Moisture Measurements as Performance Criteria for Extensive Living Roof Substrates

Abstract

Extensive living roof substrate design to promote storm-water management while balancing structural load and maintaining $>60\%$ nonirrigated plant cover is investigated through linked laboratory and field experiments in Auckland, New Zealand. Setting quantifiable goals for the 2002 FLL guidelines and agronomic testing methods resulted in successful design of multiple nonproprietary substrates. Particle size distribution and quality control in the materials’ supply chain are critical. Additional work is required to define a meaningful standard permeability test for living roofs. While the maximum water capacity guideline provides a conservative estimate for structural loading when a substrate is wet, it should not be used to predict storm-water retention. Agronomic measures of readily available water (10–100 kPa suction) plus plant stress water (100–1,500 kPa suction) provide a reasonable estimate for the maximum potential rainfall storage during individual storm events. Subject to Auckland’s frequent rainfall, an extensive living roof with 70% v/v 4–10 mm pumice, 10% v/v 1–3 mm zeolite, and 20% organic matter at a 100-mm depth is recommended to maintain plants without irrigation (excluding drought conditions) and minimize weeds while preventing runoff from storms with less than 25 mm of rainfall.