Green Roof Insulation Calculator — Full Build-Up Layers

From bottom to top: (1) Structural deck, (2) Vapour control layer, (3) Insulation (typically XPS or PIR), (4) Waterproof membrane (single-ply or liquid applied), (5) Root barrier (if not integral to membrane), (6) Drainage board (egg-crate or geocomposite), (7) Filter fleece (prevents fine particles clogging drainage), (8) Growing substrate, (9) Vegetation layer.

Extensive (sedum) green roofs weigh 80-150 kg/m² when fully saturated. Semi-intensive roofs weigh 150-300 kg/m². Intensive (garden) roofs weigh 300-1000+ kg/m². The structural engineer must verify the roof can carry the saturated weight on top of the maintenance imposed load. For a roof accessible only for maintenance and repair (Category H) with a slope below 30°, the UK National Annex to BS EN 1991-1-1 (Table NA.7) sets a uniformly distributed 0.6 kN/m² plus a 0.9 kN concentrated point load (these UK values govern; the un-nationalised Eurocode base text recommends 0.4 kN/m² and 1.0 kN, but the UK National Annex changes them). An accessible roof garden carries a higher imposed load set by its use. Most flat concrete roofs can support extensive green roofs without modification, but this must always be checked by an engineer.

Green roofs offer some thermal benefit from the substrate's mass and from reduced solar heat gain through evapotranspiration, but this varies with moisture content and collapses when the substrate is saturated — so it is not relied upon. Following BRE BR 443 convention, this calculator excludes the substrate from the U-value and counts only the insulation board plus the BS EN ISO 6946 roof surface resistances (Rsi 0.10 + Rse 0.04 = 0.14 m²K/W), giving a conservative figure. A separate insulation layer (typically 120-150mm PIR/XPS) is always needed for Building Regulations compliance — for a new-build roof the Approved Document L 2021 limit is 0.16 W/m²K.

EPDM and TPO single-ply membranes are popular for green roofs — they're root-resistant and have proven track records. Liquid-applied membranes (like GRP or polyurethane) are good for complex shapes. All membranes must have a root resistance certificate (BS EN 13948). The membrane must be tested before the green roof layers are installed — a flood test for 48 hours is standard practice.

In most cases, green roofs on existing buildings are permitted development and don't need planning permission. However, if the building is listed, in a conservation area, or the green roof significantly alters the building's appearance, planning permission may be required. Many local authorities actively encourage green roofs through planning policies and may offer incentives or reduced drainage charges.