Impervious roofs increase runoff and flooding, but green roofs can assist with flood mitigation. Intensive green roofs have deep soil and more diverse vegetation. They can retain up to 90% of rainfall from a small storm, but many factors affect retention. A higher level of organic matter in the soil increases retention, but soil will be affected by age. Aged soil is likely to be more organic, but soil compaction could decrease retention. The authors therefore studied an aged intensive green roof to determine differences in retention.

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Methods
Manchester is a large city at high risk of flooding. The chosen roof was 43 years old, had soil to 170 mm average depth, had an adjacent concrete roof for comparison, was not sheltered by higher buildings, and was not constructed specifically for this study. Vegetation included grass, moss, and shrubs. Runoff was collected from two drainage downpipes to compare the green roof with the concrete roof. Rainfall data was obtained from a nearby observatory. Soil samples were analyzed. There were 254 rainfall events over four seasons.

Findings
The soil is sandy loam, not very compacted, with good permeability. Organic content is high, 9% by weight. There was no significant difference between roofs in timing of initiation of runoff. Median retention was significantly higher on the green roof compared to the concrete roof, 65.7% to 33.6%, p < 0.001. Retention varied greatly between events, but average retention was 4 mm, 2.4% of soil depth. Observations/Conclusions The green roof may have underperformed during this study due to unusually cold wet weather. The roof's healthy ecosystem included earthworms. Other studied impervious roofs include gravel roofs with a retention of about 24% and smooth roofs 15%. The authors concluded that the intensive green roof retains about double the runoff of an impervious roof or better. Increased organic content with age improved retention. They calculated that the installation of intensive green roofs on 10% of the city's rooftops would result in at least 2.3% increased rainwater retention.

    References
  • Speak, A.F., J.J. Rothwell, S.J. Lindley, and C.L. Smith. “Rainwater Runoff Retention on an Aged Intensive Green Roof.” Science of the Total Environment, 461 (2013): 28-38.