Study site and green roof array

The study took place at the ecological research centre (ERC) at the University of Wollongong, Australia (34°25′59″S 150°52′59″E). Wollongong has an oceanic climate with humid subtropical influences. Rainfall is associated with the orographic lift caused by the escarpment; on average Wollongong, experiences 1300 mm of rain a year with the wettest months being February and March. Green roof mesocosms were constructed in December 2015 on top of a large concrete slab simulating the roof of a building. All mesocosms were North-South orientated on the slab to ensure that each setup received a comparable amount of exposure to solar radiation.

Each mesocosm was constructed using technical guidelines published by the city of Sydney [39] in an attempt to replicate green roof construction. The frame of the green roof was built using a lightweight timber, which was constructed into a rectangular box (0.5*0.6*0.3 m) (Fig 1). The green roofs were created using a layered design: -

Treatments: from the top left in a clockwise direction, shaded natural, shaded artificial, unshaded natural, unshaded (example of Lomandra longifolia and Dichondra repens pair).

Vegetation layer—the species used in the experiment were native sandstone species (forbs, low shrubs, succulents and grasses) found along the coast near Sydney.

Growing layer– 20 cm deep consisting of a mix of organic and inorganic components. We used perlite (2–3 mm), vermiculite (2 mm) and topsoil (1–4 mm) in a ratio of 4:2:1. The organic matter was kept low (<20%) to mimic the nutrient-poor sandstone soils native to the region. The soil had bulk density of ~210 kg/m3 with a water holding capacity of 72% v/v

Filter membrane—this consists of a thin membrane that prevents the soil from falling into the drainage membrane. We used a broad-gauge shade cloth

Drainage layer– 14 mm crushed terracotta

Protection mat—a Thermoplastic sheet layer to provide root and chemical protection

Waterproofing membrane- We used a liquid applied treatment of bitumen emulsion.