A.O.V Automatic opening vent specialist type of rooflight designed for the discharge of smoke and heat. Often connected to the fire alarm system (BMS) usually opens when alarm is activated.
Access Hatch A rooflight unit specifically-designed to facilitate access to the roof area – usually for maintenance.
Adaptor Kerb A type of kerb that adapts a rooflight upstand to a builder’s kerb.
Aerogel The lightest, most-effective translucent insulation available (see Lumira®)
Air Infiltration & Air Permeability Natural air seepage through unsealed voids and joints in parts of the building structure.
Asphalt Roof covering material based upon a mix of asphalt and mineral fillers.
Barrel Vault Semi-cylindrical rooflight that may be semi-circular or low rise (30° angles at kerb).
Builders Work Anything below the rooflight that involves construction.
Cassette Factory-assembled solid polycarbonate glazing panel with either double- or triple-skins.
Chain Motor Electrical actuator for hinged opening rooflights.
Cill/Sill Top face of kerb or upstand.
Clerestory Vertical glazing at the base of a rooflight. Such a unit is often called a ‘Lantern Light’.
Cold Bridge A thermal track allowing heat energy to escape by conduction.
D.G.U. Double glazed unit—refers to glass construction only.
Daylight The light produced by an overcast sky—averages 5,000 lux for 85% of daylight hours.
Daylight Factor The ratio between internal and external light levels.
Diffused/Diffusing Property of glazing material whereby transmitted light is scattered, creating better internal light distribution and preventing direct image formation.
Dome Rooflight shape. May refer to small moulded ‘modular’ units or large selfsupporting glazed constructions. Can be either ‘low rise’ (usual) or ‘high rise’ (semi-circular).
Dual Pitch May refer to ‘mono-pitch’ glazing constructed with two different angles. Sometimes used to describe a ‘ridge light’ (see Ridge Light).
E.P.D.M. Synthetic rubber used for manufacturing seals and gaskets.
Felt Bituminous-based roof covering.
Glazing Bar A structural member designed to support, restrain and seal glazing panels
High Rise Usually refers to shape or geometry of a dome or barrel vault. Actually describes semi-circular or hemi-spherical.
High-performance Felt Bituminous based roof covering upgrade for enhanced performance and longer life
Hit & Miss A type of ventilation system. Can be either open or closed.
In-Plane Rooflight set into a pitched roof so that there is no protrusion above the roof surface or plane.
Insulation Material used to increase the thermal performance of a material or system.
Kerb/Curb The roof mounting for a rooflight. May be constructed by the builder (builder’s kerb) or can be supplied as part of the rooflight. On a roof up to 17°, must protrude at least 150 mm above roof-top surface.
Lantern A rooflight set above vertical glazed sides. Invented by Victorians.
Lifting Frame The upper, supporting frame of an opening rooflight unit.
Linear Motor High-spec controllable actuator for opening rooflights.
Low Rise Usually refers to shape or geometry of a dome or barrel vault. Traditionally describes an arc of a circle or sphere with a 30° angle at the kerb or upsatnd.
Lumira® Trade name for Aerogel. Supplied in the UK by Xtralite Rooflights Limited.
Metal Deck Roof construction based upon the use of layers of corrugated metal sheets.
Mono pitch Single plane of sloping glazing.
Mullion Another name for a rafter—traditionally applies to windows or vertical glazing (see Rafter).
Multi-wall Polycarbonate Another term for structural polycarbonate.
Non-Fragile Refers to the ability of a glazing system or rooflight to resist the impact of a stumbling, falling human body. Measured and rated to ACR[M] 001: 2014 for Polycarbonate and CWCT (Centre for Window and Cladding Technology) for Glass.
Northlight A sloping rooflight which faces generally in a northern direction. This limits the amount of sunlight entering the building.
Opaque A reference to a materials inability to transmit light.
Part 6 This is a reference to the section of the Scottish Building Regulations intended to cover conservation and use of energy. Part 6 is split into two parts Non-domestic & domestic.
Part L Refers to Building Regulations Approved Document Part L—Subdivided into two parts each being further subdivided into two parts. Eire Building Regulations Part L—this is a reference to the section of the Eire Building Regulations intended to govern the use of fuel & energy. It is a single document with two distinct parts, Nondomestic & domestic.
Permanent Vent Ventilation system that is not often used. Uncontrolled and permanently open.
Powder Coating A protective coating of coloured polyester or occasionally epoxy.
Pyramid Usually refers to shape or geometry of a dome or glazed structure. The optimum construction angle is 30°.
Rafter A structural member designed to support, restrain and seal glazing panels. These members always run ‘top to bottom’.
Ridge Light Rooflight constructed with two slopes of glazing. Ends can be vertical or hipped. Can be constructed to be self-supporting.
Roof Window Window specifically designed to be set and flashed into, and be flush with, a sloping tiled or, slated roof. Velux being the generic.
Rooflight A roofing accessory designed to admit light into the building whilst weatherproofing the open roof aperture. Can also be used to introduce ventilation and/or gain access to the roof.
Security Fixing A screw or bolt with an anti-tamper head.
Self-Supporting A rooflight structure that requires no support other than that upon which it sits.
Silicone Durable synthetic resin used as a wet sealing compound.
Silicone Joint Designed joint making use of the properties of silicone.
Single-Ply Membrane Polymeric sheet roof covering.
Skin Refers to a single leaf of glazing (usually polycarbonate) may be prefixed by single, double or triple.
Skylight Another name for a rooflight (American in origin).
Smoke Vent Another term for an A.O.V.
Solar Gain Resultant internal heat gain due to transmission of solar energy (referred to as G value).
Solar Shading Device for reducing the level of transmitted solar energy. May be by blinds or louvres or by selective filtration by the glazing medium.
Solid Polycarbonate Polycarbonate produced as monolithic sheet material similar in appearance to glass.
Standing Seam A type of metal decking system that joins with a rolled metal ‘zip’ system.
Structural Self supporting system—often prefixes rooflight.
Structural Polycarbonate Lightweight polycarbonate sheet extruded in a multi-wall format which enhances strength and thermal performance.
Thermal Break A high thermal resistance barrier to heat flow. Used to insulate metal items that may run from the inside to the outside of a building.
Thermo-formed Plastic material shaped, when softened by heating.
TPR Synthetic rubber used for manufacturing seals and gaskets.
Translucent A term describing the ability of a glazing medium to transmit light without being able to form an image through.
Transom The horizontal glazing bar—runs between rafters (see transverse).
Transparent A term describing the property of a glazing medium to transmit light whilst being able to form a clear image through.
Transverse Runs across. Refers to a transom glazing bar (see Transom).
Trickle Low-level ventilation system. Usually not controlled.
Ud Value Units W/m2K² measure of the thermal efficiency of a product expressed over the developed surface area of the product.The lower the number, the higher the insulation value.
Upstand The mounting for a rooflight. May be part of the roof construction or part of the rooflight itself. Must be at least 150 mm high (see Kerb).
uPVC Un-plasticised polyvinyl chloride.
Ventilation Introduction of fresh air and exhaustion of stale air.
Weathering Cowl A hood shaped to shelter a detail and prevent weather ingress.
Wormgear Name given to hinged opening rooflight (manual).

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