Glossary of Terms

  • 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 Lumiera)

  • Air Infiltration

    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 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 60° subtended angle.

  • Lumiera

    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: 2005.

  • 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.

  • 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.

  • 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.