Completely free — including the Building-Control-ready PDF. No sign-up, no catch.
Runs entirely in your browser. Watch the roof U-value update as you build.
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.
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.