Rooflights and Building Regulations: What You Need to Know

A like-for-like replacement rooflight - replacing an existing rooflight of the same or smaller size in the same position - is generally exempt from building regulations under the Schedule 2 exemption for repairs and replacements, provided the replacement window meets the thermal performance requirements of Part L (see below).

A new rooflight inserted into an existing pitched roof - where no structural alteration is needed and the opening is small relative to the rafter spacing - is similarly low-risk and unlikely to trigger a formal building regulations application, though you must still ensure glazing safety compliance under Part N.

However, the situation changes significantly in the following circumstances:

• The rooflight is part of a loft conversion project
• The opening requires cutting through a structural rafter or purlin
• The rooflight is a large structural flat roof light over an extension
• Multiple rooflights significantly change the thermal performance of the roof

Building regulations apply in full to rooflights in the following situations:

• Loft conversions: All rooflights that form part of a loft conversion are within scope of the loft conversion building regulations application. This includes roof windows for light and ventilation, and any dormer windows or juliet balconies.
• Structural roof alterations: Cutting through a structural rafter or purlin to create a new opening requires a structural engineer to confirm the temporary and permanent structural arrangement. This triggers Part A of the Building Regulations and you must notify building control.
• Flat roof structural glazing: A large rooflight over a flat roof extension - such as a lantern light, walk-on glazing, or structural skylight - is an integral part of the extension's building regulations application. Parts A, N, and L all apply, and thermal performance calculations must be submitted.
• Significant thermal changes: If multiple new rooflights are installed as part of a wider renovation and the total area of new glazing is substantial, Part L compliance for the whole dwelling may need to be assessed.

Part N of the Building Regulations (Glazing - Safety in Relation to Impact) requires safety glazing in critical locations. All new rooflights must use glass that is safe in the event of breakage.

For rooflights, the relevant standard is BS EN 14963 (Roof windows and rooflights - continuous rooflight kits with glazing). The key requirement is that a broken rooflight does not allow broken glass to fall onto people below. There are two acceptable approaches:

• Laminated glass: When broken, laminated glass holds together rather than shattering. This is the preferred solution for most overhead glazing applications and is standard in most proprietary rooflight products.
• Catch systems: In some commercial and large-format glazing applications, a catch net or rail below the glazing can prevent falling glass. This is rarely used in domestic rooflights.

When purchasing a rooflight, confirm with the supplier that the product carries a CE or UKCA mark and meets BS EN 14963 for overhead glazing applications. Cheap or uncertified glazing units may not meet this standard.

Part L of the Building Regulations (Conservation of Fuel and Power) requires that replacement and new rooflights meet minimum thermal performance standards.

For replacement rooflights (like-for-like in England):

• The replacement unit must achieve a centre-pane U-value of 1.2 W/m2K or better
• For the whole unit (including frame), the typical target is 1.4 to 1.6 W/m2K depending on the product

For new rooflights installed as part of a larger project (loft conversion, extension), the U-value must form part of the overall energy compliance calculation. The total area of glazing relative to floor area is also regulated - excessive glazing can cause the dwelling to fail the Part L compliance calculation even if each individual unit meets its target U-value.

Most leading rooflight manufacturers (Velux, Fakro, Keylite) publish the U-values for their products and these are readily available to check against the regulatory requirement.

Part B of the Building Regulations (Fire Safety) applies to rooflights near property boundaries. Approved Document B requires that roof coverings and roof structures within 6 metres of a boundary must have a limited spread of flame rating.

For rooflights, this means that any rooflight within 6 metres of a boundary must achieve Class 1 or better (now EW or Broof(t4) classification in current terminology) for external fire performance. This is most commonly relevant for:

• Flat roof rooflights on extensions close to a garden boundary or fence
• Rooflights on outbuildings within 1 metre of a boundary

For standard pitched roof applications on a house set well back from boundaries, Part B is unlikely to raise issues. But if you are planning a flat roof extension with a glazed roof light near the back fence, check the boundary distance with your building control inspector.

Part F of the Building Regulations (Ventilation) requires that habitable rooms have adequate purge ventilation - the ability to rapidly change the air in a room, typically via an openable window. An opening rooflight can count toward the purge ventilation requirement for a loft room or any room where a conventional side-hung window is not available.

Approved Document F 2021 requires purge ventilation equivalent to at least 1/20th of the floor area of the room. For a 15m2 loft bedroom, you need at least 0.75m2 of openable vent area. A standard 780mm x 980mm roof window (Velux MK04 equivalent) has an openable area of approximately 0.55m2 - so two such windows may be needed to meet purge ventilation for a medium loft room.

Your building control inspector or architect will check purge ventilation compliance as part of the loft conversion application.

Flat roof rooflights - including lantern lights, walk-on glass floors, and large format structural skylights - are the most technically demanding rooflight applications and always require full building regulations compliance.

Key requirements for flat roof structural glazing:

• Part A (structural): The structural frame supporting the glazing must be designed by a structural engineer. Load calculations must account for snow loading, wind loading, and any imposed loads (for walk-on applications).
• Part N (glazing safety): Overhead structural glazing must use laminated safety glass throughout. The glass specification (number of plies, thickness, interlayer type) must be calculated for the span and loading.
• Part L (thermal): Structural glazing systems must meet the same U-value targets as other rooflights - typically a whole-unit U-value no worse than 1.6 W/m2K, though some high-performance systems achieve 1.0 W/m2K or better.
• Part C (weather resistance): The junction between the glazing frame and the flat roof waterproofing membrane is a critical detail for weather exclusion. This must be properly specified and executed.

If you are unsure whether your rooflight installation requires building regulations approval, the safest approach is to contact your local authority building control (LABC) and ask. Most LABC offices will give you informal guidance over the phone at no charge.

Over-notifying is always better than under-notifying. Missing building regulations sign-off for glazing or structural work can cause problems during a property sale - solicitors routinely request completion certificates for any work that should have been inspected.

For straightforward like-for-like rooflight replacements carried out by a reputable installer using certified products, building regulations compliance is largely a matter of using the right glass specification and keeping receipts and product data sheets.