A green roof does more than look green. One of its main functions is to temporarily retain rainwater. It is precisely this water-holding capacity that makes green roofs interesting to municipalities and water boards, and is often an important reason why subsidies are available.

How does water storage work on a green roof?

A green roof consists of multiple layers that together absorb and retain rainwater. The vegetation captures precipitation, after which the water is stored in the substrate. Depending on its construction, a green roof can temporarily retain a large portion of rainwater.

Excess water that cannot be stored is discharged in a delayed manner. As a result, rainwater does not flow into the sewer at once, but gradually. This helps reduce flooding during heavy rainstorms.

How much water can a green roof hold?

Water storage capacity varies by type of green roof. A standard sedum roof can store an average of 35 liters of water per square meter. Green roofs with a thicker substrate layer, such as herb or biodiversity roofs, can hold even more water.

Grants for water storage

Because green roofs are proven to help reduce flooding, many municipalities and water boards encourage their construction with subsidies. These schemes are often aimed specifically at water storage and relieving the burden on the sewer system.

The amount of the grant varies by municipality and usually depends on:

• the number of square meters of green roof
• the water holding capacity of the roof structure
• The type of green roof (for example, sedum, sedum-herbs or a flower roof)

In some cases there are additional requirements for construction or minimum water storage. Therefore, it pays to check in advance what conditions apply locally.

Cooling and delayed discharge

The water stored in a green roof gradually evaporates through the vegetation. This cools the roof surface and immediate surroundings. In urban areas, this helps reduce heat stress, while reducing the burden on the sewer system when it rains.

This combination of water storage and cooling makes green roofs an important part of climate adaptation measures.

Conclusion

Water-holding capacity is one of the biggest advantages of a green roof. By temporarily retaining rainwater and delaying its discharge, it reduces flooding and relieves the sewer system. For this very reason, many municipalities offer subsidies for green roofs. This makes a green roof not only a sustainable, but also a financially attractive long-term investment.