The water retention capabilities of growing media for green roofs

• Low water holding capacity reduces rainwater retention.
• Increased growing media depth alone did not substantially increase water retention.
• Inter-particle and intra-particle water holding pore spaces affected rainwater retention.

Green roofs can help reduce the risk of peak water flow and flooding in urban areas by reducing the amount impermeable surfaces on built land. This paper examines the contribution of growing media composition and depth to the water retention capabilities of green roof systems. Green roof simulation decks (decks) 1 m × 1 m were filled to a depth of 75 mm with growing media made with coarse crushed brick, coarse crushed tile or Lytag® amended with 10% (v/v) or 20% (v/v) composted green waste and planted with sedums (sedum decks); or to a depth of 150 mm with growing media made with fine crushed brick, fine crushed tile or Lytag® amended with 20% (v/v) or 30% (v/v) composted green waste and planted with flowering meadow plants (meadow decks). Growing media composition affected water holding capacity which in turn influenced water retention on the decks. The results indicated that both intra-particle pore spaces and inter-particle pore space distribution which was determined by particle size distribution were important determining factors of both water holding capacity and rainwater retention.