Completely free — including the Building-Control-ready PDF. No sign-up, no catch.
Runs entirely in your browser. Watch the flat roof U-value update as you build.
What a flat roof U-value is
A flat roof U-value covers the modern warm-deck build-up (insulation above the structural deck), inverted roofs (insulation above the waterproofing) and older cold-deck arrangements. Flat roofs often carry high-performance insulation such as PIR, so the build-up and the exact product conductivities drive the result.
Build-ups it handles
U-Monkey builds up warm-deck flat roofs — deck, vapour control layer, PIR or other insulation, waterproofing — cold-deck arrangements, and inverted roofs with the BS EN ISO 6946 rainwater correction, using real product λ values from the neutral 300+ material database.
How it's calculated
It calculates to BS EN ISO 6946 with BR 443 conventions, handling the insulation layers, any bridging and the air spaces, and shows the full working for Building Control.
Traffic-lit live against the Part L 2021 and Future Homes Standard targets, so you know the flat roof passes before you specify it.
Free — including the PDF
U-Monkey isn't a paywalled tool. Build up your flat roof, read the U-value and the full working on screen, and export a branded, Building-Control-ready PDF — all completely free, no sign-up. Prefer the whole SAP done for you? We're happy to help.
Condensation risk in flat roofs
Flat roofs are a classic condensation case: the waterproofing is vapour-tight and sits on the cold side, so moisture that reaches it has nowhere to go. In a warm deck the vapour control layer under the insulation carries it; in a cold deck, with the insulation between the joists, it is the ceiling VCL and the ventilation of the void — and without them the underside of the deck collects condensate year on year. BS 6229:2018 notes that cold flat roofs are not now recommended, and BS 5250:2021 relies on prescriptive rules for them rather than a calculation; U-Monkey flags one alongside its U-value and reports it that way in the condensation check, not as a pass.
U-Monkey runs a free BS EN ISO 13788 condensation check on every flat roof you build, alongside the U-value — where condensation would form, in which months, and whether it dries out again over the summer.
Worked examples: Warm flat roof · Flat roof insulation thickness · Cold flat roof · Inverted flat roof
The condensation verdict appears just under the U-value as you build — 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. It opens at 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 — so pick your nearest weather station: the result can change from one station to another.
Frequently asked questions
What is a flat roof U-value?
A flat roof U-value covers the modern warm-deck build-up (insulation above the structural deck), inverted roofs (insulation above the waterproofing) and older cold-deck arrangements. Flat roofs often carry high-performance insulation such as PIR, so the build-up and the exact product conductivities drive the result.
How do I calculate a flat roof U-value?
It calculates to BS EN ISO 6946 with BR 443 conventions, handling the insulation layers, any bridging and the air spaces, and shows the full working for Building Control. U-Monkey builds up warm-deck flat roofs — deck, vapour control layer, PIR or other insulation, waterproofing — cold-deck arrangements, and inverted roofs with the BS EN ISO 6946 rainwater correction, using real product λ values from the neutral 300+ material database.
Does it check Part L and the Future Homes Standard?
Traffic-lit live against the Part L 2021 and Future Homes Standard targets, so you know the flat roof passes before you specify it.
Is the flat roof U-value calculator free?
Yes — U-Monkey is completely free, including the Building-Control-ready PDF. No sign-up, no catch.
Common flat roof build-ups
Worked examples with typical U-values — each one opens in U-Monkey pre-filled, ready to edit:
- U-value of a warm flat roof
- 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
Other elements
U-Monkey builds up any element. Jump to another:
Or back to the main U-value calculator, or work out what U-value you need.