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What a roof U-value is
A roof U-value is the heat loss through the roof construction. It is usually one of the easier elements on which to hit a low U-value, but the detail matters — whether the insulation sits at ceiling (loft) level or follows the rafter line changes both the build-up and the bridging.
Build-ups it handles
U-Monkey handles pitched roofs insulated at ceiling level and at rafter level, with the timber fraction of the joists or rafters accounted for to BR 443, and ventilated or unventilated air spaces treated correctly. (Flat roofs have their own page.)
How it's calculated
It uses the BS EN ISO 6946 combined method with BR 443 conventions, including the bridged-layer resistances and the air-space corrections, and shows the working.
The result is traffic-lit against the Part L 2021 and Future Homes Standard roof targets live — and roofs are held to the tightest targets of the lot, so it is worth seeing the margin as you build it.
Free — including the PDF
U-Monkey isn't a paywalled tool. Build up your 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 pitched roofs
Where the insulation goes decides where the condensation risk sits. At ceiling level the loft becomes a cold, ventilated void: keep warm, moist air out of it, keep the eaves ventilation clear, and don’t lay boards straight onto the insulation. BS 5250:2021 handles a cold loft with those prescriptive rules rather than a calculation, because most of the moisture arrives on air leaking through the ceiling, and U-Monkey’s check says so rather than calling one a pass. At rafter level the cold face of the insulation sits just below the underlay, so the vapour control on the warm side and the underlay on the cold side decide it.
U-Monkey runs a free BS EN ISO 13788 condensation check on every 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: Loft insulation (270mm) · Rafter-level insulation
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 roof U-value?
A roof U-value is the heat loss through the roof construction. It is usually one of the easier elements on which to hit a low U-value, but the detail matters — whether the insulation sits at ceiling (loft) level or follows the rafter line changes both the build-up and the bridging.
How do I calculate a roof U-value?
It uses the BS EN ISO 6946 combined method with BR 443 conventions, including the bridged-layer resistances and the air-space corrections, and shows the working. U-Monkey handles pitched roofs insulated at ceiling level and at rafter level, with the timber fraction of the joists or rafters accounted for to BR 443, and ventilated or unventilated air spaces treated correctly. (Flat roofs have their own page.)
Does it check Part L and the Future Homes Standard?
The result is traffic-lit against the Part L 2021 and Future Homes Standard roof targets live — and roofs are held to the tightest targets of the lot, so it is worth seeing the margin as you build it.
Is the roof U-value calculator free?
Yes — U-Monkey is completely free, including the Building-Control-ready PDF. No sign-up, no catch.
Common roof build-ups
Worked examples with typical U-values — each one opens in U-Monkey pre-filled, ready to edit:
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.