Rooflight glossary | Fitting terms explained

Actuator: A small electric motor, driving a chain or spindle, that opens and closes an electric rooflight or smoke vent when it receives a signal from a switch, remote or sensor.
AOV: Automatic opening vent. A vent, often a rooflight, that opens automatically in a fire to release smoke from a common stair or corridor, triggered by detectors or a manual switch.
Article 4 direction: A council order that removes some permitted development rights in a defined area, often a conservation area, so work such as adding rooflights needs a planning application.
Builder's opening: The hole inside the kerb or upstand, measured timber to timber. Some makers sell rooflights by this size, others by the external frame, so always check which is meant.
Building control: The council service or a registered building control approver that checks building work meets the building regulations, including new rooflight openings and replacements not self-certified by a scheme installer.
Building warrant: The approval needed in Scotland before certain building work starts, including forming a new rooflight opening. It is issued by the local authority's building standards service.
Cold roof: A flat or pitched roof where the insulation sits between or below the joists, with a ventilated void above it. Trimming an opening must keep that ventilation path open.
Competent person scheme: A government-authorised scheme whose registered installers can self-certify certain work, such as replacement rooflights, without a separate building control application. New openings are not covered.
Condensation channel: A small gutter moulded round the inside edge of some domes and rooflights that collects condensation droplets so they evaporate rather than dripping into the room.
Conservation rooflight: A roof window designed to look like a traditional cast-iron rooflight, with a slim, flush frame and a glazing bar, used on older buildings and in conservation areas.
Controlled fitting: A building regulations term for items such as replacement windows and rooflights that must meet energy and fitting standards even when only an existing unit is being replaced.
Daylight size: The clear area of glass you see from inside the room once a rooflight is fitted. It is smaller than both the external frame size and the kerb's outside size.
Dome rooflight: A curved plastic rooflight, usually single or twin skin, fixed to a timber kerb on a flat roof. Older domes often yellow, become brittle and run with condensation.
EPDM: A synthetic rubber membrane used to waterproof flat roofs. Round a rooflight, it is bonded up the kerb and finished under the frame's drip lip, with formed corners.
Fall: The slope given to a roof or rooflight so water drains away. Many flat glass rooflights need a fall of around 5° to shed rain and dirt.
Flashing kit: The set of shaped metal and rubber pieces that weather a pitched roof window into the surrounding tiles or slates. It must match the roof covering and its profile.
Frameless rooflight: A flat rooflight where the glass covers the frame, so little or no frame is visible from inside. It needs an accurately built, square kerb to sit and seal properly.
Free area: The clear opening through which air or smoke can pass when a vent is open. For smoke vents, it is the figure that counts, rather than the unit's outside size.
Glazing bar: A bar dividing panes of glass. Conservation rooflights use slim vertical glazing bars, and roof lanterns use sloping glazing bars to carry their panels.
GRP: Glass-reinforced plastic, or fibreglass. Used as a flat roof covering laminated in place, and for some rooflight kerbs and domes.
Kerb: Another word for the upstand: the raised frame, usually timber, that a flat rooflight sits on. It lifts the joint above the roof surface and carries insulation.
Laminated glass: Glass made of two or more layers bonded with a plastic interlayer, which holds the pieces together if it breaks. Used on the inner pane of many rooflights and on walk-on units.
Lantern: A raised glazed roof structure with a frame, glazing bars and several sloping panels, sitting on a kerb. It adds height and brings in light from several angles.
Listed building consent: Permission needed to alter a listed building in ways that affect its character. Inserting a rooflight in a listed building always needs it in England.
Low-E coating: A thin metal-oxide coating on glass that reflects heat back into the room, improving the glass's insulation and keeping the inner pane warmer, which reduces condensation.
Non-fragile: A rating meaning a rooflight should not give way if someone falls onto it. It does not mean the rooflight is designed to be walked on.
Permitted development: Rights that let householders carry out certain work without a planning application, within set limits. Rooflights on houses often fall within them, depending on nation and location.
Pitch: The angle of a roof or rooflight from horizontal, in degrees. Roof windows are made for a stated pitch range, and flat rooflights for a stated fall.
Ponding: Water standing on a flat roof long after rain has stopped, usually from poor falls or blocked outlets. Ponding against a rooflight kerb is a common cause of leaks.
Rain sensor: A small sensor on an electric rooflight that detects rain and tells the control unit to close the glass. It must be kept clean to work reliably.
Reveal: The lined shaft between the ceiling and the rooflight, seen from inside the room. It should be insulated, then plasterboarded and finished, to avoid cold spots and condensation.
Roof window: A window made for a pitched roof, fitted between rafters with a flashing kit into the tiles or slates. It is assessed for energy purposes in the vertical position in England.
Sealed unit: Two or more panes of glass sealed together round their edges with a spacer between them. Misting between the panes means the seal has failed and the unit needs replacing.
Self-cleaning glass: Glass with a coating that breaks down dirt in daylight and lets rain wash it off in sheets. It works best on rooflights with a fall and open sky above.
Single-ply: A flat roof membrane laid in one layer, such as PVC or TPO, welded or bonded at its seams. Rooflight kerbs are detailed using the membrane maker's own method.
Smoke vent: A rooflight or opening designed to let smoke escape in a fire. Certified smoke vents form part of a system with detectors, controls and backup power.
Solar-control glass: Glass with a coating or tint that reduces the solar heat passing through. Useful on large or south-facing rooflights and lanterns to limit summer overheating.
Sun tunnel: A tube with a clear dome on the roof and a diffuser in the ceiling that carries daylight into rooms where a rooflight cannot reach, such as landings and corridors.
Toughened glass: Glass heat-treated so it is much stronger than ordinary glass and breaks into small, blunt pieces. Often used on the outer pane of rooflights.
Trimmer: A timber fixed across a roof opening to carry the cut ends of joists or rafters, transferring their load to the full-length timbers either side.
U-value: A measure of how much heat passes through a building element, in W/m²K. The lower the number, the better the insulation. Rooflight limits are set in building regulations.
Upstand: The raised kerb round a flat roof opening that a rooflight sits on. The height commonly cited is at least 150 mm above the finished roof.
Walk-on rooflight: A structural glass rooflight designed to be walked on, using thick laminated glass and a kerb built for the load. It needs structural calculations.
Warm roof: A roof where the insulation sits above the deck, under the waterproof membrane. A rooflight kerb must rise high enough to clear the finished surface on top of the insulation.
WER: Window energy rating. A lettered band that summarises a window's or roof window's overall energy performance, used as an alternative to a U-value in some regulations.