Glossary of Terms

Ud Value
Units W/m2K—a measure of the thermal efficiency of a product expressed over the area of the opening The lower the number, the higher the insulation value.

Ud Value
Units W/m2K—a 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.

A.O.V
Automatic opening vent—a 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-performance Felt
Bituminous based roof covering upgrade for enhanced performance and longer life

High Rise
Usually refers to shape or geometry of a dome or barrel vault. Actually describes semi-circular or hemi-spherical.

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.

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.

Lumiera
Trade name for Aerogel. Supplied in the UK by Xtralite Rooflights Limited.

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 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.

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.

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.

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.