A warm flat roof with 120mm PIR above the deck achieves a U-value of about 0.17 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 | ≈ 0.17 | Meets the 0.16 limit region — check your exact spec |
| 150mm PIR warm roof | ≈ 0.14 | The usual extension spec |
Figures computed with the BS EN ISO 6946 combined method (bridging included) using BR 443 conventional lambda values — indicative for the generic build-up shown. Your products, thicknesses and (for floors) footprint will move the number: 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.
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):
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 0.17 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) and the condensation-risk screening check 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 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 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)
- How much insulation does a flat roof need?
- 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.