Skylight or Rooflight: Is There a Difference?

Both terms describe a glazed unit fitted in a roof. Skylight is the word most homeowners use, and it is the one in most search boxes. Rooflight is the word used by architects, building control officers, manufacturers and installers in Britain, and it is the term used in Approved Document L, the official guidance on the energy rules, which notes that a rooflight is also sometimes referred to as a skylight.

If you ask a roofer for a skylight you will be understood. If you ask a manufacturer's technical desk for a rooflight you will get the same product. This site uses both terms for that reason: a page about skylights and a page about rooflights are about the same units.

The useful distinctions are between kinds of unit, not between the two names. Each kind sits in a different roof and is measured differently.

A roof window is made for pitched roofs and in most versions opens like a window, with a rated pitch range for each model. The glazed panel on a flat roof is a flat rooflight. Raise that panel into a multi-sided glazed roof and it becomes a lantern. A sun tunnel is different again, bringing daylight down a reflective tube. The roof window installation and roof lantern installation pages cover the two most common.

VELUX is a manufacturer, but many people say "a Velux" to mean any roof window, much as people say "hoover" for a vacuum cleaner. The same is true of some other makers' names. A product name tells you who made the unit. It does not tell you the type.

VELUX size codes are the clearest example of why the brand matters at the survey. The first letter of a code such as MK04 is the width and the digits are the height: M is 78 cm wide, and 04 is 98 cm high. Another maker's window of similar size has its own code system. The measured roof settles which sizes fit; the label on the box does not. The VELUX window installation page explains the range.

Part L, the energy rules for existing homes in England, names a rooflight and a roof window separately when it sets targets for heat loss. The figure for a rooflight is 2.2 W/m²K, while a roof window has two routes: 1.4 W/m²K or an energy rating in Band B.

Approved Document L defines the two by position. A roof window is installed in the same orientation as, and in plane with, the surrounding roof. A rooflight is a glazed unit installed out of plane with the roof surface on a kerb or upstand, which takes in flat-roof units and lanterns. Whether the unit opens plays no part in the definition. For you, it means the glazing you choose has a performance figure to meet, and the product data sheet will show which figure applies. The guide to Building Regulations for rooflights sets out the notification rules.

Skylight is the standard word in North America, where it covers every glazed opening in a roof. British catalogues and technical papers prefer rooflight, the word the official guidance uses. In conversation, people in Britain switch between them without noticing.

Search habits follow the same split. Homeowners tend to type skylight, and tradespeople tend to write rooflight on invoices and drawings. A quote that says rooflight and a conversation that says skylight can be about the same unit, which is why a written description of the unit's size and type matters more than its label.

Some listings add a second layer of names: lantern light, roof glazing, loft window, dormer window. A dormer is a different thing, a small structure with its own roof and vertical window that projects from the slope, and it is not a rooflight at all. A loft window might be a roof window or a dormer, so the survey sheet records which.

Either. What helps more than the word is a description of the roof and the room: pitched or flat, how many openings, what size the old one is if there is one, and what you want from the new one. A photograph from inside and one from the garden saves a conversation.

If you do not know whether your roof is pitched or flat, or what the existing unit is called, say so. At the survey we measure the roof and the opening, and the written survey sheet names the type, its size and the kerb or rafter detail, in plain words. The sheet then becomes the single reference for every later conversation about the job, so the names you started with do not matter.