Glass glossary

Acoustic glass reduces noise transmission through thicker builds, laminated interlayers tuned for sound damping, or different pane thicknesses to break resonance.

Roof lights are rarely the primary acoustic element but may matter under flight paths or beside noisy plant. Rw values come from laboratory tests — site performance depends on gaps, vents and the whole facade.

For critical acoustic projects, involve an acoustic consultant and specify tested assemblies rather than glass thickness alone.

Condensation on or around a roof light appears when warm, moist air meets a surface cold enough to reach dew point. It is common at the perimeter where frame, upstand and glass meet — not always at the centre of the pane.

Causes include high indoor humidity (kitchens, bathrooms), inadequate ventilation under Part F, cold thermal bridges and missing or failed seals.

Mitigation: thermally broken frames, adequate upstand insulation, ventilation gaps where specified, and controlling moisture at source. Persistent condensation warrants investigation — it can indicate seal failure or design issues.

Deflection is bending under load. Walk-on and floor glass must limit deflection for user comfort, perimeter seal performance and, on terraces, water run-off.

Excessive span relative to the laminated build causes noticeable spring underfoot and stresses edge seals. Manufacturers publish maximum spans for their standard builds.

Do not exceed rated dimensions. Long narrow panels may feel different underfoot than square panels — confirm with the supplier before forming the opening.

Edge cover is how far the frame or upstand overlaps the glass edge — a key retention and weathering dimension in BS 6262 and manufacturer details.

Insufficient cover risks glass slip, seal failure and wind uplift issues. Too much restriction without movement allowance can stress the glass when it expands thermally.

Follow the roof light supplier's detail for cover, setting block position and silicone bite depth.

A fixed roof light does not open for ventilation. It is the standard choice for flat roof extensions, kitchen islands and corridors where daylight is needed without smoke ventilation or access.

Fixed units are lighter and simpler than opening roof lights and must not be used as a walkable surface unless specified as walk-on structural glass.

Choose fixed unless you need ventilation, smoke extraction or roof access — opening units cost more and need additional weathering maintenance.

Float glass is the starting point for most flat glazing products. The molten glass forms a uniform ribbon with fire-polished surfaces. It is then cut, toughened, laminated or coated into finished products.

Standard float has a slight green edge tint from iron content. Low-iron float reduces the tint for clearer, more neutral transmission — popular on roof lights where true colour rendering matters.

Raw float is not safety glass and is not used as-installed on roof lights — it is processed into toughened or laminated assemblies.

A framed roof light uses a manufactured frame to carry the glazing and provide weathering. The frame may be aluminium, timber or composite, often with a thermal break on thermally rated products.

Framed units can be easier to detail on some builds and may suit refurbishments where an existing kerb matches a frame profile.

Compare frame sightlines, thermal performance and cost against frameless options for the same opening size.

A frameless roof light presents a flush internal finish with little or no visible frame at the ceiling. Externally the unit still sits on a weathered upstand — frameless refers to the internal aesthetic, not a lack of edge support.

The glass must bear on the upstand on all sides; silicone bonds weatherproof but does not carry load. Typical build: toughened outer pane, laminated inner pane, argon cavity on thermal units.

Vant Glass manufactures frameless roof lights to bespoke sizes with instant online pricing.

A glass balustrade provides fall protection with minimal visual obstruction. Panels are usually toughened laminated glass, fixed in a base channel, on standoff spigots or between posts.

Guard height must meet Part K (typically 1100 mm on balconies). Handrails may be required depending on fixing type and building control.

Vant Glass manufactures made-to-measure balustrade runs with optional slim handrails — measure total length and required glass height for quoting.

The g-value (solar factor) measures how much of the sun's energy transmits through glass into the building. It ranges from 0 to 1 — clear uncoated glass might be around 0.85; solar-control coatings reduce the figure.

On roof lights, solar gain can be welcome in winter but problematic in summer, especially on south-facing extensions. Part O addresses overheating risk in new dwellings.

