What Part L actually requires when you re-roof or convert a loft
Whether the regulations touch your roof depends on one list of five items, and almost every page ranking for this question leaves it out.
Part L bites when your work counts as renovation of a thermal element, and Approved Document L lists exactly five things that count. Stripping a pitched roof back to the rafters is one of them. Replacing the waterproof membrane on a flat roof is another. Lifting tiles without exposing the structure is not on the list.
The short version
- Part L is written in U-values, not in depths. There is no 270mm rule in the Building Regulations. 270mm is the Energy Saving Trust’s recommendation, and Approved Document L’s own worked example is 250mm.
- Strip a pitched roof back to the rafters and rebuild it and you have renovated a thermal element. Lift and re-lay the tiles without ever exposing the rafters and you have not.
- Replacing the waterproof covering on a flat roof is on the list in its own right, which catches most ordinary domestic flat roof work.
- A renovated roof is 0.16 W/(m²·K). A brand new roof element, a dormer cheek or an extension roof, is 0.15. Two different tables, and quotes routinely mix them up.
- There are two written escape routes: a 15 year simple payback test and a head room allowance at rafter level. Both are in the guidance, so neither is a favour from your inspector.
The trigger
Renovation of a thermal element is a list of five things
The question is not how much insulation. It is whether your job counts as renovating a thermal element at all, because until it does, Part L asks nothing of the roof.
The duty sits in regulation 23 of the Building Regulations 2010. It applies where the renovation of an individual thermal element either constitutes a major renovation, or amounts to the renovation of more than 50% of the element’s surface area. Where it applies, the work must be carried out so that the whole of the element complies, so far as that is technically, functionally and economically feasible.
Regulation 23 never says what renovation means. Paragraph 11.2 of Approved Document L Volume 1 does, and this is the paragraph the insulation industry’s pages leave out. It defines renovation of a thermal element as any of the following.
- Providing a new layer by cladding or rendering the external surface of the element.
- Providing a new layer by dry-lining the internal surface of the element.
- Replacing an existing layer by stripping the element down to expose its basic structural components, meaning the bricks, blocks, rafters, joists or frame, and then rebuilding.
- Replacing the waterproof membrane on a flat roof.
- Providing cavity wall insulation.
Two of those five decide almost every roofing job. Item three is the pitched roof answer. If the tiles, battens and underlay come off and the rafters are open to the sky, you have exposed the basic structural components and what goes back is a rebuild. If the roof is lifted and re-laid without the rafters ever being uncovered, you have not.
Item four is the flat roof answer and it stands on its own. No qualifier about area, no mention of structure, no exception for a small roof. Replace the covering on a garage, a dormer top or a rear extension and you have renovated a thermal element, and because a membrane is normally replaced across the whole roof, the 50% test in the next section is met the moment the old felt comes off.
The 50% test
Half of the roof, not half of the house
Once the work is on the list, paragraph 11.3 decides whether the whole element has to be improved, and what people get wrong is what the percentage is measured against.
The whole element must be improved to the Table 4.3 column (b) standard if either more than 50% of the surface of that individual thermal element is renovated, or the work constitutes a major renovation. Major renovation has its own definition in the same paragraph: more than 25% of the surface area of the external building envelope.
So the 50% is 50% of the roof, not 50% of the building. On a plain gable roof the two slopes are roughly equal, so a rear-slope-only strip sits within a few percent of the line and the answer depends on measuring it. A hip or a catslide changes the arithmetic. Ask your building control officer how they want it measured before the scaffold goes up.
Paragraph 4.11 adds a condition that is easy to miss: a renovated element must be no worse than it was before the work, as well as meeting the table.
None of this is a planning question. A like-for-like re-cover is usually not development at all, which is covered in do I need planning permission to replace a roof, and a full re-cover is notifiable building work either way.
The numbers
0.16 for a renovated roof, 0.15 for a new one
Approved Document L Volume 1 has two U-value tables, and they are not interchangeable. Table 4.3 covers existing elements in existing dwellings. Table 4.2 covers new fabric elements, meaning a dormer, an extension roof, or anything formed where there was nothing before.
| Element | Table 4.3 (a) threshold | Table 4.3 (b) improved | Table 4.2 new element |
|---|---|---|---|
| Roof | 0.35 | 0.16 | 0.15 |
| Wall, cavity insulation | 0.70 | 0.55 | 0.18 |
| Wall, internal or external insulation | 0.70 | 0.30 | 0.18 |
| Floor | 0.70 | 0.25 | 0.18 |
| Rooflight | Not listed | Not listed | 2.2 |
| Window | Not listed | Not listed | 1.4, or Window Energy Rating band B |
Column (a) is the threshold, the value at which an existing element counts as in need of improvement. Column (b) is what you improve it to. Most roofs built before the 1990s sit the wrong side of 0.35, and 0.16 is then the target.
