Rooflights bring daylight into extensions, loft conversions, stairwells and single-storey rear additions where conventional windows are not possible. The market divides into three broad product families — flat rooflights, pitched roof windows and roof lanterns — each suited to a different roof type, aesthetic and budget. Material choice (polycarbonate, GRP, glass) and glazing specification determine thermal performance, durability and maintenance requirements.
Roof Windows vs Flat Rooflights vs Lanterns
Pitched roof windows (the format popularised by VELUX and similar manufacturers) are designed for installation on sloped roofs at pitches between 15° and 90°. They sit flush with the roof plane, are individually weather-sealed by a factory-made flashing kit and are available in fixed or opening configurations. Standard sizes range from approximately 550×780 mm to 940×1600 mm. These are the default specification for loft conversions in the UK.
Flat rooflights are installed in flat or near-flat roofs (typically 0°–15° pitch) and sit proud of the deck on an upstand — either a prefabricated kerb integral to the unit or a separate timber or steel upstand built on site. They are the standard daylighting solution for modern single-storey rear extensions with flat roofs. Most UK suppliers specify a minimum 150 mm upstand height to keep the junction clear of ponding water.
Roof lanterns are three-dimensional glazed structures — a ridge-and-hip or mono-pitch aluminium or timber framework glazed on all faces and the top — that sit on a flat or low-pitched roof and can light a large floor area below. They are typically bespoke or semi-bespoke and are considerably more expensive per m² of aperture than a flat rooflight of equivalent size.
Materials
Polycarbonate
Polycarbonate (PC) is a lightweight thermoplastic used in twin-wall or multi-wall sheeting systems, and in moulded dome or pyramid rooflights. Multi-wall systems (4-wall, 6-wall or 16-wall) trap air layers and improve insulation.
- Pros: low cost, lightweight, can be cold-formed on site, impact resistant
- Cons: yellows under UV over 10–20 years; scratches easily; lower optical clarity than glass; single-dome polycarbonate units have poor U-values (2.5–3.5 W/m²K) and do not meet Part L for replacement or new work
- Application: agricultural, industrial and budget conservatory roofs; rarely specified in residential renovations where aesthetics matter
- Cost: £15–£40 per m² for twin-wall sheeting; £80–£200 per m² for moulded dome units (supply only)
Multi-wall (16-wall) polycarbonate can reach U-values of 1.3–1.5 W/m²K and is used in some conservatory roofing applications, but glass units are now competitive in cost and superior in appearance.
GRP (Glass Reinforced Plastic) Domes and Pyramids
GRP moulded units — the traditional “Perspex dome” style skylight — consist of an acrylic or GRP dome bonded to a GRP or uPVC kerb upstand. Double-skinned versions trap an air layer.
- Typical U-value (double-skin): 2.2–3.0 W/m²K — well below Part L limits for new work
- Lifespan: 15–25 years before yellowing or crazing
- Still found on 1970s–1990s flat-roof extensions; replacement with glass units is now standard when renovating
GRP domes do not comply with Part L 2022 as new installations in dwellings.
Glass Flat Rooflights
Modern flat rooflights use thermally broken aluminium frames with double or triple-glazed units. Glass specification follows the same principles as vertical glazing (Low-E + argon fill + warm-edge spacer) but solar gain management is more critical because a horizontal or near-horizontal surface receives maximum solar radiation at midday.
Toughened and laminated glass is mandatory for overhead glazing under BS EN 12150-1 and BS EN ISO 12543 respectively. Most flat rooflight manufacturers use laminated toughened units (toughened outer pane, PVB interlayer, inner pane) so that if the outer pane breaks, the laminate holds fragments in place.
Typical specifications:
| Unit Type | Centre-Pane U-value | Whole-Unit U-value | Application |
|---|---|---|---|
| DG, 4-16-4 argon, Low-E | 1.0–1.2 W/m²K | 1.2–1.6 W/m²K | Standard flat rooflight (Part L compliant) |
| TG, 4-14-4-14-4 argon, 2× Low-E | 0.5–0.7 W/m²K | 0.7–1.0 W/m²K | High-spec flat rooflight |
| DG, low g-value (solar control) | 1.0–1.2 W/m²K | 1.2–1.6 W/m²K | South/west-facing; Part O compliance |
Solar factor (g-value) for horizontal rooflights should be carefully considered: a standard double-glazed unit with g=0.63 facing south can cause summer overheating in an open-plan space. Many manufacturers now offer a mid-range g-value (0.30–0.45) glass option for south-facing installations at minimal additional cost.
