Once you’ve made the decision to invest in a rainwater harvesting tank, you then need to settle on the type of system you want to install. Here we look at the 2 types of underground tanks and aboveground models, two of the most popular types of rainwater harvesting tanks around.

Shallow Dig Underground Tank

A shallow dig rainwater harvesting tank is exactly as it sounds – a low profile tank that has a slim line design that does not require you to excavate too far into the ground. This can be particularly helpful for gardens that have wet or rocky ground that will make it difficult to dig down too far. A shallow dig system may also be your only option if there is limited space along the side of your home, which will make it difficult to get larger tanks or machinery into the back garden of the property.

Most underground tanks have a capacity of somewhere between 1,000 to 6,000 litres, although it can sometimes be more for larger properties. Of course, a shallow dig system will be at the lower end of this scale, as the slimmer profile means there is a limit on how much capacity it has available.

The hole for a shallow dig tank will be around one metre deep, compared to a few more metres or more for a larger tank. You should be able to excavate using a shovel instead of hiring a digger to make a large enough hole.

Some regions have higher water tables than others, which means you do not have to dig down too far to locate saturated soil. A shallow dig water harvesting system is a good fit for this as there is less underground water to pump away while it is being installed. It also means there is less chance of the tank floating up and breaking through the surface once work has been completed. 

Bulb-Shaped Underground Tank

A bulb-shaped underground tank again is exactly how it sounds. Bulb-shaped tanks take up minimal space at your property, making them a great option to install if you are limited for space and they can be installed in a variety of locations around the the home including; underneath the driveway or patio and in the garden.

Most bulb-shaped tanks range in capacity from 2,500 litres up to 6,500 litres and are relatively easy to install and set-up, the majority of these type of systems can be installed and ready to use within a day.

Bulb-shaped tanks are incredibly durable and tend to be made from sturdy materials such as PVC or polyethylene, making them resistant to punctures and UV damage. In contrast, above-ground rainwater tanks may take up more space than an underground tank. They also require more maintenance, as they are more exposed to the elements.

Overall, a bulb-shaped rainwater harvesting tank is a practical and sustainable choice for collecting and storing rainwater. Its underground design makes it a space-efficient and versatile option, while its durability and low maintenance requirements make it a cost-effective choice.


You’ll find aboveground rainwater harvesting tanks available in a variety of styles and sizes, including simple barrel designs to more decorative models that are intended to mimic outdoor features so they blend in more easily.

The installation process is easier than a belowground model, as you only need a firm level base and enough space. From there you connect to the downpipe and the system is ready to receive rainwater from the roof. For a small garden, an above ground system will usually hold a few hundred litres of rainfall.

One thing to be aware of with aboveground tanks is that they should either be insulated or drained down during the winter. This is because the water could freeze in very low temperatures, which can lead to the pump or other components being damaged. Adding insulation is easy and cost effective to do. You should also ensure the connected pipework is also insulated.

Things to consider to help you decide what the best solution for your requirement.

Check below the surface

Before installation begins, you should get a better understanding of what lies beneath the ground in the area you want to position the tank. If underground utility pipes and other access restrictions are in place, a shallow dig may be your only option. For a deep dig tank, you may want to look at ways of avoiding damage being caused to pathways or other areas of the garden that can be caused by heavy plant machinery.

Ground Water

A high groundwater table can also cause problems with the installation of a rainwater harvesting tank. If the groundwater table is too high, the tank may not be able to be installed deep enough and can in some cases cause the tank to pop-out of the ground. In order to stop this from happening you may need to backfill the tank in concrete which is more expensive than surrounding the tank in pea shingle.

Soil removal

You also need to plan how surplus soil will be removed from the site after work has finished. For a shallow dig installation, the hole is smaller, so the amount of excess soil will be far less. It takes time to remove earth and it’s a job that should ideally be done by professionals – so this is a cost implication that needs to be factored into your total budget. Also make plans for where the earth will be placed while it’s awaiting collection as it can cause some disruption if you are unprepared.

Trench depth

There are some health and safety implications to consider too. If you are installing a larger tank, there should be sufficient support to the sides of your trench. The Health & Safety Executives do not state the depth at which a trench needs to be supported in terms of safety, but they do say you should consider weather and ground conditions and nearby work activities, regardless of the trench depth. Guidelines on the trench depth should be provided by the product provider or taken care of by the installation company.


While maintenance requirements for underground tanks are very low, repairs or maintenance may need to be carried out at a later date. A shallow dig tank will be easier to access than a deep dig installation. Plant machinery may have to be used to access a deep dig tank, depending on the type of work that needs to be carried out.

Callum Vallance-Poole

Marketing Coordinator - Based at our UK HQ in Banbury, Oxfordshire, Callum is responsible for promoting Water Management Systems, Attenuation Tanks, Treatment Plants, Rainwater Harvesting Systems and more!

Marketing Coordinator - Based at our UK HQ in Banbury, Oxfordshire, Callum is responsible for promoting Water Management Systems, Attenuation Tanks, Treatment Plants, Rainwater Harvesting Systems and more!