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IV. Rainwater Harvesting 

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Rainwater Harvesting

Rainwater usage from metal surfaces

Rainwater is precious – and useful! By using rainwater, e.g. for use as garden irrigation or as domestic water (e.g. for washing clothes), you effectively save costs.

Incidentally, this also applies to rainwater that flows over metal surfaces on buildings made of zinc (or copper): the trade association for industrial and rainwater use confirms that rainwater flowing out of copper and zinc surfaces can be used for rainwater use without any problems.

Rainwater seepage

If the rainwater from draining building surfaces is not used, it should be returned to the natural water cycle. This is happening more and more often through local infiltration. The rainwater contributes to the maintenance of the local water balance and thus to groundwater recharge. Rainwater can seep away above or below ground. Various possibilities have proven themselves, which we present to you here:

Rainwater seepage

There are two types of percolation:

  1. Wide drainage over vegetated topsoil

    This can be, for example, your own meadow or specially prepared areas on your own property. Rainwater helps to improve the small climate of built-up areas and helps to absorb flood peaks in streams and rivers.

  2. Infiltration via an infiltration trough as a technical system

    This solution is structurally very easy to implement and inexpensive, requires little space and enables attractive design variants. In principle, rainwater is led from a paved area into a flat, green basin, where it is briefly collected before it seeps into the ground. As a rule of thumb for the installation of infiltration troughs: 10% of the area to be drained should be available as trough area.

In Baden-Württemberg , permission-free infiltration of rainwater from metal roofs and surfaces via above-ground infiltration troughs is expressly supported.
For more information, please visit:

You can get the answer to the question of whether you are allowed to infiltrate rainwater in Bavaria without a permit via the online query program “BEN” of the Bavarian State Office for the Environment:

Underground seepage

Various technical options are available for the underground infiltration of rainwater, such as:

  1. Drainage trench

  2. Soakaway

  3. Filter System

These are underground rainwater storage tanks, in which rainwater is briefly collected, filtered through natural materials such as gravel and sand, and then infiltrated over the subsoil.

In Bavaria, for example, the infiltration of rainwater from copper and zinc roof areas up to 50 m² (horizontal projection) in underground infiltration systems such as infiltration wells and trenches is free of permission.

With larger roof areas, drainage is possible via several systems (e.g. infiltration shaft or infiltration system). You can find more information on:

Derivation of very large metal roofs (50-1000 square meters)

There are also two license-free variants for rainwater drainage:

  1. Above-ground drainage with suitable soil conditions

    The drainage can take place via an overgrown topsoil layer, which has the following guide values: pH 6 to 8, humus content 1 to 3%, clay content <10%; Check and, if necessary, correct the pH value every 3 years.

  2. Underground infiltration via filter systems

    These correspond to the type approved under Art. 41f BayWG. A list of the type-approved systems for subsequent discharge into the groundwater can be found here:

If none of the options listed here are used, rainwater can also be discharged into the sewage system and – if available – into a separate rainwater sewage system or surface water such as flowing water, canals or lakes can be supplied. More information can be found in the DWA leaflet M 153:$file/vorschau_DWA-M_153.pdf

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