Living Roof Calculator

Media depth moves Total Dead Load (psf) two ways at once: it directly scales Green Roof System Weight (psf) through the depth x density formula, and it ALSO steps Vegetation Weight up at fixed 4 in and 8 in breakpoints as the planting palette shifts from sedums to shrubs. Saturated Media Density only affects the first of those two mechanisms, so the same percentage change moves Total Dead Load by less.

Yes -- both figures are multiplied by the same Slope Factor, which drops as Roof Slope increases (water sheds faster off a steeper roof before the media can absorb it). At the calculator's minimum near-flat slope the factor sits close to 1.0 (little penalty); it falls toward a floor of 0.3 as slope approaches 45 degrees, reducing both Storm Retention (gal) for a single event and Annual Retention (%) across the year by the same proportion.

Annual Retention (%) is calculated as 30% plus 8 percentage points per inch of media depth, but the formula caps that figure at 90% before the slope adjustment is applied. So additional media depth beyond roughly 7.5 in (where the uncapped formula would already reach 90%) no longer raises Annual Retention (%) further, even though it still keeps adding to Green Roof System Weight (psf) and Total Dead Load (psf).

Roof Type reads directly off Growing Media Depth (in): 6 in or less is classified Extensive (sedums, low maintenance), 6-12 in is Semi-intensive (grasses, perennials), and beyond 12 in is Intensive (shrubs, trees, walkable garden). Estimated Cost per square foot steps up at those same thresholds ($15 extensive, $25 semi-intensive, $45 intensive), so crossing a depth breakpoint changes both labels at once, not just one.