The short answer
Replacing more than 25% of a roof slope brings the work under Building Regulations — mainly Part L (insulation) and Part A (structure). You either notify building control before the work and have it inspected, or use a roofer registered with CompetentRoofer who can self-certify that the work meets the standard. Small repairs and re-covering less than 25% of a slope with the same type of covering usually do not need approval. This is separate from planning permission, which most like-for-like re-roofs do not require.
Building Regulations and planning permission are two different things, and roof work often involves one without the other. Planning is about appearance and impact; Building Regulations are about safety and performance — insulation, structure, fire and weatherproofing. For most re-roofs the regulation that matters is Part L, triggered by the amount of the roof being renewed. This guide explains when the regulations apply, what they require, and the two routes to compliance. It is general information, not a substitute for advice from building control or a registered roofer.
Building Regulations at a glance
- Trigger threshold >25% of a roof slope
- Main regulation Part L (insulation)
- Also relevant Part A (structure)
- Self-certify route CompetentRoofer
- Alternative route building control notice
- Small repairs usually exempt
The 25% rule
The key threshold is straightforward: if you re-cover more than 25% of a roof slope, the work falls under Building Regulations. Below that, a like-for-like repair or partial re-cover generally does not. The logic is that once a significant share of a slope is being renewed, it is a sensible point to bring the roof's thermal performance up to current standards rather than rebuild it to the old, less efficient specification. Because a full re-roof always exceeds 25%, full replacements are regulated by default.
Part L — insulation
Part L (conservation of fuel and power) is the regulation most re-roofs engage. When triggered, it can require the roof's insulation to be improved to current standards as part of the work — typically by adding or upgrading insulation at loft or rafter level. This is why a re-roof quote may include an insulation line. Alongside Part L, ventilation requirements under Approved Document F apply so the upgraded roof does not trap moisture. Our insulation and ventilation guide explains how the two work together.
Part A — structure
Part A (structure) becomes relevant where the new covering is significantly heavier than the old one — for example replacing lightweight felt or some concrete tiles with natural slate, or vice versa. A heavier covering can overload rafters and trusses designed for something lighter, so the structure may need checking or strengthening. A competent roofer will flag this; where there is doubt, a structural assessment is the safe route before the new covering goes on.
Two routes to compliance
There are two ways to satisfy the regulations on a re-roof. The first is the building control route: you (or your roofer) submit a building notice or full plans to your local authority building control or an approved inspector, and the work is inspected and signed off. The second is the CompetentRoofer self-certification route: a roofer registered with the scheme certifies that the completed work meets the regulations and notifies building control on your behalf, which produces a compliance certificate without a separate application. The self-certification route is usually simpler and is one practical reason to use a registered contractor for a full re-roof.
| Work | Building Regulations? |
|---|---|
| Replacing a few slipped tiles | No |
| Re-covering less than 25% of a slope, like-for-like | Usually no |
| Re-covering more than 25% of a slope | Yes (Part L) |
| Full re-roof | Yes (Part L; Part A if heavier covering) |
| Switching to a much heavier covering | Yes (Part A) |
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Frequently asked questions
Does a new roof need Building Regulations approval?
Yes if you re-cover more than 25% of a roof slope, which a full re-roof always does. The work falls mainly under Part L (insulation), and Part A (structure) if the new covering is much heavier. You can satisfy the rules through building control or by using a CompetentRoofer-registered roofer who self-certifies the work.
What is the 25% rule for roofs?
If more than 25% of a roof slope is re-covered, Building Regulations apply — principally Part L, which can require an insulation upgrade to current standards. Re-covering less than 25% like-for-like, or carrying out small repairs, generally does not trigger the regulations.
What does CompetentRoofer self-certification mean?
CompetentRoofer is a government-authorised scheme that lets registered roofing contractors self-certify that their re-roofing work meets Building Regulations, and notify building control on your behalf. It produces a compliance certificate without a separate building-control application, which is usually simpler than the building-control route.
Sources & further reading
- GOV.UK / Building Regulations Approved Document L — conservation of fuel and power
- GOV.UK / Building Regulations Approved Document A — structure
- CompetentRoofer — government-authorised self-certification scheme for re-roofing
- GOV.UK Planning Portal — guidance on roof work and building control
This is general information, not advice for your specific property or roof. Whether and how the regulations apply depends on your home and the work. Confirm the position with your local building control or a CompetentRoofer-registered roofer, and use a vetted roofing contractor.