Which size / design for rainwater cistern?

Hello,

we would also like to have a rainwater cistern installed solely for garden irrigation. Using the water for toilet flushing or the washing machine is not an option for us for the reasons mentioned in the other thread.

Our property is about 400 sqm in size; once the garages and the house are in place, approximately 260 sqm of garden area will remain.

How large should our cistern be?

The rainwater is collected from both roof sections of the house and both garages.

Is it better to choose a concrete or a plastic cistern?

The tank is to be buried beneath our driveway – could this possibly also influence the choice of material?

Regards,

Dirk

That depends on the precipitation in the region. If it rains rarely but heavily, a larger one makes sense.

Our architect had to include that on the drainage application for the city.

According to the development plan, we have to build a cistern. The calculated size (according to regulations) is around 2,200 liters. However, it will be built for (I don't remember exactly) a bit over 4,500 liters. The garden is 430 sqm.

We are building with concrete.

What you choose can be related to your groundwater level. If it is high and "surrounds" the cistern and the thing is almost empty and therefore too light, it can happen that the thing surfaces like a submarine.

Our architect had to include that on the drainage application for the city.

Ok, interesting. I just checked our drainage application again, and there is nothing like that on it. However, the cistern is marked in the approved building application. Maybe it’s different in BW than in Hessen?

We build concrete.

What you use can be related to your groundwater level. If it is high and "surrounds" the cistern and the thing is almost empty and therefore too light, it can happen that it surfaces like a submarine

Exciting thought...

Our property is slightly on a slope, you could almost say on the "mountain" top. The geologist did not find any groundwater until he had to abandon his planned five-meter drilling at just over three meters depth due to massive rock.

Personally, I also prefer concrete...

Regards,

Dirk

Hello Dirk

You get the greatest benefit if all roof surfaces can be directed into the cistern (to be noted in sloping locations as water does not flow uphill) so to a low point of the property.

Hello Doc,

I can’t give you exact information on this. It’s just from my own experiences and what I’ve heard.

Material: My father has a plastic cistern that he installed himself and he’s very satisfied with it even after more than 10 years. Sometimes he complains that in hot and dry periods the cistern is too small (as far as I know 6 cubic meters - on a 650 sqm property including the house).
Our general contractor installs only concrete cisterns. When I asked him about plastic cisterns, he said that he had never really dealt with this type. I also got the impression that he prefers to stick with what is proven.
I think it is a "matter of belief." However, I can imagine that a cistern in the driveway would be better made of concrete. At least we were informed that our 10 m³ concrete cistern (see picture) requires a different cover if it is to be drive-over capable. So I assume a drive-over cistern should be made of a more stable material.
We discussed cisterns of 5 m³, 7.5 m³, and 10 m³ in the meantime. When I asked about the price difference between the 5 and the 10, I was told - not quite a thousand. So it was clear that it would be a 10.

If you have a permeable substrate, make sure that the cistern lies in a gravel bed. Then the overflow seeps away first until it reaches a certain height and then is directed into the sewer. If the substrate is not permeable, it doesn’t matter in my opinion since the cistern overflow goes into the sewer anyway.

If the cistern is empty, it could float up - whether concrete or plastic - with high groundwater. Maybe the heavier concrete keeps it down a little longer.

Hello Dirk,

Our plot is about 400 sqm in size, when the garages and the house are built, there will be about 260 sqm of garden area left. How big should our cistern be?

That depends on the total roof area as well as the design of the remaining 260 sqm. If you want to plant many flowers or even a small vegetable garden, the cistern should be larger. If only a lawn with occasional shrubs as a border is planned, a smaller model that takes the roof area into account is sufficient.

Is it better to choose a concrete or a plastic cistern?
The tank will be buried beneath our driveway – does this possibly also influence the choice of material?

Concrete cistern

The concrete cistern has the advantage that, due to its stability, it is basically drivable over. Therefore, it is also suitable for installation under courtyards and driveways. Another plus for the concrete cistern is the fact that the slightly acidic rainwater is somewhat neutralized by the concrete. This slightly reduces the corrosive effect of the rainwater. A major disadvantage of the concrete cistern is the greater effort for transport and installation, which is due to the higher weight. A concrete cistern with a capacity of 6000 liters weighs about 5 tons.

Plastic tank

Plastic tanks for storing rainwater are made from polyethylene. These tanks offer the advantage over concrete cisterns that they are much lighter, making transport and installation considerably easier. However, plastic tanks have significantly lower stability, so the lid is not drivable over.

Basically, for both tank types, it must be ensured that the lid is sufficiently secured to prevent accidents, especially with small children. For the concrete cistern, this usually does not require additional measures due to the heavy weight of the lid. For the plastic tank, an appropriate locking device must be installed.

Source: my homepage

Rhenish greetings