Depth Is a Load Before It Is a Sponge: Substrate Depth, Retention, and the Structural Cap on Green Roofs

Extensive green roofs are specified by substrate depth, and deeper is routinely sold as better for stormwater. This paper computes what depth buys and what it costs structurally. For a single design event arriving on a partly wetted substrate, retention is 14.4 mm at 80 mm depth and 27.0 mm at 150 mm. Across a synthetic temperate year of daily balance, annual runoff retained rises from 72.8 percent at 40 mm to 86.9 percent at 100 mm and 92.4 percent at 200 mm — a flattening curve. Weight rises without flattening: saturated load is 1.23 kN per square metre at 100 mm and 1.84 at 150 mm, so under a stated spare allowance of 1.5 kN per square metre for an existing roof, depth is capped at 122 mm before retention preference is even consulted. The design rule that follows is to set depth by the structural allowance first and check retention second, because retention gains beyond about 100 mm are small while load gains are linear and unforgiving. All results are simulated, not measured.