Soka-Bau Green Roof Case Study - SCHL

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Courtesy of Wigbert Riehl. Translated by Goya Ngan.
Figure 4: Extensive green roof (single-layer construction) cross-section
In this case, the substrate is composed of lava rock, pumice and zeolite with a particle size
distribution of 1–10 mm (.03–.39 in.) It does not contain any organic material, a composition
German landscapers have used successfully for more than 10 years. It has several advantages
over organic-rich growing media. It does not decompose over time and the particle size
distribution remains stable. It is slump-free after installation and compression. Other
characteristics include a silica-buffering system, pH regulation, high cation-exchange capacity,
low dissolvable-salt content and a high nutrient-retention capacity. After fertilizer has
been applied very little of it washes away. In fact, water off these extensive green roofs was
tested at a laboratory and found to conform to regulation drinking-water quality.
Because the roofs are high and subject to strong wind load, the growing medium also
had to conform to requirements for resistance to wind uplift. Finally, the growing
medium has a high frost-tolerance, which limits physical weathering and the hydraulic
changes that would otherwise reduce the permeability of the substrate.
The growing medium for the intensive green roofs is lava rock, pumice and zeolite with a
particle size distribution of 0-8 mm (0–.3 in.) (see Figure 5). Added to it is a 6.5 per
cent weight organic component of steer manure and green compost. Many of the
characteristics are similar to the single-layer, extensive-growing medium. However, it has a
higher water retention capacity, higher nutrient retention capacity and supports stronger
plant germination and growth.