What is a roof light?

A roof light (also called a skylight) is a window installed in a roof slope or flat roof to admit natural light - and in opening versions, ventilation - to rooms below. In-slope roof lights (Velux-type) sit within the roof slope between rafters, roughly flush with the tile surface. Flat roof lights are sealed units set into or on top of a flat roof membrane. Roof lanterns are glazed structures projecting above a flat roof, forming a pyramid or ridge shape to maximise light and headroom. Sun tunnels (or light tubes) use a reflective tube to pipe daylight from a small roof-mounted dome to a diffuser in the ceiling below.

Do roof lights need planning permission?

Roof lights are generally Permitted Development (PD) on the rear or side slopes of a house, provided they do not project more than 150mm above the plane of the roof slope and do not overhang the outer wall of the house. Front-elevation roof lights are also PD unless they project more than 150mm above the roof plane. In conservation areas, roof lights on the principal or side elevation visible from a highway are not PD and require planning permission. On listed buildings, any roof lights require listed building consent. Flat roof lanterns on extensions may also be PD, but large roof lanterns that materially alter the appearance of a house may require a householder application.

What thermal performance do roof lights need to achieve?

Approved Document L sets a limiting U-value for roof windows of 1.4 W/m2K (centre-pane) for replacement units and 1.6 W/m2K for new roof windows in new dwellings. Modern Velux-type centre-pivot roof windows with triple glazing achieve around 0.7-1.0 W/m2K. Flat roof lights and lanterns typically achieve 1.0-1.4 W/m2K with argon-filled triple glazing. The total glazed area of a building (including roof lights) is subject to an area limit in Part L compliance calculations - adding large areas of glazing in an extension increases the thermal demand and may require compensating improvements elsewhere.

What is the difference between a Velux window and a roof lantern?

A Velux window (Velux is a brand name - the generic term is 'in-plane roof light') is a window installed flush within a pitched roof slope, between or across rafters. It sits at the same angle as the roof and protrudes minimally above the tile plane. A roof lantern is a glazed structure rising above a flat roof, typically in a pyramid, ridge, or ridged hip shape, bringing light and a sense of volume to the room below. Velux-type windows are for pitched roofs; lanterns are for flat roofs. Both achieve natural light but lanterns are more expensive and architecturally distinctive.

Roofing noun

Roof Light

/ ruːf laɪt /

Also known as: skylight, Velux window, roof lantern, sun tunnel, flat roof light

Definition

A roof light (or skylight) is a window set into a pitched or flat roof to admit daylight and sometimes ventilation. In-slope roof lights (Velux-type) fit between rafters and lie roughly flush with the tiles. Flat roof lanterns rise above the roof surface in a pyramid or ridge shape. Sun tunnels pipe daylight from a roof dome to a ceiling diffuser below. Roof lights are common in loft conversions, single-storey extensions, and deep-plan rooms that cannot be lit from side windows.

In practice

In-slope roof lights require careful installation to maintain weathertightness - they are fixed with a factory-made flashing kit (tiles or slates fitting tight to the frame) or a low-profile tile replacement flashing. The window unit sits on a kerb or directly on the rafters, and the surrounding tiles are cut to fit the frame perimeter. The internal reveal (lining) splays out below the window to maximise light spread into the room below. Opening roof lights provide essential ventilation in loft conversions - Building Regulations require minimum ventilation areas in habitable rooms.

Flat roof lanterns are fitted on an upstand above the waterproofing membrane, with the lantern frame kerb standing clear of the roof surface so that the membrane can terminate properly around it. The kerb must be at least 150mm high to prevent wind-blown water from backing under the frame. Condensation management is important in both in-slope and flat roof lights - modern triple-glazed units have warm-edge spacers and low-E coatings to reduce condensation risk. Roof lights in bathrooms or over showers should be obscure-glazed for privacy.

Building Regulations

Thermal (Part L)U-value limit 1.4 W/m2K for replacement; 1.6 W/m2K new build

Ventilation (Part F)Opening roof lights can provide rapid ventilation to habitable rooms

Planning (PD)PD if projecting under 150mm above roof plane; front elevation restrictions apply

Glazing (Part N)Safety glazing required where roof light is in a critical location

Approved Document L specifies limiting U-values for roof windows: 1.6 W/m2K for new dwellings and 1.4 W/m2K for replacement units. Triple-glazed units are typically required to meet these standards. Approved Document F requires habitable rooms to have background ventilation (trickle vents) and rapid ventilation - an openable roof light can provide rapid ventilation for a loft room. Approved Document N requires glazing in critical locations (within 800mm of floor level or in certain door positions) to be safety glass - check whether a low-mounted roof light or floor-to-ceiling extension glazing falls within a critical location.

Full Building Regulations guidance

Roof Light

Just so you know...