The idea of constructed wet roofs CWR is to connect (extensive) green roofs and constructed wetlands for domestic wastewater (so-called grey water) treatment. Besides, constructed wet roofs retain storm water for a certain period of time, gradually releasing rainwater and reducing the overall runoff. Furthermore, CWRs have positive impacts on the microclimate. Constructed wet roofs consists of precultured mats with evergreen vegetation that are installed on rooftops. The plants are irrigated with storm- and wastewater to ensure the surface layer remains moist. Water impurities are filtered during their way through the vegetation layer and absorbed as plant nutrients. Roofs need to have a moderate to high slope gradient to enable the water runoff. The processed water is used for irrigation as well as for disposal into receiving water or for toilet flushing. Besides the wastewater maintains the green space on the rooftop.

*different term for constructed wet roofs used in literature is wetland roofs (source:

Basic information

Retrofitting + Creation


Evapotranspiration 2 2
Shading none 2
Reflection (Albedo) none
Water Conveyance 1
Water Infiltration none
Water Retention 1
Water Storage 1
Water Reuse 1
Water Filtering 1
Water Bio-remediation 1
Deposition 1
Bio-filtration none
Habitat Provision none
Connectivity none
Beauty / Appearance 1
Usability / Functionality none
Social Interaction none
Role of Nature / Mode of Action: 
Constructed wet roofs can provide a variety of benefits, replicated from natural processes especially in soils. The most important service in the context of constructed wet roofs is the treatment of wastewater e.g. domestic or industrial wastewater. Water impurities in grey water are filtered during their way through the vegetation layer and absorbed as plant nutrients. Another important service is “storm and wastewater storage and retention”. As a result, the risk for flooding during or after a storm water event is lowered. Water evaporates from the water surface and transpires from the plants surface and stomata. This process leads to a decrease of the air temperature.
Technical & Design Parameters: 
- horizontal flow constructed wet roof (depth: 9 cm: shallow bed depth corresponds to an extensive green roof) with four beds (3,0 x 25,5 m) - roof slope: 14,3 degrees - half retention time (HRT): 3,8 days - CW construction (top - down): - turf mats (height: 1,5 cm): sandy, highly fertilized soil (here: organic soil) and grass roots/seed (3 mixture of 3 types) - stabilization plates (height: 3,5 cm) - substratum (height: 7,5 cm): sand, light expanded clay aggregates (LECA), polylactic acid beads (PLA) - (Waterproofing surface (bituminous waterproofing)) - type of wastewater: domestic wastewater (effluent of kitchen-, bathroom-, toilets sink and dishwater from considered building) - additional technical devices (tanks and pumps): septic tank, inlet tank, pumps for each bed, pressure pipes (influent and effluent pipe), infiltration pond
Conditions for Implementation: 
- Waterproofing surface/roof - sufficient roof load-bearing capacity - slope gradient to water outlets - (emergency) overflows
Benefits & Limitations: 
Benefits: - effect on microclimate: cooling of air temperature - decreased probability and consequential effects of flooding (water retention) - habitat for insects and birds/urban wildlife - improves water quality - (relative) water quantity (water can be used for different purposes after natural treatment)

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 730052 Topic: SCC-2-2016-2017: Smart Cities and Communities Nature based solutions