Most people expect a re-roof to cost them a covering and some labour. Fewer expect an insulation bill. Yet the energy rules in Approved Document L (Part L, covering fuel and power) can demand that a renovated roof reach a set insulation standard once the work goes beyond certain limits. Whether it applies depends on how far the work goes, not on what you call it.
This guide explains the insulation rule. Whether a job needs a building control application in the first place is a related but separate matter, covered in our guide to the 25% and 50% thresholds. The VAT side of insulation has its own page, and the wider service is on roof replacement. All figures here come from Approved Document L Volume 1, the 2021 edition incorporating 2023 amendments. Take it as orientation only, and confirm the detail with your building control body.
What counts as renovating a thermal element
Part L calls a roof a thermal element: a part of the building that separates warm from cold. It treats the work in two ways. Paragraph 11.2 lists what counts as renovation, and for a pitched roof the relevant item is replacing an existing layer by stripping the element down to expose its basic structural components, such as rafters, and then rebuilding. For a flat roof, replacing the waterproof membrane also counts.
So a full re-cover that takes the roof to bare rafters is renovation. Replacing a few broken tiles is not, because nothing is stripped back to the structure. The same goes for a ridge re-bed or a flashing repair.
Why the word matters
The test turns on the structure being exposed. A re-roof that lifts the covering and battens but leaves the underlay and everything below untouched is a different case from one that takes the lot down. We note at survey which of these your job will be, because it changes the work.
Roofs also differ in how they are insulated. A cold roof has insulation at ceiling level, in the joists, with the rafters left cold and ventilated. A warm roof has insulation at the rafters. Which you have decides where an upgrade would go.
The 50% and 25% tests
Paragraph 11.3 asks for the entire element to be upgraded to the Table 4.3 column (b) U-value in two situations. One is where over 50 per cent of that individual element's surface is renovated. The other is where the job amounts to a major renovation, meaning more than 25 per cent of the external envelope's surface area is renovated.
Regulation 23(2), as printed in the document, adds a parallel rule: part-replacement that covers more than 50 per cent of an element's surface area means the entire element must comply so far as feasible technically, functionally and economically.
| Trigger | What it means for a roof |
|---|---|
| More than 50% of the roof renovated | Whole roof upgraded to the improved U-value |
| More than 25% of the envelope renovated | Counts as major renovation, same upgrade |
| Below both | No thermal upgrade is required by this paragraph |
Notice the wording differs from the application test. There the limits are "less than" 25 and 50 per cent; here it is "more than". At exactly the boundary the tests differ, which is another reason to measure and ask building control. A re-roof of a whole house exceeds both.
The 0.16 W/m²K target and where it comes from
Table 4.3 sets, for an existing roof, a threshold U-value of 0.35 and an improved U-value of 0.16 W/(m²·K). A U-value measures how fast heat passes through a construction: the lower the number, the better the insulation. The threshold figure is the level above which an existing element is considered poor enough to bring up; the improved figure is the target.
Do not confuse this with Table 4.2, which applies to new or replacement elements. There, a roof is given 0.15 W/(m²·K). The difference between 0.15 and 0.16 is easily mixed up, and the page you are reading uses 0.16 because this is a renovation of an existing roof.
What the number demands in practice
Hitting 0.16 on a pitched roof means a significant thickness of insulation. We do not quote depths here, because they depend on the material and on how the roof is built. Your survey and the insulation manufacturer's data decide it. The calculation, and the building control sign-off that follows, record that the target has been met.
It also helps to know what the figure does not require. It does not require a specific brand or a specific thickness. It requires a result, and a result can be reached in more than one build-up.
Head-room relief for loft rooms
Note 3 to Table 4.3 recognises that insulating a roof can eat into space. It says that where meeting the improved U-value would limit head room, a lesser standard may be appropriate. It also says the insulation plus the air gap should be at least to the depth of the rafters.
This matters for anyone with a bedroom or a study in the roof. Add a thick layer under the rafters and a room that was just high enough may stop being so. The allowance lets the designer settle for less in that case, but not for nothing: the insulation should still fill the rafter depth.
