemicals like lead, copper or asbestos can get into rainwater from roofing materials, gutters, piping and storage (CDC). WHO also lists lead, copper and certain metallic paints as materials that can affect the safety of collected rainwater. Our note: many UK roofs have lead flashing around chimneys and in valleys, and some older garages and outbuildings have asbestos cement roofs or gutters.
• Roof treatments. Our suggestion: if your roof has recently been treated for moss or algae, or cleaned with chemicals, don't collect from it, and disconnect the butt until several good downpours have rinsed the roof. Check the product label for any advice on water butts.
• Overhanging trees and aerials. The DWI advises removing or avoiding them over the collection area, and discouraging birds from perching on the roof.

Collecting it well

Most of what keeps rainwater cleaner is simple housekeeping. The DWI's advice for rainwater systems includes:

• Screens or filters on the inlet, and a regular routine for cleaning gutters, to keep leaves and muck out.
• Covers on storage tanks to keep out particles, insects and small animals. A water butt with a fitted lid does this.
• A first-flush diverter, which sends the first rain of a shower to waste. The CDC explains that this first flow picks up the most dirt, germs and chemicals from the roof.
• Approved materials for anything the water runs through, to avoid contaminating it.

Two more points from the CDC: keep the system clean and fix broken parts, and make sure rainwater can never get into the pipes that carry your drinking water. Our suggestion: never connect a water butt to your indoor plumbing, and label it "Not drinking water" so visitors and children know.

If you have no water butt

Our suggestion: in an outage, a clean bucket or storage box left out in the open, away from roofs and trees, collects rain directly without the roof contaminants. It fills slowly, but it is the cleanest rainwater you can get at home.

Storing it safely

• Use the right containers. The CDC recommends food-grade water containers with a tight lid, made of durable, unbreakable material (not glass). Never use anything that has held chemicals, such as bleach or pesticides. Wash with soap and rinse well before filling.
• Keep rainwater separate. The CDC warns that mixing rainwater into treated water, such as bottled or tap water, can make the treated water less safe. Never top up your drinking water store with rainwater.
• Cool, dark and labelled. The CDC advises keeping stored water out of direct sunlight and away from chemicals such as fuel or pesticides, labelling it with the date, and replacing water you have filled yourself every six months.
• Boiled water doesn't keep long. The DWI says to store cooled boiled water in a clean container in the fridge, and throw it away if it isn't used within 24 hours.

What rainwater is good for in a water outage

During an outage, GOV.UK says to keep bottled water for drinking, washing your hands and preparing food. That leaves rainwater for the other jobs:

Based on GOV.UK and CDC advice (see sources below). *Our suggestion, alongside GOV.UK's advice that ready-to-feed formula is the safest option in an outage.

Flushing with a bucket. Our suggestion: you don't need the cistern to work. Pour a bucket of water steadily into the bowl and it will flush. Only flush when you need to, so your rainwater lasts longer.

Making rainwater safer to drink: a last resort

Before you reach for the water butt, check your water company's website. GOV.UK says it will show updates for your area and where you can pick up other water, and if you're on the Priority Services Register, your water company will arrange supplies for you. Soft and fizzy drinks from the cupboard are also safe to drink (GOV.UK).

If there really is no other option, GOV.UK says boiling or water purification tablets can help make other water safer to drink in an emergency. Work through these steps:

1. Settle and filter

If the water is cloudy, the CDC says to filter it through a clean cloth, paper towel or coffee filter, or let it settle and draw off the clear water. GOV.UK adds that most alternative water sources need filtering before you use tablets.

2. Boil: the main method

Boiling is the most reliable way to kill germs. WHO says heating water to a rolling boil is enough to inactivate bacteria, viruses and parasites, and the DWI says boiling also works against parasites such as Cryptosporidium. The DWI's advice is to bring the water to a rolling boil, take it off the heat and let it cool naturally. The CDC goes a little further and suggests keeping it at a rolling boil for 1 minute, which is easy in a pan. Take care, as boiling water scalds.

In a power cut you may not be able to use the kettle or hob. Our suggestion: a camping stove used outdoors works, but never use a camping stove or barbecue indoors, because of the risk of carbon monoxide.

3. Or use purification tablets

GOV.UK says tablets are sold at pharmacies, outdoor and camping shops and online, and come in different strengths, so always follow the label. The CDC explains their limits: chemical disinfectants kill most bacteria and viruses but don't work as well as boiling against the parasites Cryptosporidium and Giardia. Chlorine dioxide tablets do kill Cryptosporidium if you follow the instructions. Avoid iodine-based treatment if you are pregnant or have thyroid problems, and no one should drink iodine-treated water for more than a few weeks (CDC).

4. Filters help, but aren't enough on their own

The CDC says portable water filters don't remove viruses, and only remove bacteria if the absolute pore size is 0.3 micron or smaller (1 micron or smaller for parasites). It advises adding a disinfectant after filtering. Our note: a jug filter made for improving the taste of tap water is not designed to make untreated water safe.

What none of these can fix

Boiling, tablets and filters deal with germs, not chemicals. The US EPA says boiling or disinfection won't destroy heavy metals, salts and most other chemicals, and the CDC says water containing fuel or toxic chemicals can't be made safe by boiling or disinfecting. So rain off a roof with lead flashing, after a moss treatment, or from a butt that has been under flood water is not drinking water even after boiling. Our suggestion: if your water butt has been flooded, don't use it for anything until you have emptied and cleaned it.

Thinking of drinking rainwater long term?

That is a different job from an emergency backup. The DWI says a rainwater system used for domestic purposes, which includes toilet flushing, counts as a private water supply and falls under the private water supply regulations, which local councils enforce. The CDC advises testing rainwater regularly for germs and chemicals if you drink it. Talk to your council's environmental health team before you rely on it.

Further reading: Waterphile's guide to drinking rainwater, which looks at rainwater from the drinking water side.

Related: how much water to store, the 72-hour kit checklist and our Water Outage page.

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