Rainwater tanks are usually buried in the garden soil. Only very rarely are they set up in the house cellar. The collected rainwater can be used in many ways, for the plants in the garden and in the house, but also for flushing the toilet and the washing machine. Whether you use a water tank or a cistern depends on numerous factors. There is a large selection of containers. Their size depends on the roof area and how much water is required. You shouldn’t choose too big. We have compiled everything else that needs to be considered for you.


  • A distinction is made between tanks used above ground and underground
  • Above ground includes rain butts, rain barrels, small to medium garden tanks…
  • Underground include cisterns and tanks
  • Two dominant materials – plastic and concrete
  • Both have advantages and disadvantages
  • The underground tanks are mostly connected to the water system of the house.
  • There are different opinions about the sense and economic viability of this
  • It always depends on why you want to install a rainwater collection tank.
  • Just to save money or for environmental reasons?
  • The price is often the decisive factor in the purchase.

Collect rainwater – how does it work?

In principle it is very simple. Regardless of whether rainwater is collected above ground in decorative collection containers or underground in larger tanks. First of all, the rainwater has to go from the roof into the gutter and from there through a downpipe and preferably a filter. This cleans the water from coarse dirt such as leaves, moss and insects and directs the water on. An overflow valve is usually built into the filter, which prevents the container from overflowing. As soon as it is full, the water is simply passed down the downpipe without taking the branch into the container. The water is stored in the tank and stored until it is used.

Advantages of underground rainwater tanks

  • Large capacity
  • Rainwater has good water quality and you can collect a lot of it to use it
  • Tanks made of polyethylene (plastic) – recyclable, food-safe and thanks to the smooth surface, no dirt can stick
  • Better water values ​​than concrete cisterns (study by the Bavarian State Institute for Viticulture and Horticulture)
  • Easy to install due to their lighter weight
  • Have generous access openings, so that you can easily do any maintenance work.
  • Concrete cisterns – durable (decades), extremely stable and can also be buried in places that are exposed to high loads.
  • Mostly produced locally and recyclable.
  • Safe against buoyancy – when the groundwater level is high, the empty cistern does not rise, as can sometimes happen with plastic tanks
  • However, the installation of a concrete cistern is more complex. A crane is necessary because of the weight. The costs are higher.
  • Leaks can occur if well ring cisterns are installed improperly.
  • If they are connected to the household circuit, the consumption of drinking water is significantly reduced
  • Even if you only use them for garden irrigation, water and waste water can be saved.
Tip: Use filter systems that clean themselves as much as possible. Dirt particles are passed directly into the sewer. The water remains poor in nutrients.

The size of the water tank

The size of the water storage tank depends on the people living in the household, the collection area and the average rainfall for the area. The rainwater yield is made up of the roof area in m² x the precipitation value in m (500mm=0.5m) x 0.9 (roof coefficient – ​​losses through evaporation and filtering).
The rainwater requirement, on the other hand, is determined from the number of people x 8.8 cbm (for toilet flushing) + number of people x 4.5 cbm (for washing machine and cleaning) + m² garden x 0.06 cbm.
At the end, the smaller number of results is multiplied by the loading coefficient 0.06. The result is the volume of the storage tank in cbm. Take the next larger tank size.

Note: Don’t plan the tank too big. First, it’s more expensive, but second, and more importantly, the tank should be overflowed several times a year to flush out dirt particles and pollen that float to the water’s surface.

Rainwater tank – models and costs

An underground rainwater tank is located underground, cannot be seen from above and can only be seen through the tank cover. The tanks are frost-proof. Because it is dark and cold underground, bacteria and algae should not be able to grow. That may be true for algae, but certainly not for bacteria.
In any case, fine dirt particles that are not sorted out by the filter sink to the bottom of the container. Lighter ones, on the other hand, remain on the water surface and are passed on to the sewage system or infiltration via the overflow.
There are numerous suppliers of rainwater tanks. The three largest among the plastic tanks are: GRAF, GARANTIA and 4rain. All also offer the right accessories for the tanks.

The smallest from GRAF is the Carat underground tank with a capacity of 2,700 litres. The tank costs around €900, but you also get a 25-year guarantee. The tank can be driven on up to 8 t, but an appropriate earth cover is necessary for this

  • Length 208cm
  • Width 156 cm
  • Height 201 cm

The largest of the Carat series has a capacity of 13,000 liters and costs a whopping €3,800. This also comes with a 25-year guarantee.

  • Length 239 cm
  • Width 219 cm
  • Height 271 cm
  • Maximum axle load 8 t
  • Maximum total weight 12 t

Flat tanks are also offered by Graf, for example the Platin 5000. These tanks are flat, so you don’t have to dig so deep. There is also a 25-year guarantee for this. The 5000 liter tank costs about 1,600 euros.

