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The substrates in G1–G7 were of dual layer, and the dual substrate layer consisted of two parts: (1) the uppermost was a nutrition substrate layer (50 mm thick) for plant growth; and (2) the lower was an adsorption substrate layer (50–100 mm thick) for water retention and preventing pollutants (especially nitrogen and phosphorus) leaching from the nutrition layer. BAOLVSU and turfy soil (1:1) were used as the nutrition substrate. BAOLVSU is a commercial substrate especially made for green roofs. The fertility of BAOLVSU can be slowly released, which contributes to the prevention of nutrient leaching into the green roof outflow. However, compared to turfy soil, the heat preservation property of BAOLVSU in winter is relatively poor, and the cost is slightly higher. Therefore, the nutrition substrate in this study combined BAOLVSU and turfy soil to complement each other. The adsorption substrates used in the green roofs of G1–G7 were activated charcoal, zeolite, pumice, lava, expanded perlite and vermiculite. In environmental protection, these materials are generally used as filtering media and adsorbents. The substrate in G8 was of a single layer to be compared with the performance of G1–G7, and the mixture of expanded perlite, turfy soil and vermiculite, which is a very common substrate/media for a traditional green roof (DB11 2015), was selected as the substrate for G8. The water-saturated densities (kg/m3) of the substrates in G1-G8 varied from 570 to 970 and the corresponding weight (kg substrate/m2 roof) varied between 85.5 and 145.5, which met the building roof's weight restrictions (250 kg/m2 roof) in this study. The substrate properties in the green roofs are provided in Table 1.

Table 1

Substrate properties in the green roofs of G1–G8

MaterialParticle size (mm)Porosity (%)Packing density (kg/m3)
BAOLVSU 45 ± 5 360 ± 10 
Turfy soil – 60 ± 5 750 ± 15 
Pumice 25 ± 5 76 ± 4 350 ± 10 
Activated charcoal 10 ± 2 90 ± 2 560 ± 14 
Zeolite 10 ± 1 43 ± 5 1,470 ± 25 
Lava 25 ± 5 40 ± 2 820 ± 20 
Expanded perlite 8 ± 3 50 ± 2 150 ± 5 
Vermiculite 10 ± 2 85 ± 5 300 ± 4 
No. Nutrition substrate Adsorption substrate Total void fraction (%) 
Material Depth (mm) Void fraction (%) Material Depth (mm) Void fraction (%) 
G1 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Pumice, activated charcoal and zeolite (4:4:2) 50 50 ± 5 48 ± 5 
G2 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Pumice, activated charcoal and zeolite (4:4:2) 100 50 ± 5 48 ± 5 
G3 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Lava, activated charcoal and zeolite (4:4:2) 100 50 ± 5 48 ± 5 
G4 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Lava, pumice and zeolite (4:4:2) 100 55 ± 4 52 ± 3 
G5 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Expanded perlite, activated charcoal and vermiculite (2.5:5:2.5) 100 45 ± 3 45 ± 3 
G6 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Expanded perlite, activated charcoal and vermiculite (1.5:7:1.5) 100 45 ± 3 45 ± 3 
G7 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Pumice and activated charcoal (1:1) 100 50 ± 5 48 ± 5 
G8 Expanded perlite, turfy soil and vermiculite (2.5:5:2.5) Depth = 200 mm Void fraction = 45 ± 3 45 ± 2 
MaterialParticle size (mm)Porosity (%)Packing density (kg/m3)
BAOLVSU 45 ± 5 360 ± 10 
Turfy soil – 60 ± 5 750 ± 15 
Pumice 25 ± 5 76 ± 4 350 ± 10 
Activated charcoal 10 ± 2 90 ± 2 560 ± 14 
Zeolite 10 ± 1 43 ± 5 1,470 ± 25 
Lava 25 ± 5 40 ± 2 820 ± 20 
Expanded perlite 8 ± 3 50 ± 2 150 ± 5 
Vermiculite 10 ± 2 85 ± 5 300 ± 4 
No. Nutrition substrate Adsorption substrate Total void fraction (%) 
Material Depth (mm) Void fraction (%) Material Depth (mm) Void fraction (%) 
G1 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Pumice, activated charcoal and zeolite (4:4:2) 50 50 ± 5 48 ± 5 
G2 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Pumice, activated charcoal and zeolite (4:4:2) 100 50 ± 5 48 ± 5 
G3 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Lava, activated charcoal and zeolite (4:4:2) 100 50 ± 5 48 ± 5 
G4 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Lava, pumice and zeolite (4:4:2) 100 55 ± 4 52 ± 3 
G5 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Expanded perlite, activated charcoal and vermiculite (2.5:5:2.5) 100 45 ± 3 45 ± 3 
G6 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Expanded perlite, activated charcoal and vermiculite (1.5:7:1.5) 100 45 ± 3 45 ± 3 
G7 BAOLVSU and turfy soil (1:1) 50 45 ± 5 Pumice and activated charcoal (1:1) 100 50 ± 5 48 ± 5 
G8 Expanded perlite, turfy soil and vermiculite (2.5:5:2.5) Depth = 200 mm Void fraction = 45 ± 3 45 ± 2 

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