Application of Living (Green) Roof Design Recommendations for Stormwater Management

Living (or green) roofs are a Water Sensitive Design device particularly suited to attenuating stormwater peak flow rate and volume in areas where space for more traditional at-grade devices, such as raingardens and swales, is limited. Applying a living roof, i.e., a substrate (growing media) and vegetation cover, transforms impervious, often under-utilised spaces into stormwater assets. The at-source control eliminates runoff generation for most storms.

The Auckland Council’s Living Roof Review and Design Recommendations for Stormwater Management (Technical Report 2013/045) updates the TP10 (2003) chapter on Green Roofs. Stormwater calculations are based on a 5-year, multi-scale, multi-roof research programme, with results published in peer-reviewed international journals. Research included development of substrates from locally available resources and plant trials. For planning purposes, results enable design that completely retains up to approximately 30 mm of rainfall, with limited substrate depths. Peak flows are effectively detained, regardless of storm size.

The primary purpose of the report is practical guidance for design of extensive (50–150 mm-deep) living roofs in Auckland. This paper outlines the design objectives on which the research was based. These objectives inform how living roof performance may differ when they are not primarily designed for stormwater mitigation, and how to enhance their performance.