Look at the roof row again. There is one of it. The 2021 edition does not split the roof into pitched-at-ceiling, pitched-at-rafter and flat, and gives rafter level no softer number. Between the joists, between and under the rafters, or above the deck on a warm flat roof, column (b) reads 0.16, and how you get there is a build-up question rather than a regulatory one, which is the subject of warm roof against cold roof.
The way out
Two escape routes written into the guidance
Approved Document L expects that 0.16 is sometimes unreachable and says so twice. Both allowances are printed in the document.
The 15 year payback test
Paragraph 4.13 covers it. If achieving the column (b) U-value is either not technically or functionally feasible, or would not achieve a simple payback of 15 years or less, the element should instead be upgraded to the best U-value that is feasible and does pay back inside 15 years. It then adds a floor: generally, an upgraded thermal element should not have a U-value worse than 0.7 W/(m²·K). That is the backstop. It is not a licence to leave the roof as you found it.
The head room allowance
This is note 3 to Table 4.3, and it matters because it governs most rafter-level work on an older house. Where meeting the standard would limit head room, a lesser standard may be appropriate, provided two things are true: the depth of the insulation plus any air gap the construction requires is at least to the depth of the rafters, and the insulant is chosen to achieve the lowest practicable U-value.
Read the second half of that carefully. The note does not let you underfill. It requires the rafter depth to be used up, and then the best insulant you can get into that depth. On a 100mm rafter with a 50mm ventilation gap you have 50mm to work with, and in 50mm the difference between a good board and a poor one decides whether you pass. Thermal conductivity stops being a price question and becomes a compliance one, which is why the choice between PIR board and mineral wool matters more at rafter level than anywhere else.
Before you sign anything. Insulating at rafter level on a roof built to breathe through the loft is how cold roofs start running with condensation in February. If the ventilation path is being closed up, somebody has to say in writing what replaces it. The symptoms are in telling loft condensation from a roof leak, and by the time you can see them the plasterboard is already wet.
Loft conversions
A conversion triggers Part L with no percentage test at all
Converting a loft works differently and is stricter. There is no 50% test to argue about, because the trigger is not the amount of work but the change in what the space is for.
Paragraph 11.6 treats bringing a previously unheated loft into the heated envelope as a change to a building’s energy status, and the requirements in 11.7 then follow automatically. Existing thermal elements that now bound heated space, the roof slope and the gable, must meet Table 4.3. Anything new or replaced, a dormer cheek or a new flat roof over it, must meet Table 4.2. That is the 0.16 and 0.15 split doing real work on one job.
Two further rules in 11.7 catch people out on the glazing.
- Existing windows, roof windows and rooflights separating the new heated space from unheated space must be replaced to Table 4.2 if their U-value is worse than 3.30 W/(m²·K) for windows and roof windows, or 3.80 W/(m²·K) for rooflights. An old single-glazed rooflight in the way of the conversion is coming out.
- The total area of windows, roof windows, rooflights and doors in the newly created dwelling should be no more than 25% of its total floor area, so a design that wants a wall of glass at the gable end has to find that area from somewhere.
A loft conversion goes through building control as a whole, by building notice or by an application with full plans, and the difference is set out in building notice, full plans or a competent person scheme. A straight roof re-cover is different: still notifiable, but Schedule 3 of the Building Regulations 2010 lets a registered roofer self-certify it.
Corrections
Four things you will read that are not true
“270mm is required by Building Regulations”
270mm is not required by Building Regulations. Part L does not specify a depth anywhere, for any element. It specifies U-values, and how you reach one depends on the product and the construction. 270mm is the Energy Saving Trust’s recommendation for mineral wool at ceiling level, which is sensible advice and not a legal requirement.
Approved Document L’s own worked example uses 250mm. A quote saying 270mm because the regulations demand it has the reason wrong even where the depth is fine.
