A free tool for flat roofs
Flat roof U-value calculator.
Add the layers of a flat roof, from the outside in, and see its U-value. Then set a target and see roughly how much insulation it needs. A quick check for early design and condition surveys, not a calculation to issue.
Simplified method: plain layers only, no corrections. Written 13 October 2026. Next review by 13 April 2027.
The calculator
Layers, from the outside in.
Pick each layer and its thickness. The values are typical design values: use the chosen product's declared value before relying on the result.
How it works
Add up the resistances, then turn it over.
A U-value says how readily a roof lets heat through: the lower the number, the better the roof holds heat in. It is measured in watts per square metre for each kelvin of difference between inside and outside (W/m²K).
For a roof built of plain, even layers, the method of BS EN ISO 6946, as set out in the BRE's conventions for U-value calculations (BR 443), is:
- Each layer's resistance is its thickness in metres divided by its thermal conductivity (lambda, in W/mK). 120 mm of an insulation board at 0.022 gives 0.12 ÷ 0.022 = 5.45 m²K/W.
- Add the surface resistances. For a roof, where heat flows upwards, the inside surface adds 0.10 and the outside surface 0.04.
- Add everything up to get the total resistance, and divide 1 by it. That is the U-value, reported to two decimal places.
Very thin layers, such as a liquid-applied membrane or a polythene sheet, are taken as adding nothing, which errs on the safe side. Gravel, paving and green roof build-ups on top of an inverted roof are left out of the U-value altogether.
What it leaves out
A quick check, not a calculation to issue.
A real roof is rarely made only of plain layers. This calculator does not include:
- Mechanical fixings through the insulation, which can raise the U-value.
- Gaps between boards (the air gap correction).
- Bridged layers, such as insulation between timber joists.
- Tapered insulation, where the thickness changes across the roof.
- Inverted roofs, where rainwater running under the insulation needs its own correction. Inverted roof boards are left off the list for that reason.
Each of these usually makes the true U-value a little higher than the figure here. Before a specification is issued, the full calculation should be done with the chosen products' declared values and checked by a competent person. The current targets for new and refurbished roofs are in Approved Document L on GOV.UK.
In Pro
Every correction, in Pro.
The Pro web app keeps specification templates and detail drawings with each building, alongside the survey that found the problem. Explore DXF drawings, data exports and the specification tools, including U-value calculations. Calculations and specifications need competent review before issue.
Questions
U-values, answered.
What is a U-value?
How readily a part of a building lets heat through, in watts per square metre for each kelvin of difference between inside and outside. Lower is better.
How do you calculate the U-value of a flat roof?
Divide each layer's thickness in metres by its thermal conductivity, add the inside and outside surface resistances (0.10 and 0.04 for a roof), add everything up and divide 1 by the total.
Why does a manufacturer's calculation give a different answer?
Their calculation includes the corrections this one leaves out, such as fixings and gaps between boards, and uses their product's declared value rather than a typical one.
What U-value should a flat roof reach?
It depends on whether the building is new or being refurbished, whether it is a home, and which edition of Approved Document L applies. Check the current document on GOV.UK.
Can I use this result in a specification?
No. Use it as a quick check. A specification needs the full calculation with the chosen products' declared values, checked by a competent person.
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