A warm flat roof with 120mm PIR above the deck achieves a U-value of about 0.18 W/m²K; 150mm reaches about 0.14, which is why 150mm is the common specification for new extensions. "Warm" construction — insulation above the deck, VCL below the insulation — is the default because it keeps the structure warm and dry.
Every layer editable · live U-value · Part L & Future Homes Standard targets · free PDF report
The build-up (outside → inside)
- Waterproof membrane
- 120–150mm PIR
- Vapour control layer
- 18mm deck (OSB/ply)
- Firrings/void + plasterboard ceiling
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 120mm PIR warm roof | ≈ 0.18 | Short of the 0.16 limiting value — a new roof needs about 140mm |
| 150mm PIR warm roof | ≈ 0.14 | The usual extension spec |
Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork 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
Board thickness is nearly everything. The VCL sits below the insulation on the warm side — get that wrong and the deck gets wet, which the free condensation check will show. Tapered schemes (falls cut into the insulation) are handled by U-Monkey’s tapered mode.
Condensation risk in a warm flat roof
A warm deck puts vapour-tight waterproofing on the cold side of the insulation, so moisture that reaches it has nowhere to go — which is why the vapour control layer under the insulation matters. As pre-filled, U-Monkey’s free BS EN ISO 13788 check fails it. With the roof surface taken 2 K below the air every month, the standard’s simplified method for roofs, a little condensation builds up under the waterproofing through the winter, peaking at about 7 g/m² in April, and about 3.5 g/m² of it is still there at the end of the year, so it accumulates. The waterproofing is entered at BS 5250’s figure for roofing felt laid in bitumen, sd 90 m, and a 500-gauge polythene VCL, sd 50 m, is simply less vapour-resistant than the roof above it.
The fix is the one warm-deck specifications use: a high-performance vapour control layer, such as a foil-cored bitumen VCL. Left on the generic felt, the check asks for a VCL rated at least sd 2,400 m, because the result has to hold with the waterproofing anywhere up to BS 5250’s sd 200 m for a reinforced bitumen membrane; enter your membrane’s declared value and that falls, to about 950 m for a declared 90 m. Swap the PIR for mineral wool slabs and it fails by far more with the polythene VCL. Model a cold deck instead — insulation between the joists, deck on top — and it fails outright, with or without a VCL at the ceiling; even a cold deck that clears the calculation is reported as outside its scope, because BS 5250:2021 relies on prescriptive rules for cold roofs. BS 5250 counts a warm deck with a sealed vapour control layer among the arrangements experience shows are safe, and the calculation is harder on it than experience is: its simplified roof method leaves out the sun, which dries a roof in summer. At colder or wetter stations than Manchester it asks for more, and at some none of the standard fixes clears it, so run it at your own.
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.
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?
A warm flat roof with 120mm PIR above the deck achieves a U-value of about 0.18 W/m²K; 150mm reaches about 0.14, which is why 150mm is the common specification for new extensions. "Warm" construction — insulation above the deck, VCL below the insulation — is the default because it keeps the structure warm and dry.
Warm roof or cold roof for my extension?
Warm (insulation above the deck) is the modern default: simpler ventilation, structure stays warm, condensation risk is lower. Cold roofs need a ventilated void and careful detailing. U-Monkey models both — but if you have the build-up height, warm wins.
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 air-gap corrections, ISO 13370 for ground and suspended floors, and ISO 13788 for the condensation screening check.
Other common build-ups
- U-value of a cavity wall with full-fill insulation
- U-value of a cavity wall with PIR partial-fill insulation
- U-value of a timber frame wall
- U-value of a solid brick wall with internal insulation
- U-value of a one-and-a-half-brick (317.5mm) solid wall
- U-value of a solid wall with external insulation
- What is the U-value of an uninsulated cavity wall?
- U-value of 270mm loft insulation
- U-value of insulation between rafters (loft conversion)
- Flat roof insulation thickness — about 150mm PIR for 0.15, 140mm for 0.16
- U-value of a cold flat roof (cold deck)
- U-value of an inverted flat roof
- U-value of an insulated concrete ground floor
- U-value of an insulated suspended timber floor
- U-value of a beam and block floor with PIR
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.