Re-roofing insulation checker
Find out whether recovering or replacing a commercial roof means upgrading its insulation under the Building Regulations, the U-value it has to reach, and roughly how much insulation gets it there.
The rules come from Approved Document L for buildings other than dwellings, with the equivalents for Wales and Scotland. Building control has the final say on any project, and a core sample shows what insulation the roof has now.
The work
What’s planned for the roof, and how much of it.
Leave at 0 if it’s being stripped or you don’t know. A core sample shows what’s there and whether it’s dry.
Result
Whether the work triggers an upgrade, and what it takes.
- U-value to reach
- Share of the roof being renovated
- New PIR insulation, roughly
- Allowing for the existing 25 mm
The thickness is worked out by BR 443’s method for Kingspan Thermaroof TR27, and leaves out the deck and membrane, which errs on the thick side.
Before the work is specified
- What’s there now: the build-up, insulation and whether it’s wet. Core sampling
- How much is wet, so dry insulation can stay and wet can come out. Moisture mapping
- Condensation risk for the new build-up, checked to BS 5250 by the designer.
- Heights at upstands, doors and outlets, which extra insulation raises. Measured survey
- Building control, notified before work starts, unless the roofer is registered to self-certify a replacement covering.
These results are a guide only and don’t replace a survey or professional advice.
Sources
- Approved Document L, Volume 2, Buildings other than dwellings, 2021 edition incorporating 2023 amendments, for England, regulation 23, paragraphs 4.7, 4.8, 10.2 and 11.2 to 11.4, and Tables 4.1 and 4.2
- Welsh Government, Approved Document L, Volume 2, Buildings other than dwellings, 2022 edition, for Wales, regulation 23, paragraphs 4.7, 4.8, 10.2 and 11.2 to 11.4, and Tables 4.1 and 4.2
- Scottish Government, The Scottish Building Standards Technical Handbook, Non-domestic, April 2026 edition, for Scotland, standard 6.2, clause 6.2.11 and Table 6.3, and schedule 3, types 2 and 25
- The Building Regulations 2010, schedule 3, paragraph 16, for roofers registered to self-certify a replacement covering
- BRE, BR 443, Conventions for U-value calculations, 2019 edition, the method, a roof’s surface resistances and how a U-value is rounded
- Kingspan, Thermaroof TR27, Nineteenth Issue, March 2026, its thermal conductivity, for PIR board
- ROCKWOOL, HardRock Multi-Fix (DD) technical datasheet, Version 2.08, August 2026, its thermal conductivity, for mineral wool
- BBA, Agrément Certificate 21/5874, Ravatherm XPS X 300 SL Inverted Roof Insulation, its design thermal conductivity and its correction for rain with a water flow reducing layer, for XPS
- BBA, Information Bulletin No 4, Inverted roofs, drainage and U value corrections, the correction for rain reaching the membrane
England’s 2026 edition of Approved Document L, Volume 2, takes effect on 24 March 2027 for work other than higher-risk building work, and sets 0.18 W/m²K for every roof, renovated or replaced.
How to use it
Describe the work
Where the building is, what’s planned, and how much of the roof it covers. The roof measurement tool gives the areas.
Pick the insulation
The type for the new layer, and any existing insulation that will stay because it’s dry.
Read the result
Whether an upgrade is triggered, the U-value to reach, a rough thickness, and the checks to make before the work is specified.
What it tells you, and what it cannot
The trigger is a rule. The right build-up for a particular roof is a design.
It tells you
- Whether the planned work is likely to trigger an insulation upgrade
- The U-value the regulations set for a renovated roof
- Roughly how thick the new insulation would need to be
- What to check before the work is specified
It cannot tell you
- Whether an upgrade is feasible on this roof, which building control decides
- The exact build-up, which a designer calculates for the real layers
- Whether the existing insulation is dry enough to keep
- Whether the deck can take the extra weight and height
Questions about re-roofing and insulation
When does re-roofing mean upgrading the insulation?
When the work renovates more than half of the roof’s area, or more than a quarter of the whole building’s envelope, the Approved Document expects the roof to be brought up to the standard for a renovated roof, where that is technically, functionally and economically feasible. Small repairs don’t trigger it.
Does a recover over the existing roof count?
Yes. Adding a new layer over the existing roof is renovation in the Approved Document’s terms, the same as stripping it and laying a new one.
Can the existing insulation stay?
If it’s dry and sound, it counts towards the U-value and the new layer can be thinner. Wet insulation has lost most of its value and usually has to come out. Moisture mapping and core samples show which is which.
What if the upgrade isn’t feasible?
The Approved Document allows a lesser standard where meeting it in full isn’t technically, functionally or economically feasible, for example where the extra height can’t be accommodated at upstands, or the structure can’t carry the extra load. Building control agrees what is reasonable for the project.
Are the rules the same in Wales and Scotland?
Not entirely. Wales has its own Approved Document, with the same triggers and the same U-value for a renovated roof as England’s. Scotland’s Technical Handbook sets no share of the roof: where work alters a roof’s build-up, its insulation should be improved as far as is reasonably practicable, to a lower U-value. The tool uses the right set for the country you choose.
Get the right survey for your roof
Whether you’re investigating a problem or planning ahead, we’ll help you choose the right survey for your roof. Get in touch for a free quote.
Core sampling
What the roof is built from, layer by layer, and whether the insulation is wet.
About core samplingMoisture mapping
How much of the insulation is wet, so dry areas can be kept when the roof is recovered.
About moisture mappingAll measurements, calculations and estimates provided by Roofcheck’s online tools are for guidance purposes only and should not be relied upon as definitive or technically accurate. Results should be independently verified through a professional roof survey or assessment before undertaking any works or making decisions. Roofcheck accepts no liability for any loss, damage or costs arising from reliance on these results, to the fullest extent permitted by UK law.