Rainwater normally lands on the roof of a house, flows through the gutters and disappears into a drain. At the same time, treated drinking-quality mains water is being used for jobs that do not necessarily require drinking water.
Rainwater harvesting attempts to make better use of this free resource.
At its simplest, this means connecting a water butt to a downpipe and using the collected water on the garden. At the other end of the scale, a whole-house rainwater-harvesting system can collect thousands of litres in a storage tank and automatically pump it back into the house for suitable non-drinking uses.
For a homeowner trying to substantially reduce water bills, a properly designed rainwater-harvesting system is much more interesting than a single water butt.
What Can Collected Rainwater Be Used For?
Untreated harvested rainwater should not simply be connected to every tap in the house. Drinking, cooking and personal hygiene require appropriate water quality.
However, a significant amount of household water is used for purposes that do not require drinking-quality water.
Depending on the design of the system, harvested rainwater can potentially be used for:
- Toilet flushing.
- Garden irrigation.
- Greenhouse watering.
- Washing cars.
- Outdoor cleaning.
- Other suitable non-potable uses.
- Washing machines where the system has been appropriately designed and installed for that use.
This makes rainwater harvesting particularly interesting for a home with a productive vegetable garden, greenhouse or other significant outdoor water demand.
Three Levels of Rainwater Collection
1. Water Butt – Cheapest and Simplest
A water butt is connected to a roof downpipe using a rainwater diverter.
When it rains, some of the roof water is diverted into the container. Once the container is full, excess water continues through the normal drainage system.
This is inexpensive and particularly useful for:
- Watering plants.
- Filling watering cans.
- Greenhouses.
- Washing the car.
- Outdoor cleaning.
Multiple water butts can also be linked together.
A pump can be added if enough pressure is required to operate a hose.
Best for: Someone who wants a cheap and easy way to reduce mains-water use in the garden.
Disadvantage: A few hundred litres can disappear remarkably quickly during a dry summer, and it does little to reduce water consumed inside the house.
2. Large Garden Rainwater System
The next step is a much larger storage tank collecting water from one or more roof areas.
This might contain hundreds or thousands of litres and use a pump to supply garden taps, hoses or irrigation systems.
For a Homestead Project-style productive garden, this can make considerably more sense than several small water butts.
The system could potentially supply:
- Vegetable beds.
- Greenhouses.
- Fruit trees.
- Containers.
- Automatic irrigation.
- Garden taps.
- Car washing.
Best for: Homes with substantial gardens, greenhouses or high summer watering requirements.
It is still relatively simple because the collected water doesn’t necessarily need to be connected to the plumbing inside the house.
3. Whole-House Rainwater Harvesting
For someone seriously trying to reduce a metered water bill, this is the most interesting option.
A typical system works approximately like this:
House roof
↓
Gutters and downpipes
↓
Leaf/debris filtration
↓
Large storage tank
↓
Pump and controls
↓
Separate rainwater pipework
↓
Toilets / garden / other suitable non-potable uses
The tank is commonly installed underground, although above-ground systems are also possible.
When rain is plentiful, the house uses stored rainwater for the connected services.
When the tank becomes too low, a correctly designed system can provide a mains-water backup so the toilets and other connected appliances continue working.
The homeowner therefore doesn’t need to manually switch between rainwater and mains water every time the tank empties.
What Is the Best System?
For a typical existing house where the objective is to make a meaningful reduction in mains-water consumption, the most capable solution is likely to be:
A filtered, underground rainwater tank with automatic pumping and mains-water backup, supplying toilets and outdoor water uses.
Where appropriate and correctly designed, additional non-potable uses may also be considered.
An underground tank has several advantages:
- It doesn’t occupy valuable garden space above ground.
- Large volumes can be stored.
- The stored water is protected from sunlight.
- It can supply the house automatically.
- It can collect from a substantial roof area.
- It can support garden irrigation.
- The system can automatically fall back to mains water when necessary.
However, it is substantially more expensive and disruptive to install than a water butt.
For someone who only wants cheaper garden watering, it would usually be excessive.
How Much Rainwater Could My Roof Collect?
The amount potentially available depends principally on:
- Roof collection area.
- Local annual rainfall.
- Roof material and runoff characteristics.
- Filtration losses.
- Tank capacity.
- How regularly the stored water is used.
A useful first approximation is:
1 mm of rain falling on 1 m² of roof = 1 litre of water.
So, before allowing for losses, a 100 m² collection area receiving 900 mm of rain in a year represents:
100 × 900 = 90,000 litres
of rainfall landing on that collection area.
Not all of that will end up as usable stored water. Some is lost through splash, overflow, filtration and other inefficiencies.
Nevertheless, the calculation demonstrates why a house roof can potentially collect a surprisingly large quantity of water over a year.
Bigger Isn’t Always Better
It may seem logical to install the largest tank possible, but this is not necessarily economical.
An oversized tank costs more and may spend much of its life partly empty. A tank that is too small, on the other hand, will frequently overflow during wet periods and then run dry during prolonged dry weather.
