U-value of a green roof (sedum or extensive green roof)

What the plants and substrate add to a flat roof’s U-value (nothing, by the rules), and the insulation the roof needs underneath.

A green roof’s U-value comes from the roof under it: BR 443 says to calculate it as for any other flat roof, disregarding the landscaping, so the substrate, plants and drainage layers count for nothing. A warm deck with 150mm of PIR under a sedum roof has a U-value of about 0.14 W/m²K, inside the 0.15 for a new roof on an extension; 120mm gives about 0.17. Built as an inverted roof, with the insulation over the waterproofing, some of the rain reaches the waterproofing and carries heat away: 200mm of XPS (at its design λ, as BR 443 asks for inverted roofs) under a green roof gives about 0.21 with Manchester’s rainfall, and about 0.17 under a certified water flow reducing layer.

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The build-up (outside → inside)

Warm deck: joists at 12%, the void between them unventilated. Inverted rows: Bauder XPS 300 Inverted at its design λ, 0.035, in two layers (BR 443’s air-gap level 0), on a 150mm reinforced concrete deck, with U-Monkey’s rain correction (BS EN ISO 6946 Annex F.4), f·x 0.02 for a green roof (BR 443 §4.8.6) and Manchester’s October–May rainfall, 2.16 mm/day, U-Monkey’s default station — pick yours in the app. The landscaping is left out throughout.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm PIR warm deck≈ 0.20
120mm≈ 0.17
140mm≈ 0.15Misses 0.15: 0.153
150mm≈ 0.14As pre-filled: 0.144
160mm≈ 0.14
Inverted: 200mm XPS (2 × 100) on a concrete deck, green roof f·x 0.02, Manchester rain (2.16 mm/day)≈ 0.21The rain adds 0.040
Inverted: 200mm XPS at 3 mm/day (a wetter site)≈ 0.22
Inverted: 200mm XPS under a water flow reducing layer (f·x 0.001)≈ 0.17
Inverted: 250mm XPS (150 + 100), f·x 0.02, Manchester≈ 0.18
Inverted: 300mm XPS (2 × 150), f·x 0.02, Manchester≈ 0.15Misses 0.15: 0.154

Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and the air-gap correction included — exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for.

What changes the number

In a warm deck the board does all the work, as on any flat roof, and the green roof sits outside the waterproofing. For a roof that isn’t inverted, BR 443 says to leave every kind of roof garden out of the calculation until a method has been defined; for an inverted green roof, to proceed as for other roofs, disregarding the landscaping, with a design λ for it only where an accredited certificate gives one. The GRO Green Roof Code (2021 edition) says a green roof “cannot as yet be quantified” to influence the insulation a building needs, and Kingspan’s GreenGuard tables give a green roof the same U-value as a gravel-ballasted one. In an inverted roof the insulation’s design λ counts, not its declared one (BR 443 §4.8.6, after ETAG 031) — Bauder’s XPS 300 Inverted is 0.035 against 0.033 declared — and so does the rain correction. BR 443 takes f·x as 0.02 under a roof garden or green roof — less than the 0.03–0.04 for boards under an open covering such as gravel, because less of the rain reaches the waterproofing — and a certified water flow reducing layer brings it down to about 0.001 (BBA guidance takes 2.5% of the rain as the minimum). The weight is a structural question: an extensive sedum system with 80mm of substrate weighs about 130 kg/m² saturated (Bauder’s system summary), and the GRO Code puts extensive roofs at typically under 250 kg/m².

Next: check the junctions

A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT a junction):

Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.

Frequently asked questions

What is the U-value of a green roof (sedum or extensive green roof)?

A green roof’s U-value comes from the roof under it: BR 443 says to calculate it as for any other flat roof, disregarding the landscaping, so the substrate, plants and drainage layers count for nothing. A warm deck with 150mm of PIR under a sedum roof has a U-value of about 0.14 W/m²K, inside the 0.15 for a new roof on an extension; 120mm gives about 0.17. Built as an inverted roof, with the insulation over the waterproofing, some of the rain reaches the waterproofing and carries heat away: 200mm of XPS (at its design λ, as BR 443 asks for inverted roofs) under a green roof gives about 0.21 with Manchester’s rainfall, and about 0.17 under a certified water flow reducing layer.

Does a green roof add insulation?

Not in a U-value calculation. BR 443 leaves roof gardens out of the calculation for a roof that isn’t inverted, and for an inverted one says to calculate it as for any other roof, disregarding the landscaping, unless an accredited certificate gives the landscaping a design λ; the GRO Green Roof Code says its effect on the insulation needed “cannot as yet be quantified”. So the roof under it has to meet the U-value on its own: in England 0.15 W/m²K for a new roof on an extension, 0.16 for a new house (Approved Document L 2021, Tables 4.2 and 4.1). A green roof’s benefits lie elsewhere — holding back rainwater, wildlife, and shielding the waterproofing from sun and frost.

Is this calculator free?

Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.

Does it follow the official calculation method?

Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging, mortar joints in blockwork (and in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Roofs

Walls

Floors

Different build-up? See the full flat roof U-value calculator, or start from scratch in the main U-value calculator.

Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are, and why two calculators can give different answers for the same wall.

Your build-up won't be exactly this one.

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