Rooflight & Skylight Glossary: Terms Explained

Rooflight terms A–Z64 terms

Access hatch: A hinged rooflight used to reach a roof or terrace.
ACR[M]001: The industry test for non-fragility of roof assemblies, including rooflights.
Actuator: The motor that opens an electric rooflight or vent.
Anti-slip frit: A ceramic pattern fused to glass to add grip on walk-on surfaces.
AOV: Automatic opening vent — a smoke vent that opens on a fire signal.
Article 4 direction: A council order removing some permitted development rights in an area.
Barrel vault: A curved continuous rooflight used on commercial buildings.
Box rooflight: A rooflight with glazed sides, often sliding, used for roof terrace access.
BS EN 12101-2: The standard for natural smoke and heat exhaust ventilators.
Builder's upstand: An upstand built on site, usually timber with insulation, rather than supplied by the rooflight maker.
Building warrant: Scotland's approval for building work, issued by the local authority verifier.
Competent person scheme: A government-authorised scheme allowing registered installers to self-certify certain work.
Conservation rooflight: A low-profile rooflight styled on traditional cast-iron designs.
Diffuser: The ceiling panel of a sun tunnel that spreads light.
Dome rooflight: A moulded plastic dome rooflight on an upstand.
EPDM: A synthetic rubber flat-roof membrane.
Fall: The slope built into a roof or rooflight so water runs off.
Flashing: Shaped metal or flexible material that weathers a roof window or rooflight into the surrounding roof covering.
Flashing kit: The set of flashings supplied for a roof window to suit a tile or slate profile.
Fragile roof: A roof or rooflight that would not safely support a person; HSE advises treating roofs as fragile until confirmed otherwise.
Frameless rooflight: A rooflight with no visible frame from inside; the support is concealed.
Gas strut: A gas-filled spring that supports the lid of an access rooflight.
Glazing bar: A bar dividing a rooflight or glass roof into panes.
GRP: Glass-reinforced plastic — used for flat roof coverings and for rooflight sheets.
Horizontal plane: How rooflight U-values are quoted in England and Wales, because the unit lies flat.
Imposed load: The load a structure is designed to carry from people and use, as stated by the manufacturer or engineer.
In-plane rooflight: A translucent sheet matching a profiled metal roof, laid in the same plane.
Kerb: Another name for an upstand.
Laminated glass: Two or more panes bonded with an interlayer so the glass holds together if broken.
Lawful Development Certificate: A certificate confirming that work is lawful without planning permission.
Listed building consent: Consent needed for works affecting the character of a listed building.
Low-E coating: A thin coating that reflects heat back into the room.
Low-iron glass: Extra-clear glass without the green tint of standard float glass.
Non-fragile: A safety classification meaning a roof element should resist a person falling onto it; not the same as walk-on.
Party wall: A wall shared with a neighbour; work affecting it may need notice under the Party Wall etc. Act 1996 in England and Wales.
Permitted development: Building work allowed without a planning application, within set limits.
Plaster box: The plastered lining around a roof lantern or rooflight opening inside the room.
Purge ventilation: Rapid ventilation to clear pollutants or heat, defined in Approved Document F.
Rain sensor: A sensor that closes an opening rooflight when it detects rain.
Responsible person: The person with fire safety duties for a non-domestic building in England and Wales.
Reveal: The internal sides of the opening between the ceiling and the rooflight frame.
Ridge rooflight: A dual-pitched glazed roof meeting at a ridge.
Ring beam: A beam around a large opening that supports a roof lantern or glass roof.
Roof lantern: A raised, hipped glazed structure with glass on each face, meeting at a ridge, sitting on a flat roof.
Roof window: A skylight made for a pitched roof, fitted in the plane of the tiles with a flashing kit; usually opens.
Rooflight: A glazed unit fitted in a flat or low-pitch roof, usually on an upstand, to bring daylight into the room below.
Sealed unit: An insulated glass unit (IGU) of two or three panes with a sealed cavity.
Self-cleaning glass: Glass with a coating that helps rain wash dirt away.
Single-ply membrane: A flat roof waterproofing sheet such as PVC or TPO, often dressed up rooflight upstands.
Skylight: The everyday word for any window in a roof. In the trade it usually means a rooflight or roof window.
Solar-control glass: Glass with a coating that reduces solar heat gain while admitting visible light.
Splayed reveal: A reveal angled outward to spread light into the room.
Structural glass: Glass designed to carry load, used in walk-on, frameless and glass-link products.
Sun tunnel: A tubular skylight with a reflective tube carrying daylight to a ceiling diffuser.
Toughened glass: Heat-treated safety glass that breaks into small pieces.
Trimmer: A joist or rafter fixed across an opening to carry the cut members.
Triple glazing: A sealed unit with three panes and two cavities.
U-value: The rate of heat loss through an element, in W/m²K. Lower is better.
Ud: The U-value of a whole rooflight or door unit, including frame.
Upstand: The raised kerb on a flat roof that a rooflight sits on; also called a kerb.
Vertical plane: How window and roof-window U-values are quoted; Northern Ireland also assesses rooflights this way.
Walk-on rooflight: A structural glass rooflight designed for foot traffic.
Warm-edge spacer: A low-conductivity spacer bar in a sealed unit that reduces edge condensation.
WER: Window Energy Rating — a band (A++ to E) for windows and roof windows.

Rooflight and skylight glossary

Plan the light before you build