Aluminium Roof Lanterns
Roof lanterns use thermally broken aluminium extrusions for the structural ridge, hip and eaves members, with double or triple-glazed units in the panels. The thermal performance of the aluminium frame and the number of glass faces both affect the overall U-value.
A quality double-glazed aluminium lantern typically achieves a whole-unit U-value of 1.3–1.8 W/m²K depending on size (larger lanterns have more glass relative to frame and perform better). Triple-glazed lanterns reach 0.8–1.1 W/m²K.
Fixed vs Opening Configurations
Fixed rooflights are lower cost, simpler to waterproof and have no mechanical components to fail. They are appropriate where ventilation is provided by other means.
Electrically opening rooflights (driven by 24 V DC tubular motors, typically with rain sensors) provide stack-effect ventilation and can contribute to summer cooling strategies. Rain sensors are standard on most mid-range and premium units. Cable runs need to be planned into the build — allow 6–10 metres from the rooflight motor to a 240 V transformer typically located in the ceiling void.
Manually opening units (friction stays or spring-balanced) are lower cost but require accessible opening — suitable for loft conversions where the rooflight is at reach.
Part F (ventilation): in habitable rooms without openable windows, a rooflight with an opening area of at least 1/20th of the floor area satisfies the rapid ventilation requirement of Approved Document F.
Cost (Supply Only, 2026 UK)
| Product | Size | Approx Supply Cost |
|---|---|---|
| Fixed flat rooflight (DG, alum) | 600×900 mm | £200–£350 |
| Fixed flat rooflight (DG, alum) | 1000×1500 mm | £380–£600 |
| Electric opening flat rooflight | 1000×1500 mm | £700–£1,200 |
| VELUX GGL (pivot, pine) | CK04 (550×980 mm) | £380–£480 |
| VELUX GGU (pivot, uPVC) | MK04 (780×980 mm) | £430–£550 |
| Aluminium roof lantern (DG) | 1000×2000 mm | £1,500–£2,800 |
| Aluminium roof lantern (DG) | 2000×3000 mm | £3,500–£6,000 |
Installation costs add £300–£800 for a single flat rooflight (including upstand and waterproofing membrane work) and £1,500–£4,000+ for a roof lantern depending on structural complexity.
Planning and Building Regulations
Planning permission is generally not required for rooflights that do not protrude more than 150 mm above the roof plane (Permitted Development Class A for dwelling houses). However:
- On listed buildings, Listed Building Consent is required for any rooflight
- In Article 4 Direction areas or conservation areas, flat-on-slope or highly visible rooflights may need planning permission
- Rooflights on the principal elevation of a house visible from a highway require permission where they exceed 150 mm protrusion
Building Regulations:
- Part L: Replacement or new rooflights must achieve ≤1.4 W/m²K whole-unit U-value. Most modern double-glazed aluminium rooflights just meet this; specify the unit U-value (not centre-pane) in writing.
- Part O: For new dwellings, south and west-facing rooflights are a significant overheating risk. CIBSE TM59 or the simplified check in AD O should be carried out at design stage.
- Part K: Overhead glazing must use safety glass to BS EN 12150-1 (toughened) or BS EN ISO 12543 (laminated). Laminated units are standard for flat rooflights; check that the specification confirms compliance.
- Part B (fire): Rooflights within 1 m of a boundary must achieve Class B (or better) fire resistance — polycarbonate and some GRP units do not meet this; glass units generally do.
For flat-to-pitched conversions where a new rooflight replaces an existing opening, a Building Regulations application (Full Plans or Building Notice) is required if the work involves structural alteration. A straightforward like-for-like replacement of an existing approved rooflight in England is notifiable under Part L only and can be self-certified by a FENSA or CERTASS registered installer.
