Flat roof work meets the Building Regulations in three places: how much of the building is being changed, the insulation standard in Approved Document L (Part L), and the way the roof drains and keeps moisture out. Each can apply to a replacement membrane, and each has its own test. Patching a small area usually sits outside all of them. A full re-cover of a flat roof is different, because Part L names it directly.
Approved Document L Volume 1 puts a replacement top layer on a flat roof under renovation. Renovating a roof brings an insulation standard, set out as a U-value, a figure for the heat that leaks through a roof. The Planning Portal's re-roofing test, which looks at how much of the building envelope and of the roof is affected, sits alongside that.
This guide sets out how those rules apply to a flat roof replacement, with the figures as published. It covers the 25 per cent and 50 per cent test, the Part L renovation figures, the date the next edition takes effect, the finished fall required by BS 6229:2025, and who to ask. Treat it as a general guide: your building control body decides how the rules apply to your roof.
Why a new membrane is renovation
A thermal element in the Building Regulations means a wall, floor or roof, and the definition leaves out windows, doors, roof windows and rooflights. Regulation 2(3) of the Building Regulations 2010 defines renovation of a thermal element as adding a fresh layer or swapping an existing one.
What Part L says about flat roofs
Approved Document L Volume 1 (2021 edition, 2023 amendments) takes effect for work in England from 15 June 2022. Its examples of renovation include a replacement flat-roof membrane. A new felt, EPDM or GRP covering over an existing deck therefore falls within the document, and the insulation has to be looked at.
What triggers the work
The trigger is the renovation, not the size of the roof, and that surprises owners of small garage roofs. If you strip the old covering and lay a new membrane, the roof is being renovated. If you only patch a small area, you are repairing, not renovating. The line is not always sharp, which is why a survey and a conversation with building control help.
Who the rules reach
The rules apply to homeowners and landlords alike, and the work must comply whoever pays. For Rushmoor and Hart addresses, one team handles applications from the Farnborough offices, on 01252 398715. Private firms registered as building control approvers, under rules in force since 6 April 2024, are an alternative.
Our guide to warm and cold flat roofs explains the two build-ups that these figures steer you towards, and why the choice matters.
The 25 per cent and 50 per cent test
The Planning Portal gives a simple national test for re-roofing. It asks two questions about the work, and if both answers are yes, a building control application is not normally needed.
| Question | Limit |
|---|---|
| Share of the total building envelope affected | Less than 25 per cent |
| Share of the roof affected | Less than 50 per cent |
What the building envelope is
Together, the envelope is the whole shell of the building: its walls, floor, roof and every opening in them. A flat roof over a small extension may be a modest part of the envelope but the whole of that roof, and it is the roof share that decides the outcome. Replacing all of a flat roof therefore usually fails the second question, because it affects more than half of that roof.
A word on councils
No council in this area publishes its own percentage. Bracknell Forest and Basingstoke and Deane include roof re-covering among works that may also need approval, and Windsor and Maidenhead names re-tiling and re-felting. Each speaks for its own area only. Rushmoor and Hart publish no percentage, and Rushmoor points to the Planning Portal. None of these says approval is always needed.
Weight and fire
Whatever the percentages, approval also comes into play where the new covering is 15 per cent or more heavier, or burns very differently from the old. Flat roof coverings are generally light, but a switch to a heavy finish such as paving on supports is worth checking.
For a garage roof or an extension roof, the sensible step is to ask your building control body before the work, and keep the answer in writing.
Insulation under Part L: the renovation figures
When a flat roof is renovated, Part L asks you to look at the existing insulation and to improve it if it falls short. The measure is the U-value, given in watts per square metre per degree; the smaller the number, the better the insulation.
| Situation | Figure |
|---|---|
| Existing roof, threshold for renovation (Table 4.3) | 0.35 W/(m²·K) |
| Existing roof, improved standard (Table 4.3) | 0.16 W/(m²·K) |
How to read it
Renovation of an existing roof uses Table 4.3. If the roof is worse than 0.35, it has to be improved towards 0.16. Table 4.2 carries a separate, stricter figure for brand-new and replacement elements, and it is not the one for a renovation job.
