Designing green roofs for stormwater management

This lightweight green roof uses retention layers for high water storage capacity.

When an incentive policy mandates a green roof to have a minimum of 100-mm (4-in.) growing medium, it will achieve a water-retention capacity of 65 mm (2 9/16 in.) when GM#1 is used but only 40 mm (1 9/16 in.) if GM#2 is employed. While both growing media will meet the prescriptive requirements of this green roof policy, GM#1 can retain 25 per cent more water on a volume basis compared to GM#2. While it is easy to specify a minimum growing medium depth, prescriptive policy does not necessarily achieve specific stormwater performance.

Additionally, soil is heavy when wet. Typical green roof growing medium weighs 24 to 34 kg/m2 (5 to 7 psf) per 25-mm (1-in.) depth when fully saturated. Prescriptive green roof incentive programs in North America generally require a minimum growing medium depth of 75 to 100 mm (3 to 4 in.), which translate to an additional loading of 72 to 136 kg/m2 (15 to 28 psf). This extra weight often prevents lightweight construction such as factories and warehouses from adopting vegetative roofs. Unfortunately, these buildings often have large footprints, which exert a considerable burden on the municipality’s stormwater infrastructure.

Water-retention fleece and horticultural mineral wool offer lightweight alternatives to growing media to achieve water retention in green roof systems. Figure 3 compares the saturated weights of typical growing media and water-retention materials (again, normalized to 25 mm for ease of comparison). Each bar represents the saturated weight composed of the dry component (red) and the stored water (blue).

The saturated weight of the water-retention layers is an average of about 25 per cent lighter than typical growing media. Additionally, only 9.8 kg/m2 (2 psf) out of the saturated weight of 38.8 kg/m2 (7.9 psf) or 25 per cent by weight, for the lava/pumice/dolomite growing medium is water.

Figure 3: Water-retention capacity of different green roof components.

In comparison, a high fraction of the saturated weight for the retention layers comes from the stored water as evident from the high blue fraction compared to the small red fraction in each bar. For example, out of the 24 kg/m2 (4.9 psf) saturated weight of the mineral wool, 23.4 kg/m2 (4.8 psf) or 98 per cent of which is water. It is clear the water-retention-to-weight ratio for the water-retention materials is significantly higher than growing media. Replacing all or part of the growing medium in a green roof with materials such as fleece and mineral wool can achieve equal or better water-storage capacity while keeping the system weight low.

A prescriptive-based policy mandating a minimum growing medium depth can impose too much loading on the roof structure and prevent buildings with limited structural capacity (e.g. factories and warehouses) and the existing building stock from adopting green roofs. However, an objective-based policy that requires specific stormwater performance (e.g. water-retention capacity) enables these buildings to adopt green roof systems based on lightweight water-retention layers instead of growing media and contribute positively to the overall stormwater management goal in the municipality. (For more, see “Sherway [Rooftop] Gardens”)

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