A cold flat roof — insulation between the joists, a ventilated void above it, deck and waterproofing on top — needs far more insulation than it looks. With 175mm of mineral wool between the joists it reaches only about 0.28 W/m²K, because BS EN ISO 6946 ignores the ventilated void and everything above it, and the joists bridge the whole insulation layer. Adding 70mm of PIR under the joists brings it to about 0.14, inside the 0.15 a new roof on an extension has to meet. But BS 6229:2018, the flat roofing code of practice, notes that this construction is not now recommended, and BS 5250:2021 says it should not be used where the void spans more than 5m, can’t be cross-ventilated from both sides, or can’t have a sealed vapour control layer — which is why most new flat roofs are built warm.
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The build-up (outside → inside)
- Waterproof membrane on an 18mm deck (OSB/ply)
- 50mm ventilated void, cross-ventilated from side to side
- 175mm mineral wool quilt between 225mm joists (~12% timber)
- 70mm PIR under the joists (continuous)
- Sealed vapour control layer
- Plasterboard ceiling
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 100mm quilt between joists | ≈ 0.45 | Older cold roof — worse than 0.35, so re-covering it triggers an upgrade |
| 175mm quilt between joists | ≈ 0.28 | Joist-depth insulation alone — well short of a new roof |
| 175mm quilt + 50mm PIR under | ≈ 0.16 | Just over the 0.16 limiting value for new dwellings (0.163) |
| 175mm quilt + 70mm PIR under | ≈ 0.14 | Meets the 0.15 for a new roof on an extension |
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
The ventilated void is the catch. BS EN ISO 6946 ignores it and every layer above it, so the deck and waterproofing add nothing, and the quilt between the joists is bridged by timber through its full depth — which is why a continuous layer under the joists does more per millimetre than extra depth between them. The void itself has to stay open: BS 5250:2021 asks for at least 50mm clear above the insulation, cross-ventilation equal to a continuous 25mm opening on each side, a sealed air and vapour control layer on the warm side, and no cold-deck void spanning more than 5m.
Condensation risk in a cold flat roof
A cold deck leaves the vapour-tight waterproofing on a deck that sits outside the insulation, so everything rests on the void between them staying ventilated. By the standard’s convention a well-ventilated void counts as outside air, so a condensation calculation takes the ventilation on trust, and BS 5250:2021 draws the conclusion: calculation methods don’t model air movement and are not suitable for cold flat roofs, which it treats by prescriptive rules instead (and BS 6229:2018 notes they are not now recommended). U-Monkey’s free BS EN ISO 13788 check follows it. As pre-filled, with a 50mm void open to outside air, it finds no condensation, with or without the vapour control layer, but it reports the roof as outside the scope of a calculation rather than as a pass.
What the check can show is what happens when the ventilation isn’t there — blocked vents, a void too shallow, a span too long to cross-ventilate. Model the void as unventilated, with just the quilt between the joists and no vapour control, and condensate builds up under the deck year on year, far faster than it can dry: the app’s own timber check flags the OSB. Add a sealed VCL at the ceiling and the peak drops to about 45 g/m², but it still builds up year on year. Keep the pre-filled foil-faced PIR under the joists as well and it falls to a few grams, and even that does not quite clear each year with the waterproofing at BS 5250’s figure for bitumen felt. If you have to build a cold roof, the ventilation and a continuous, sealed VCL are not optional.
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 cold flat roof (cold deck)?
A cold flat roof — insulation between the joists, a ventilated void above it, deck and waterproofing on top — needs far more insulation than it looks. With 175mm of mineral wool between the joists it reaches only about 0.28 W/m²K, because BS EN ISO 6946 ignores the ventilated void and everything above it, and the joists bridge the whole insulation layer. Adding 70mm of PIR under the joists brings it to about 0.14, inside the 0.15 a new roof on an extension has to meet. But BS 6229:2018, the flat roofing code of practice, notes that this construction is not now recommended, and BS 5250:2021 says it should not be used where the void spans more than 5m, can’t be cross-ventilated from both sides, or can’t have a sealed vapour control layer — which is why most new flat roofs are built warm.
Can I still build a cold flat roof?
You can, but only as a last resort. BS 6229:2018, the flat roofing code of practice, notes that this type of roof is not now recommended: a sealed vapour control layer and enough cross-ventilation are hard to achieve and keep. BS 5250:2021, the code of practice on moisture in buildings, says it should not be used where the void spans more than 5m, can’t be ventilated at both ends, or can’t have a sealed air and vapour control layer. Warranty providers may refuse it on new work too: LABC Warranty, for one, does not accept cold deck flat roofs over 3m². Where one is unavoidable — for example an existing roof being re-covered with no height to add insulation above the deck — it needs a sealed air and vapour control layer on the warm side, a clear 50mm air gap above the insulation and cross-ventilation equal to a continuous 25mm opening on each side. If you can gain the height, converting it to a warm deck is the better answer.
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)
- U-value of a warm flat roof
- Flat roof insulation thickness — about 150mm PIR for 0.15, 140mm for 0.16
- 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.