“Insulation at rafter level only has to reach 0.18”
That was the 2013 edition, which split the roof row by where the insulation sits. The 2021 edition has one roof row and it reads 0.16. Plenty of current pages, manufacturers included, still quote the superseded split, so a spec written to 0.18 at rafter level today is written to a document that no longer applies.
“You have to insulate whenever you replace roof tiles”
No, and the five-item list shows why: go back to it in paragraph 11.2. Replacing tiles without stripping down to the structural components is not on it. Then the 50% test has to be met, and then the payback test and the head room note both apply. A roofer telling you the law forces a full rafter-level upgrade for a tile swap has cleared none of those hurdles.
“The Future Homes Standard applies now”
It depends on the date of your job. Approved Document L 2026 was published on 24 March 2026, but the amending regulations come into force on 24 March 2027, with transitional provisions running to 24 March 2028. Before 24 March 2027 the document that governs a re-roof or a loft conversion is Approved Document L Volume 1, 2021 edition incorporating 2023 amendments. After that, whether your job falls under the old or the new edition turns on the transitional provisions, so ask your building control body which one it is applying. The trap is that both editions sit on the same gov.uk publication page and the 2026 file is the more prominent download. Check the cover of the document anyone quotes at you before you accept a number from it.
Next steps
What to do, in order
Part L on a re-roof turns on what is coming off: tiles only, tiles and battens, or everything back to bare rafters. Settle that trigger first, then get the target U-value and its build-up written on the quote. Everything follows from it.
- Write down what is coming off: tiles only, tiles and battens, or everything back to bare rafters. That sentence decides whether Part L is in play.
- For a flat roof, assume it is in play: replacing the covering is on the list.
- Measure the slope you are doing against the whole roof, not the house, and get your building control officer to confirm how they measure it.
- Ask for the target U-value on the written quote with the build-up that achieves it: the product, its thickness, its thermal conductivity and the ventilation gap. A number with no build-up behind it is not a specification. The rest is in what a roofing quote should include.
- If 0.16 will not fit, ask which written allowance is being relied on, the payback test or the head room note, and have it put on the quote. Neither survives being invented on site.
- Keep the completion certificate. It is what a buyer’s conveyancer asks for.
This page sets out the general position in England. It is an explainer, not legal advice, and your building control body decides your job. If you want somebody to say plainly whether the work you are being sold triggers any of this, Thomas comes out and puts what he found in writing. What we cover is on the insulation page and the roofing page, and you can ask for a free written itemised quote.
Questions
Common questions
Do I legally have to insulate my roof when I have it re-covered?
Only if the work counts as renovation of a thermal element under paragraph 11.2 of Approved Document L, and then only if more than 50% of the roof's surface is renovated or the job is a major renovation. Stripping back to bare rafters counts. Lifting and re-laying tiles without exposing the rafters does not.
What U-value does Part L require for a renovated roof?
0.16 W/(m2K), from column (b) of Table 4.3 in Approved Document L Volume 1, 2021 edition incorporating 2023 amendments. A brand new roof element, such as a dormer or an extension roof, is 0.15 W/(m2K) from Table 4.2. The 2021 edition has one roof row and does not give rafter level a different figure.
Is 270mm of loft insulation a Building Regulations requirement?
No, part L is written in U-values and never specifies a depth. 270mm is the Energy Saving Trust's recommendation for mineral wool at ceiling level, and Approved Document L's own worked example uses 250mm.
What if there is not enough room to reach 0.16 between the rafters?
Note 3 to Table 4.3 allows a lesser standard where meeting it would limit head room, provided the insulation plus any required air gap fills the depth of the rafters and the insulant chosen achieves the lowest practicable U-value. Paragraph 4.13 separately allows a lesser standard where the upgrade would not pay back within 15 years, down to a floor of about 0.7 W/(m2K).
Does the Future Homes Standard apply to my loft conversion now?
Not before 24 March 2027: approved Document L 2026 was published on 24 March 2026 and comes into force on 24 March 2027, with transitional provisions to 24 March 2028. Until then the 2021 edition incorporating 2023 amendments governs the work. After that, ask your building control body which edition applies to your job.
Read next
Related advice
The guides below pick up where this one leaves off, and the insulation page covers the work itself and the places we cover, if you would rather have the job looked at.
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
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Not sure whether the regulations touch your roof?
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