Hydrologic Impacts of Detention Layers within Extensive Vegetated Roof Assemblies

First-generation extensive green roof systems included only vegetation, growing media, and drainage materials. However, these simple systems can be too heavy for roofs that were not originally intended to support green infrastructure, and the growing media may not provide adequate stormwater detention to meet desired flooding mitigation targets. Today, many commercial green roof manufacturers and suppliers aim to increase the retention and detention provided by green roofs by using lightweight growing media alternatives. This study evaluates the hydrologic impacts of generic vegetated roof assemblies (VRA) that use ultra-lightweight and soilless materials—fleece, mineral wool, and a combined reservoir-detention system—throughout a complete growing season under the natural rainfall conditions in Toronto, Ontario. Discharge from test beds was continuously measured from July to November 2022 and compared to discharge produced by a conventional green roof assembly and an impervious roof covered with stone ballast. The total rainfall for the monitoring period was 219.2 mm, and the stone control retained 48% (105.4 mm). The green roofs improved retention, retaining an additional 73–95 mm of rainfall depending on the roof assembly. All green roofs provided some detention and decreased peak discharge. The extent of detention provided ranged substantially based on the VRA, particularly for large rain events. All green roofs drastically reduced peak flow, averaging greater than 95% regardless of the assembly. The best-performing system (combined reservoir-detention) retained 95% of all received rainfall, reduced peak flows by 98%, and had the greatest discharge delay (9.6 hrs) and duration (15 hrs). This work demonstrates that detention layers improve the hydrologic performance of a green roof system to varying degrees.