What are the layers in a green roof build-up?

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.

How much does a green roof weigh?

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 that the roof structure can support the saturated weight plus a maintenance access load (typically 1.0 kN/m²). Most flat concrete roofs can support extensive green roofs without modification.

Does a green roof provide insulation?

Green roofs provide some thermal insulation from the substrate layer (R-value of 0.2-1.0 depending on depth and moisture content) and reduce solar heat gain through evapotranspiration. However, the insulation value varies with moisture content and should not be relied upon as the primary insulation. A separate insulation layer (typically 100-150mm PIR/XPS) is always needed for Building Regulations compliance.

What waterproof membrane is best for green roofs?

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.

Do green roofs need planning permission?

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.

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Green Roof Insulation Calculator — Full Build-Up Layers

Calculate green roof layer quantities: waterproof membrane, insulation, drainage board, filter fleece, and substrate weight per m².

Roof Area

Plan area of green roof

Green Roof Type

Insulation Thickness

PIR or XPS below membrane

Price Per m² (optional)

Complete system installed

How We Calculate This

This calculator determines the quantities of each layer in a green roof build-up, including the total weight and thermal performance.

The formula

Each layer area = Roof area × Wastage factor

Substrate volume = Roof area × Depth (m) × Wastage

Saturated weight = Area × Weight per m² (by type)

U-value = 1 ÷ (Surface R + Insulation R + Substrate R)

For example, 50 m² extensive green roof with 120mm PIR insulation: Membrane = 50 × 1.27 = 63 m². Substrate = 50 × 0.1 = 5 m³. Weight = 50 × 120 = 6,000 kg saturated. U = ~0.17 W/m²K.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.