Green Roof Calculator

Extensive (sedum): 80-100mm substrate, lightweight (80-120 kg/m² saturated), low maintenance. Semi-intensive (wildflower): 150mm, moderate weight. Intensive (garden): 300mm+, heavy (350+ kg/m²), requires irrigation. Biosolar combines sedum with solar panels for dual benefit.

Sedum roofs add approximately 80-120 kg/m² when fully saturated. Wildflower: 150-200 kg/m². Intensive gardens: 350-500+ kg/m². The structural engineer must confirm the building can support the additional dead load. Most flat-roofed buildings can handle extensive green roofs.

Extensive green roofs typically retain around 40-60% of annual rainfall, reducing stormwater runoff; intensive roofs around 60-80%. The GRO Green Roof Code 2014 (citing Mentens, Raes & Hermy 2005) gives roughly 50% retention at 50mm substrate, 55% at 100mm and 60% at 150mm. The figures shown by this calculator are indicative typical values — actual retention varies with rainfall intensity, season and substrate moisture (up to ~100% in summer, ~20-30% in winter). For SuDS design, follow the GRO Code and your drainage engineer rather than a single annual percentage.

No — they are indicative mid-range values from the GRO/FLL bands (sedum 80-100mm and 80-120 kg/m² saturated; wildflower 150mm; intensive 300mm+, 350+ kg/m²). A different green-roof tool or manufacturer system may use slightly different presets within the same valid range, so figures can vary between calculators. Always confirm the actual substrate, saturated dead load (plus snow and imposed loads per BS EN 1991-1-1/-1-3) and build-up with the chosen manufacturer and a structural engineer before construction.

In most cases, a green roof on an existing flat roof does not need planning permission if it does not alter the height significantly. However, check with your local authority. Listed buildings and conservation areas may have restrictions. Building regulations approval may be needed for structural loading.

Sedum/extensive: 1-2 visits per year (remove weeds, check drainage outlets, apply slow-release fertiliser). Wildflower: 2-3 visits. Intensive: regular maintenance like any garden. Irrigation may be needed in the first year for establishment and during extended dry spells.