Guide to Underground Water Tanks

Underground water tanks in the UK are becoming increasingly popular due to their various benefits, such as saving space, reducing the environmental impact, and providing a reliable water source during droughts or water restrictions. Here is a guide to help you understand and install an underground water tank in the UK.


Determine the Purpose

Decide why you want an underground water tank. It could be for rainwater harvesting, irrigation, or to store water for emergencies. Knowing the purpose of the tank will help you choose the right size and type of tank. required.


Choose the right tank

There are different types of underground water tanks available in the UK, including polyethylene tanks, concrete tanks, and fibreglass (GRP) tanks. Considering factors such as durability, capacity, space availability, and cost when selecting the tank.

Polyethylene Tanks

Polyethylene underground water tanks have gained popularity due to their numerous advantages. Firstly, these tanks are lightweight, making them easy to transport and install. They require less labour and equipment during installation, resulting in cost savings. Additionally, their flexibility allows them to withstand ground movements, reducing the risk of cracking or damage.

On the downside, polyethylene tanks have limitations in terms of capacity. They are available in smaller sizes, making them more suitable for residential or small-scale applications. Moreover, they are not as durable as concrete tanks and may be susceptible to punctures or damage from sharp objects.

Cost is another consideration, as polyethylene tanks can be relatively more expensive compared to other materials. However, their lower installation and maintenance costs may offset this initial investment.


Concrete Tanks

Concrete underground water tanks have their own set of advantages and disadvantages. One of the major advantages of concrete tanks is their durability. They are known for their strength and longevity, with the ability to withstand harsh weather conditions and external forces. Concrete tanks also have a larger capacity, making them suitable for commercial or industrial applications.

Another advantage of concrete tanks is their thermal properties. They have excellent insulation, which helps to maintain the temperature of the stored water. 

Along with advantages, there are some disadvantages to consider with concrete underground water tanks. Firstly, concrete tanks are heavy and require heavy machinery for installation, resulting in higher installation costs. They also have a longer installation time compared to other options.

Another drawback is the possibility of cracks or leakages over time. Concrete is prone to cracking due to ground movement or settling, which can lead to water loss or contamination.

Fibreglass (GRP) Tanks

GRP (glass reinforced plastic) underground water tanks are lightweight in nature, making them easier to handle and install compared to concrete tanks. Their lightweight construction also reduces transportation costs. Additionally, fibreglass tanks are highly resistant to corrosion, making them suitable for long-term use without the risk of rust or degradation.

Another advantage is their versatility in terms of shape and size. Fibreglass tanks can be customised to fit specific requirements, allowing for a tight fit in limited spaces. They also have excellent insulation properties, helping to maintain water temperature.

However, fibreglass (GRP) tanks do have some disadvantages to consider as well. They can be more expensive compared to other options such as plastic tanks, also if they are exposed to UV light, they can degrade over time and weaken the structure, however if in the ground, this will minimise the UV rays affecting it.



Plastic Underground Water Tanks
GRP Underground Water Tanks

Calculate the required size

Determine the amount of water you need to store based on your intended use and the average rainfall in your area. This will help you choose the right capacity for your tank. The most common sizes in the UK range from 1,000 to 10,000 litres. If it's rainwater you are storing, you can use the following link to calculate a rough amount of storage you might require. - 


Check Local Regulations

Before installing an underground water tank, check with your local authorities about any permits or regulations you need to comply with. These regulations may vary depending on your area.


Choose a suitable Location

Look for a location on your property that is easily accessible yet hidden from view. Ensure that the area is away from tree roots, utility lines, and potential flooding zones. Consider the terrain, as the tank will need to be installed on level ground.


Excavation and Installation

hire a professional excavation contractor who specializes in installing underground water tanks. They will excavate the area, prepare the base, and install the tank according to the manufacturer's instructions. It is crucial to follow all safety protocols during the installation process.


Connect the tank to the water source

Depending on your purpose, you may need to connect the tank to a rainwater harvesting system or divert the water from the main water supply. This can be done through the use of gutters, down spouts, and filters.


Set up a filtration system

To ensure the water is clean and safe for use, install a filtration system. This can include a sediment filter, carbon filter, and UV sterilisation if required. Regular maintenance and cleaning of the filtration system are essential to keep the water quality high.


Pump and distribution system

If you plan to use the stored water for irrigation or other purposes, you will need a pump and distribution system. Choose a reliable pump that suits your needs and install pipes or hoses to distribute the water as required.

Water Pumps
Grey Water Pumps

Regular maintenance

Schedule regular maintenance for your underground water tank. This includes cleaning the tank, checking for leaks, and inspecting the filtration system. Additionally, consider installing an overflow system to prevent flooding during heavy rainfall.



Remember that it is always advisable to consult with a professional if you are unsure about any aspect of installing an underground water tank. They will have the expertise and knowledge to help you make informed decisions and ensure a successful installation.