U-value of a warm flat roof on a concrete deck

PIR over a bitumen vapour control layer on a concrete slab: the thickness for 0.15 and 0.16, and why one layer or two matters.

A warm flat roof on a 150mm concrete slab needs about 150mm of foil-faced PIR (λ 0.022) for the 0.15 W/m²K Approved Document L sets for a new roof on an existing home — laid in two staggered layers and held with telescopic fasteners, or bonded. As 2 × 75mm it gives 0.14; the same 150mm in a single layer gives 0.1507, which shows as 0.15 but misses, because BR 443’s default air-gap correction (level 1) applies to a single layer of butt-jointed boards, and none (level 0) to two staggered layers. Plain screws count too: five galvanised ones per m² through both layers take the two-layer roof to 0.164, which misses. 160mm in one layer meets it (0.142), and the 0.16 for a new home takes about 140mm in two layers (0.151). The slab itself adds almost nothing: 200mm of concrete instead of 150mm changes the U by 0.0005.

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The build-up (outside → inside)

Foil-faced PIR at λ 0.022: Unilin Thin-R FR/ALU (NSAI Agrément 23/0436) in U-Monkey’s library; any foil-faced flat-roof board at 0.022 gives the same. The boards are held with telescopic tube fasteners, 5 per m² (steel recessed at least half the fixing length, no more than 15 per m²), which BR 443 §4.8.3 exempts from the fixings correction; bonded boards give the same. Fixed as Unilin’s NSAI table assumes — five 4.8mm galvanised screws per m² through both layers — the same roof is 0.164 and misses 0.15; with stainless screws it is 0.149. Stone wool: ROCKWOOL HardRock Multi-Fix DD (λ 0.039). The link opens as a new roof on an existing home, judged against Approved Document L’s 0.15 (2021 edition, Table 4.2); a new home’s roof is 0.16 (Table 4.1).

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
100mm PIR, one layer≈ 0.22
120mm, one layer≈ 0.18Misses 0.18: 0.184
140mm, one layer≈ 0.16Misses 0.16: 0.1604
150mm, one layer≈ 0.15Shows 0.15 but misses it: 0.1507
160mm, one layer≈ 0.14Meets 0.15
150mm as 2 × 75mm, joints staggered≈ 0.14As pre-filled: two layers take air-gap level 0, and telescopic fasteners no correction
2 × 75mm, 5 galvanised screws per m² through both layers≈ 0.16Misses 0.15 (0.164): fixed as Unilin’s NSAI table assumes
2 × 75mm, 5 stainless screws per m²≈ 0.15Meets 0.15, just (0.149)
180mm as 2 × 90mm≈ 0.12
150mm in one layer, 5 galvanised screws per m² through it≈ 0.17The screws add 0.022
150mm in one layer, 5 stainless screws per m²≈ 0.16They add 0.008
150mm in one layer, telescopic tube fasteners≈ 0.15Steel recessed at least half the fixing length, up to 15 per m²: no correction (BR 443 §4.8.3)
150mm in one layer on a 200mm slab≈ 0.15The deck barely matters
Stone wool (λ 0.039), 150 + 60mm≈ 0.18Meets 0.18 (a non-domestic flat roof)
Stone wool, 150 + 100mm≈ 0.15Misses 0.15 by a hair (0.1501); ROCKWOOL’s table gives 150 + 105mm

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

Thickness, and whether it goes down in one layer or two. BR 443 (§4.8.1) adds an air-gap correction of level 1 to a single layer of boards — about 0.009 here — unless the joints are lapped or sealed or the boards are dimensionally tight to 5mm, and level 0 to double-layer insulation, so two staggered layers of 75mm beat one of 150mm. Fixings are the other lever. On a concrete deck the boards are bonded, or held with telescopic fasteners whose steel is recessed at least half the fixing length, which need no correction at up to 15 per m² (BR 443 §4.8.3); five galvanised screws per m² straight through 150mm of PIR add 0.022, stainless ones 0.008. The deck hardly matters: 200mm of reinforced concrete (λ 2.3) instead of 150mm improves the U by 0.0005, and a 50mm screed laid to falls by about 0.001 — just enough to take a single 150mm layer from 0.1507 to 0.1497. Makers’ and building-control tables are worked to two decimal places with no air-gap correction: Unilin’s NSAI certificate for FR/ALU on concrete (23/0436) puts 160mm with galvanised fixings, or 150mm with stainless, at 0.15, and U-Monkey reproduces both on that basis; a Hertfordshire building-control guide lists 150mm for 0.15. Stone wool (λ 0.039; A2-s1, d0) needs about 255mm: ROCKWOOL’s table for HardRock Multi-Fix gives 150 + 105mm for 0.15.

Condensation risk with a warm roof on concrete

For the pre-filled roof, U-Monkey’s free BS EN ISO 13788 check finds a trace of seasonal condensation under the PVC membrane — a peak of 2.1 g/m² in February — that evaporates in summer: a conditional pass, which holds as long as the vapour values are not all at the unfavourable ends of their tested ranges at once. The bitumen vapour control layer is the one to rely on — the boards’ foil is broken at every joint — so it has to be fully bonded and sealed at laps and upstands, and a new slab or screed can only dry downwards once it is on, so let it dry first.

Stone wool is vapour-open, which puts all the load on that layer: with U-Monkey’s generic bitumen vapour control layer (sd 90m) the 250mm stone-wool roof fails, with about 7 g/m² of condensate still there at the end of the year, so enter the declared value of the layer you will use before relying on it.

Check this build-up for condensation — free

Opens pre-filled. The condensation verdict sits just under the U-value — tap it for the full month-by-month check, the surface-mould (fRsi) check included. If it isn’t a pass, How to fix it tests the usual remedies on your build-up — a vapour control layer, a vapour-open underlay, a ventilated void, a warm deck — and applies the one you pick in a tap, with Undo.

Need it in writing for Building Control? The full condensation risk analysis report is £25 +VAT (£30 inc VAT), or included on Monkey Pro Unlimited — see an example report (PDF), or read what building control expects the analysis to show.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.

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 warm flat roof on a concrete deck?

A warm flat roof on a 150mm concrete slab needs about 150mm of foil-faced PIR (λ 0.022) for the 0.15 W/m²K Approved Document L sets for a new roof on an existing home — laid in two staggered layers and held with telescopic fasteners, or bonded. As 2 × 75mm it gives 0.14; the same 150mm in a single layer gives 0.1507, which shows as 0.15 but misses, because BR 443’s default air-gap correction (level 1) applies to a single layer of butt-jointed boards, and none (level 0) to two staggered layers. Plain screws count too: five galvanised ones per m² through both layers take the two-layer roof to 0.164, which misses. 160mm in one layer meets it (0.142), and the 0.16 for a new home takes about 140mm in two layers (0.151). The slab itself adds almost nothing: 200mm of concrete instead of 150mm changes the U by 0.0005.

Can PIR go on the flat roof of a block of flats over 18m?

Yes, on the roof itself. Regulation 7(2) of the Building Regulations 2010 requires materials in the external walls and “specified attachments” of a relevant building — one with a storey at least 18m above ground containing dwellings, an institution or rooms for residential purposes — to be Euroclass A2-s1, d0 or A1. A flat roof isn’t an external wall, so that doesn’t reach it, and regulation 7(3)(b) makes clear that even the parts of a roof connected to an external wall are exempt, unless pitched at more than 70° beside a space people use. A balcony is a specified attachment, so it is caught; a terrace over a heated room is a grey area to settle with building control. The roof covering still has to meet Approved Document B’s requirements for external fire performance.

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

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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.

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