Overheating regulations for rooflights changed permanently in 2022 when Approved Document O entered the Building Regulations for the first time.
The flat rooflight is both the best daylighting tool available for a single-storey extension and the most likely source of summer overheating if incorrectly specified. A standard 1,000mm x 1,000mm clear double-glazed flat rooflight with a g-value of 0.60 admits the equivalent of a 540-watt heat source into the room below on a clear June afternoon.
The good news is that the overheating problem is entirely solvable at the specification stage. The solution is not smaller rooflights or fewer rooflights. It is correctly specified glass, correctly sized openings, and correctly designed ventilation. This guide explains exactly what Approved Document O requires and how to meet it.
1. What Approved Document O Actually Requires
Approved Document O (Overheating) came into force in June 2022 and applies to all new dwellings in England, including extensions that create new habitable rooms. It introduces two compliance pathways:
Pathway 1 — The Simplified Method
The simplified method sets fixed limits on the total area of solar-gain glazing based on the room's orientation and the presence of cross-ventilation. For rooflights, the simplified method applies maximum glazed area limits that vary by whether the room has adequate cross-ventilation through opening windows and whether solar control glazing is specified.
Pathway 2 — Dynamic Thermal Modelling (CIBSE TM59)
Where the simplified method cannot be satisfied - typically on larger extensions with generous rooflight areas - a dynamic thermal model using CIBSE TM59 methodology must demonstrate that the operative temperature in the occupied space does not exceed 26°C for more than 3% of occupied hours during the cooling season. This is a performance-based test modelled against a 2026 UK climate dataset.
TM59 modelling is commissioned from a building services engineer or energy assessor. It costs £500–£1,500 for a typical residential project and is increasingly required on extensions with total rooflight areas exceeding 10% of floor area.
2. The g-Value: The Only Number That Matters for Compliance
The total solar energy transmittance - the g-value - is the single most important glass specification for Approved Document O compliance. It defines what percentage of total incident solar energy passes through the glass into the room. A lower g-value means less solar heat gain.
|
Glass Specification |
Typical g-Value |
Approved Doc O Suitability |
|
Standard clear double-glazed |
0.60–0.70 |
Poor — high solar gain, likely fails simplified method |
|
Low-E double-glazed |
0.45–0.55 |
Marginal — borderline on south-facing horizontal glazing |
|
Solar control soft coat double-glazed |
0.25–0.38 |
Good — meets simplified method on most orientations |
|
Triple-glazed with solar control |
0.20–0.32 |
Excellent — best performance for compliance and comfort |
For flat rooflights in south-facing or horizontal orientation, a g-value of 0.35 or below is the practical target for simplified method compliance. This requires a soft coat solar control coating on the inner surface of the outer pane - not a standard low-E coating, which primarily reduces winter heat loss rather than summer solar gain.
Our rooflight triple glazed self-clean units and glass rooflight fixed and custom sizes can be specified with solar control glass - contact our team to confirm the g-value of the specific unit configuration for your project.
3. The 25% Glazing Rule and Its Interaction with Document O
Approved Document L's 25% glazing rule - limiting total rooflight and window area to 25% of extension floor area - interacts directly with Approved Document O compliance. A larger glazed area generates more solar gain, making it harder to satisfy the overheating thresholds.
The practical implication is that exceeding the 25% threshold triggers a double burden: you need SAP calculations for Document L compliance and TM59 modelling for Document O compliance. Neither is insurmountable - but both add cost and programme time to your project.
4. Ventilation: The Other Half of the Overheating Solution
Glass specification alone cannot satisfy Approved Document O on a large south-facing extension. Ventilation is the second essential compliance mechanism and the one most frequently omitted from extension designs.
Approved Document O requires that habitable rooms have either cross-ventilation (openings on two sides of the room allowing air movement from one side to the other) or single-aspect ventilation with a total opening area of at least 8% of the floor area.
For a flat-roofed single-storey extension with no openable wall windows - a common configuration where the extension abuts a boundary on two sides - an opening rooflight is the primary ventilation mechanism. A single opening rooflight on a 15 m² extension must provide an opening area of at least 1.2 m² to satisfy the 8% rule. Most standard opening rooflights open to 15°–25° of the sash width - calculate the free area at the maximum opening angle before confirming compliance.
5. Document O and Existing Extensions: Does It Apply to You?
Approved Document O applies to new dwellings and to material changes of use creating new habitable rooms - it does not retrospectively apply to existing extensions built before June 2022. However, where a Building Regulations application is submitted for a new extension after June 2022, Document O compliance is mandatory regardless of whether the rest of the house predates the regulation.
For homeowners retrofitting rooflights into an existing pre-2022 extension - replacing an existing rooflight with a new unit of the same size - Document O does not apply as a standalone requirement, although Building Control may request evidence that the change does not materially worsen the thermal performance of the existing installation.
Conclusion
Overheating in a flat-roof extension is not inevitable - it is a specification problem with a glass-and-ventilation solution. At Skylights Roof Lanterns, our flat rooflights range includes solar control glazing options and opening rooflights designed to meet 2026 Approved Document O requirements. Call 0204 538 3079 or email sales@skylights-rooflanterns.co.uk to discuss the right specification for your project before your Building Regulations application is submitted.
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