What types of green roof are there?

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.

How much weight does a green roof add?

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.

What rainfall does a green roof retain?

Extensive green roofs retain 40-60% of annual rainfall, reducing stormwater runoff. Intensive roofs retain 60-80%. This helps meet SuDS (Sustainable Drainage Systems) requirements and can reduce drainage infrastructure costs. Peak flow rates are also significantly reduced.

Does a green roof need planning permission?

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.

What maintenance does a green roof need?

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.

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Green Roof Calculator

Calculate build-up depths, weights, and material quantities for green roofs

Green Roof Type

Roof Area

How We Calculate This

Select the green roof type to determine substrate depth and saturated weight. Enter the roof area and the calculator computes substrate volume, drainage board and filter fleece quantities (with overlap allowances), total system build-up depth, and total saturated weight for structural assessment. Rainfall retention percentage is shown for SuDS calculations.

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.