  • Length 289 cm
  • Width 230 cm
  • Height 95 cm
  • All GRAF tanks are black

GARANTIA’s underground rainwater tanks are all green. The smallest tank is the CRISTALL 1.600, with the same capacity. The tank costs a good 500 €. There is a 15-year guarantee for this. The normal tanks are not passable. But there are also options for this, albeit for a considerable surcharge.

  • Length 210cm
  • Width 105cm
  • Height 122 cm

The largest water storage tank on offer is the COLUMBUS 9,000, consisting of two 4,500 liter tanks that are connected to each other. You will have to pay almost €2,700 for this, also with a 15-year guarantee. Dimensions per tank:

  • Length 244cm
  • Width 184 cm
  • Height 214 cm

The largest separate tank is COLUMBUS 6500 l, which is available for around €1,800.
There are two different model series from 4rain. Mono tank system of 3,000 l, 4,000 l, 5,000 l and 6,800 l. These cost between €680 and €1,330. This seller gives a 10-year guarantee. The tanks are black.

There is also the Modularis series. A tank holds 2,500 liters. Several tanks are connected together for a higher capacity. There is also a 10-year guarantee here. The tanks look like thermos flasks and are blue. They are installed on edge, so you have to dig deeper. One tank costs around €500, four just under €1,900.

  • Length 119 cm
  • Width 147 cm
  • Height maximum 279 cm

There are three types of concrete cisterns, the shaft ring cistern, the two-part concrete cistern and the monolithic concrete cistern. The shaft ring cistern is made up of individual shaft rings. At the bottom there is a pure base plate and at the top there is a tapered end part. In terms of price, this is the cheapest option. The two-part cistern, on the other hand, consists of a cast container and a conical end piece. The monolithic cistern consists of only one tank, to which all connections are already prepared. It’s the most expensive model.
When it comes to concrete cisterns, there are no comprehensive providers like those made of plastic. Regional providers predominate here, which has to do with transport.
At www.regendsammler.de you will find providers nationwide, a network so to speak, depending on the postal code.

  • A basic version of a monolithic concrete cistern with a capacity of 3200 liters costs €1,100 for the rain collectors, including delivery costs.
  • A similar cistern, only with 17,400 liters is priced at €2,425, also with delivery.
Tip: If you compare the prices, you will quickly see that the plastic tanks in the lower size classes are significantly cheaper than the concrete cisterns. However, this price advantage evens out with increasing volume. From a capacity of around 5,000 liters, the prices are almost the same. However, the freight costs for concrete cisterns are significantly higher. In any case, a price comparison is worthwhile.

Rainwater tank – the installation

We generally recommend hiring a specialist company to install the rainwater tank. This is particularly important if the collected water is to be used for water supply in the household. The installer for the technology should be on site when the tank is installed.

For reasons of cost, however, many homeowners do it themselves. Some suppliers of the tanks offer installation instructions on the Internet, see http://www.graf-online.de/download/einbauanleitungen.html   This describes in detail how to proceed.
It’s not just a matter of digging a hole in the ground, putting the tank in, and then attaching some connections. There is a lot to consider, especially on difficult properties, such as those on slopes and with high groundwater levels. If you follow the instructions, you shouldn’t have any problems. Equivalent instructions are also available from other manufacturers.

Prices for professional installation and connection

These prices cannot be given as a flat rate. It always depends on what is to be fed from the water, how far the pipes go and on the local conditions. It is also important to know whether emergency connections have to be laid if the tank is empty and drinking water is needed for the toilet and washing machine. Some things you can certainly do yourself, for other things a specialist is much better. In any case, you should get more than just an offer.

Frequently Asked Questions

The installation of the water tanks and the use of the water in them for running the house is controversial. Why is that?
It is clear that it makes sense to collect and use rainwater. When it comes to gardening, no one disputes this. On the other hand, there are critics when it is used for the toilet or the washing machine. On the one hand, installation is expensive and the cost-benefit factor is low. Such a system only pays off after 10 to 15 years, sometimes not even then. In any case, you are doing something for the environment.

However, there are repeated warnings about bacteria. They occur when the water is in the tank for a long time, but they can also be in the water beforehand. Rainwater is now quite contaminated with pollutants, more in one area, less in another.

The installation is controversial because the supporters and opponents cannot agree. Nobody wants to give in. As with all things in life, there are not only advantages. However, many “experts” do not want to know anything about the disadvantages or risks. On the other hand, not everything is to be seen negatively, as opponents often portray it and only suspect greed. Perhaps the two parties will soon meet something.

Does the installation of a rainwater tank or cistern require approval?
According to the building regulations (respective state building regulations), installation up to 50 m3 does not require a permit. The tank must be approved if it is a nuisance or if it poses a risk to the general public.

As long as it’s not about the seepage of harmfully contaminated wastewater, the district office and the lower water authority stay out of it. Since the tank has no repercussions on the public network, the water supply companies are not interested either. b you can save sewage fees depends on different things. There are numerous principles in this regard. I would definitely apply for a rainwater exemption.

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