When the full figure is not feasible
Paragraph 4.13 allows a relaxation. Where the full improvement is out of reach in practice, or would need over 15 years to pay for itself on a simple basis, the roof goes as far as that payback allows. Generally a thermal element once upgraded should not exceed 0.7. The point is that it is a judgement, and the reasoning belongs on the file.
Upstands and details
An upstand built on site, the raised edge where the roof meets a wall, should have a maximum U-value of 0.35. Edges, upstands and the junction with the house wall are where a flat roof loses heat and where moisture collects, so the insulation detail there deserves as much attention as the main surface.
How much insulation fits in the build-up is then a design question for the survey.
The next edition: AD L 2026
Approved Document L is being revised, and the date matters if your work might start near it. The 2026 edition takes effect on 24 March 2027 for use in England, for building work that is not in connection with higher-risk building work.
What changes in the numbering
In the 2026 edition the tables are renumbered 3.2 for new elements and 3.3 for existing elements, and the payback rule sits in paragraph 3.14. Until the new edition takes effect, the 2021 edition with its tables 4.2 and 4.3 applies. From 24 March 2027, the 2026 edition applies to work it covers.
Which applies to your job
Which edition governs a particular job is something to confirm with building control, and we ask rather than assume. For a flat roof job near that date, the answer goes on the file.
Why it matters for a roof
A figure that is right today may be replaced. If you are planning work for spring 2027, ask early and keep your paperwork. The insulation thickness you specify now may need to change if the figures tighten, and a decision made on the wrong edition can mean redoing design work.
Do not assume the 2026 figures apply now. The edition in force today is the 2021 edition incorporating 2023 amendments, and the date for the new one is the date quoted. Our flat roofing page lists the work these figures affect.
Until 24 March 2027, AD L 2021 Tables 4.2 and 4.3 apply, and from that date AD L 2026 Tables 3.2 and 3.3.
Falls and drainage under BS 6229:2025
A flat roof is never truly flat, because water must run off it. BS 6229 is the British Standard for membrane flat roofs laid on a continuous deck: felt, EPDM and single-ply. Lead sheet is outside it, and follows BS 6915 through Approved Document C.
Which edition is current
BS 6229:2018 was withdrawn on 16 December 2025. BS 6229:2025 is the current edition, written for roofs pitched at 10 degrees or less. Citing the 2018 edition as current is a common error.
Design fall and finished fall
BS 6229:2025 asks for a finished fall of no less than 1:80 over general roof areas and internal gutters. For design, IKO's advice is to draw the roof with a 1:40 fall. The two numbers answer different questions: what to design for, and what the finished roof must achieve. Mixing them up is a common error, and so is calling 1:80 the design fall.
Why the fall matters
A roof that holds water ponds. Standing water ages the membrane, gathers dirt and can find weak laps. Approved Document C expects the roof both to resist rain and to avoid harmful condensation within its layers. Our answer on flat roof minimum fall goes through the figures.
On the survey
We measure the fall of your existing deck, and photograph ponding marks and outlets. Where the deck falls short, tapered insulation can add fall, and the quote says so.
Ventilation, condensation and who checks the work
The third strand of the regulations is moisture. Under paragraph 6.10 of Approved Document C, a roof passes the condensation test if it follows Clause 8.4 of BS 5250 (2002 edition) and BS EN ISO 13788:2002, while paragraph 6.11 looks to ventilation as the answer for cold deck roofs. The current BS 5250 is the 2021 edition, which BSI lists as current but under review.
Cold decks and warm decks
A cold deck roof has insulation between the joists with a ventilated void above it. A warm deck has insulation above the structure, with no void to ventilate. The two behave differently, and the regulations treat them differently. Paragraph 6.11 also leaves room for roofs too small to need vents, such as over a porch.
Who checks the work
Your building control body inspects work that needs approval. In Rushmoor and Hart that is the shared partnership. Wokingham's service is Building Control Solutions, Elmbridge's is EBCS and Surrey Heath's is SHBCC, and other councils run their own. A registered building control approver can be used instead.
Before the job
Our flat roofing hub lists the work involved. Ask whether your work needs an application, which edition of Part L applies, and what insulation the roof must reach. Keep the answers in writing. We raise these at the survey and give one written price after it.
Planning is separate. A new covering on a house normally falls under Class C permitted development, but flats have no such rights, and listed buildings and conservation areas add checks. Our answer on flat roof planning permission covers that side.