Professional rainwater-harvesting systems are therefore sized using factors including:
- Local rainfall.
- Roof collection area.
- Expected demand.
- Number of occupants.
- Intended uses.
- Seasonal variations.
Current UK sustainable-drainage guidance refers to BS EN 16941 for rainwater-harvesting system design.
The objective is not necessarily to capture every drop that ever lands on the roof. It is to find a sensible balance between storage cost and useful water savings.
What Happens During a Drought?
This is an important limitation.
The period when the garden needs the most water is also the period when the least rain may be falling.
A rainwater tank therefore cannot guarantee unlimited free water.
A large tank can carry stored water from wetter periods into dry spells, but a sufficiently long dry period can eventually empty it.
For an indoor system, an appropriately designed mains-water backup can maintain essential services when stored rainwater is unavailable.
For a productive garden, additional strategies can make stored rainwater last much longer:
- Drip irrigation.
- Mulching.
- Soil with good organic matter.
- Watering at appropriate times.
- Larger storage capacity.
- Collecting from garages, sheds and greenhouses as well as the house.
- Choosing drought-tolerant crops where appropriate.
Rainwater harvesting therefore works particularly well as part of a wider water-efficiency strategy.
Is Rainwater Free?
The rain is free.
The system isn’t.
A simple water butt can be inexpensive, whereas a large underground system requires equipment and potentially significant excavation and plumbing work.
There can also be a small electricity cost for operating the pump.
For this reason, a rainwater-harvesting system should not automatically be viewed as a financial investment with a rapid payback.
Its value can also come from:
- Reduced dependence on mains water.
- Greater resilience during dry weather.
- A readily available supply for food growing.
- Reduced demand for treated drinking water.
- Reduced rainwater runoff from the property.
- Potentially lower metered water bills.
Someone without a water meter will not normally see the same immediate financial saving from reducing the volume of mains water consumed.
Do I Need a Licence to Collect Rainwater?
Ordinary harvested rainwater is treated differently from taking water from a river, stream or groundwater.
The Environment Agency states that an abstraction licence is not required where the water consists solely of harvested rainwater and has not entered inland surface water or groundwater.
Different rules can apply if harvested rainwater is combined with water abstracted from another source.
Can Rainwater Be Connected Directly to the Mains Plumbing?
This needs to be done correctly.
A rainwater supply must not be allowed to contaminate the drinking-water supply. Appropriate separation, backflow protection, identification of pipework and other requirements therefore apply.
A whole-house rainwater system should consequently be properly designed and installed rather than created by simply connecting a rainwater tank to existing cold-water pipes.
This is particularly important where the system includes automatic mains-water backup.
What About Greywater?
Rainwater harvesting and greywater recycling are different.
Rainwater harvesting collects water falling onto the property.
Greywater recycling attempts to reuse water that has already been used, such as water from showers, baths and wash basins.
Greywater can potentially reduce mains-water consumption further, but treatment, storage and hygiene requirements make the system more complicated.
For many homes, rainwater harvesting is the simpler place to start.
Is Rainwater Harvesting Worth It?
The answer depends on what the homeowner is trying to achieve.
A water butt is probably worthwhile if:
- There is a garden.
- The house already has accessible guttering.
- The objective is inexpensive water for plants.
- Installation needs to be simple.
A large garden system may be worthwhile if:
- A lot of food is being grown.
- There is a greenhouse.
- Summer water demand is high.
- There is sufficient roof area.
- There is room for a larger tank.
A whole-house system becomes more interesting if:
- The property is on a water meter.
- Several people live in the house.
- Toilet flushing represents significant mains-water consumption.
- There is substantial roof area.
- There is space for a tank.
- Excavation can be carried out reasonably easily.
- The property is already undergoing major renovation.
- Reducing dependence on mains water is an important objective.
Retrofitting an underground system purely to save money may be difficult to justify if extensive excavation and replumbing are required.
Installing one while a house, extension, driveway or garden is already being substantially renovated can make considerably more sense.
The Homestead Approach
For someone attempting to create a genuinely lower-cost, more self-sufficient home, there is no need to jump immediately from mains water to an expensive underground system.
A sensible progression is:
Stage 1 — Reduce demand
Fix leaks, use efficient fixtures and avoid unnecessary water consumption.
Stage 2 — Collect easy rainwater
Connect water butts to available roofs.
Stage 3 — Increase storage
Add larger tanks and collect water from the house, garage, greenhouse and other suitable roofs.
Stage 4 — Make garden watering efficient
Use stored rainwater with drip irrigation and other low-water growing techniques.
Stage 5 — Consider whole-house harvesting
If the numbers justify it, investigate an automatically pumped system supplying toilets and other appropriate non-potable uses.
The objective should not be to install the most impressive system available.
It should be to determine how much mains water the household can realistically replace, how much storage is needed to achieve it and whether the resulting saving justifies the installation cost.