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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) 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 — and cold-deck arrangements, using real product λ values from the neutral 200+ 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.
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) 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 — and cold-deck arrangements, using real product λ values from the neutral 200+ 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:
Other elements
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Or back to the main U-value calculator, or work out what U-value you need.