Rainwater is a free, renewable source of water used in domestic and commercial purposes. More recently, more people are seeking more effective ways to ensure that they adequately harvest more rainwater to reduce their reliance on municipal water supply and thus minimize their water bills. Underground rainwater tanks are one of the popular methods being used to store harvested rainwater.

Underground rainwater tanks mainly store water for huge commercial enterprises, but some homeowners may install underground tanks to keep more than enough water for domestic water uses, depending on the tank installed. Some homeowners, however, experience challenges in installing underground water tanks, probably due to incorrect measurements, poor base excavations, and improper digging techniques, among other issues.

This article is thus meant to guide the DIY home-improvement enthusiast the proper procedures of installing an underground water tank. However, we do urge all homeowners to get their rainwater tank supplier to install the underground tank for them. This way, they won’t have to worry about getting the correct equipment and permits, breaking any local legislation, etc., when they start digging.

Choosing a rainwater tank fit for underground installation

First and foremost, you will have to consider the type of rainwater tank you’ll be using for your rainwater collection system. When consumers think of installing their rainwater tanks underground, the primary material with which the tank is built is concrete. Concrete is an ideal material since concrete is a natural insulator, which will keep stored water cooler for longer.

The lower temperature inside the concrete underground rainwater tank inhibits the growth of fungi and other bacteria, which means the water may be safe for drinking (provided you install the correct purification equipment). However, this does not mean you will not have to expel the stagnant water from time to time. This keeps the rainwater stored in the tank fresh, which means lesser wear and tear for your pumping, filtering, and purifying equipment.

For a less complicated build, consider under deck rainwater tanks instead as they can be installed in your basement or under your deck. This way, you won’t have to dig holes in your property and risk damaging gas or water pipes.

Starting excavation

After selecting the right rainwater tank for your needs, measure its dimensions first before you start digging. You need to get the correct measurements of both the length and width of your water tank. Remember to leave room for adjustments as you will need some more space for connecting the pipes and pumps, where necessary. You should add about 18 to 24 inches to the tank’s measurements to get the correct size of the hole. This ensures that the water tank fits perfectly in the hole.

Preparing the base of the hole

You need to make a flat base in the hole being excavated; otherwise, roots or other objects could cause punctures on the water tank, which then leads to leakages. Consider pea gravel elements to provide a good cover at the base of the hole. However, the gravel should be leveled to effectively fit the tank on the hole. Be aware of sharp objects in the pea gravel as this would also puncture the tank as well.

Setting the tank

Insert the tank in the middle of the hole and ensure that it fits correctly, leaving equal spaces on all sides. Push the tank in back-and-forth movements to ensure that it effectively fits in the hole.

Installing connections

Make installations of bulkhead fittings to ensure that the water tank is firmly set on the ground and the tank is not subject to movement or stress. You also need to be careful as extreme compression may also cause some punctures on the tank. If your underground rainwater tank is not directly beneath your rainwater harvesting system, you may need additional pumps and fixtures to transfer the rainwater from the conveyance system (i.e., your roof gutter) to your tank.

Adding risers

Remove the gasket and the water tank lids to add the risers. You may put apply silicone around the tank and screw the risers with the supplied bolts and nuts. You can add the lid and the gasket after all risers have been installed in the water tank.


Fill the remaining spaces with pea gravel to ensure that the tank is firmly set on the ground. You should also mound the soil near the tank surface to avoid drainage from the underground water tank. You should also be careful to avoid directing objects into the tank. Once the tank is installed, you need to correctly fit your rainwater harvesting system to direct the rainwater into the tank.

Installing an underground rainwater tank can be a simple process if all procedures are strictly followed. Regular checks on the underground tanks are also needed to see that the tank is safe from any punctures. Make sure the rainwater being collected into the tank is properly filtered and treated for any bacterial growth, if the harvested rainwater will be used for drinking or kitchen prep.

Although there is a sense of accomplishment in installing fixtures or upgrading your house by yourself, the best way to go about installing rainwater tanks—underground or above ground, for that matter—is to hire professionals to do it for you. This way, you can be sure your tank is installed properly and all parts are working according to standard.