Using the relief sensibly
A loft used only for storage has no head-room problem, so the full target is the one to aim for. The relief is a judgement for building control, not a right we can claim. We raise it at survey if the loft is lived in. The usual sequence is to measure the head room, decide the build-up and agree it with the body that signs the work off before the strip begins.
If you are planning to convert the loft later, that is a different job with its own requirements, including means of escape and, in Stroud's conservation areas and the Cotswolds National Landscape, planning rules for any dormer. Doing the insulation properly at re-roof stage can save opening the roof twice.
Where head room is tight, we measure and agree the build-up with building control before stripping, not after.
Insulation and ventilation have to be designed together
Insulation without a plan for moisture can do harm. Warm, damp air from the house rises into the roof and, if it meets cold timber, condenses. BS 5250, the code of practice for moisture in buildings, sits behind the ventilation guidance that roof-product manufacturers publish.
Marley, citing BS 5250, sets out figures for a cold roof. Under a non-breathable (HR) underlay the eaves need a 25 mm opening along their length at pitches up to 15 degrees, and 10 mm when steeper. Add a continuous 5 mm opening high up if the pitch is steeper than 35 degrees, the span runs past 10 m, or the roof slopes one way only. Under a breather (LR) underlay, with an airtight ceiling below and a covering that passes air, ordinary eaves construction at 3 mm may be all that is needed, or 5 mm at the ridge.
Where we look first
We look at the eaves, where insulation is often pushed tight against the wall plate and blocks the air path, and at the ridge. Spray foam on the underside of an old roof is a recurring worry. RICS issued a consumer guide in March 2023 cautioning that faulty installation can lead to damp, and in November 2024 reporting from the BBC found some lenders unwilling to lend on homes with it. If yours has foam, mention it before the survey.
Detailing the build-up on a stripped roof
With the roof open, there are several ways to reach the target. Insulation can go between the rafters, over the rafters, under them, or in a combination. Each has trade-offs.
- Between the rafters. This keeps the roof profile and the eaves as they are, which matters on an older house where the look of the edge is part of its character. The depth is limited by the rafter size, so it may need help from a layer below.
- Over the rafters. A layer above the timbers can reach the target without losing any room height, but it raises the roof line slightly, which can affect the eaves, verge and flashings, and may need a planning check.
- Under the rafters. A layer on the underside is straightforward from inside but costs head room.
Questions to put to any roofer
Ask which U-value the quote is aiming for, what the insulation is made of and how thick it will be, where the ventilation openings are and how they are kept clear, and who tells building control the work is starting. A quote that answers those in writing is one you can compare with another. Silence on them usually means the thermal side has not been thought through.
Keeping the air path clear
Whatever the method, ventilation paths at the eaves and ridge must stay open, and the underlay must be correct for the design. These details are decided before the strip, so they can be allowed for in the quote. The cost of a re-roof is covered in our materials guide, and the VAT treatment of the insulation is on its own page, insulation and VAT when you re-roof.
A stone roof is a special case. Heavy slates and old timbers, particularly in a listed building, can limit what is allowed, so the council's planning officers come first.
What we do about it on your job
At the survey we establish how far the work will go. If it will pass the 50 per cent or 25 per cent tests, we tell you the insulation requirement is likely to apply and describe the build-up we would propose. We photograph the loft and the rafters so the discussion rests on your own roof.
The quote then covers insulation as well as the covering, scaffold, labour, waste and making good. If head room or a listed status complicates the picture, that goes in the quote as a note, with a suggestion to ask building control or the planning team before you commit.
Any notifiable work comes with an explanation of the consent involved, and we make the Building Regulations notification. The building control body looks at the insulation before it is covered. Afterwards the site is cleared of waste, and a 10-year workmanship guarantee is given to you on paper.
To get a first view before anyone visits, send pictures of the loft that show the rafters and whatever sits between them, along with the address. A reply will say what the photographs reveal and what still needs checking in person.