Balance g-value against light transmission and U-value when choosing glass. Low-E and solar-control coatings can cut solar gain without sacrificing all daylight.

An insulating glass unit (IGU, double or triple glazed unit) seals two or more panes around a spacer with a dry gas cavity. The cavity and Low-E coatings reduce heat transfer.

Standard flat roof lights use IGUs for thermal performance. Walk-on glass uses a thicker laminated structural build — which may also include an IGU cavity when thermally broken.

Product standard for IGUs: BS EN 1279. Always quote whole-unit U-value, not glass-centre only, for compliance checks.

A Juliet balcony is guarding across a door opening on an upper floor — you cannot step out onto a platform. It provides safety and an open-door feel without planning the projection of a full balcony.

Usually a glass panel balustrade fixed to the building face or within the reveal, with doors opening inward.

Glass must meet impact and guard height requirements. Vant Glass supplies made-to-measure Juliet balcony glass to your opening width and guard height.

Laminated glass consists of two or more panes bonded with a plastic interlayer. Under impact, the glass may crack but the assembly tends to stay in place — essential for overhead glazing and structural walk-on applications.

Roof lights commonly use a toughened outer pane and laminated inner pane. Walk-on units use multi-pane toughened laminated builds rated for foot traffic.

Laminated glass also contributes to security and can reduce UV transmission depending on interlayer type. Product standard: BS EN 14449.

Light transmission (visible transmittance) measures how much daylight passes through the glass, typically as a percentage. Clear low-iron units achieve the highest figures; tinted, coated or opaque finishes reduce transmission.

On roof lights, balance brightness against g-value (solar gain) and privacy needs. Anti-reflective or self-cleaning coatings have minor effects on transmission.

Specify transmission when designing rooms that rely on roof light daylight — basements, corridors and north-facing extensions.

Low-E (low emissivity) coatings reduce radiant heat loss through the glass. The coating reflects interior heat back inside in winter and can be combined with solar-control layers for summer performance.

On roof lights, Low-E is applied to a surface within the insulating glass unit cavity — not a coating you can feel on the room side.

Pair Low-E glass with argon-filled cavities and warm-edge spacers for the best whole-unit U-values. Balance against g-value and daylight needs.

Low-iron glass (extra-clear glass) uses reduced iron in the melt, removing the green cast visible in standard float especially on thick edges. Light transmission is higher and views through the glass look more neutral.

It is popular on frameless roof lights, glass floors and contemporary extensions where the glazing is a design feature.

Low-iron is a base material — it is still toughened and laminated into safety assemblies for overhead use. It does not by itself improve U-value; coatings handle thermal performance.

An opening roof light incorporates hinged or sliding vents. Uses include bathroom ventilation, kitchen extract, smoke ventilation (subject to fire strategy) and occasional roof access.

Seals, hinges and actuators need periodic maintenance. Weathering detail at the upstand is critical — more complex than a fixed roof light.

Electric actuators can link to rain sensors and building management systems. Confirm opening size and free area for ventilation calculations under Part F.

Part K covers guarding, stairs, ramps and glazing in locations where people could fall or collide with glass. Safety glass is required in defined critical locations — including low-level glazing and doors.

Walk-on glass and glass floors must resist imposed loads and meet guarding requirements. Balustrades typically need a minimum guard height (often 1100 mm on balconies).

Always confirm the exact layout with building control or an approved inspector — Part K interacts with structural design and product certification.

Part L sets energy efficiency requirements for buildings in England. For extensions and new openings, roof lights must meet limiting U-values for the whole unit unless a justified trade-off is used in SAP or SBEM calculations.

Renovation work may have different thresholds than new build. Wales and Scotland use separate guidance — always confirm jurisdiction.

Declared U-values should come from the manufacturer for the exact unit and size. Vant Glass publishes thermal data for its made-to-order roof light builds.

A roof lantern (rooflight lantern) sits above the roof plane with sloped glazing on one or more sides. It suits orangeries, kitchen extensions and features where a flush flat light would lack presence.

Thermal performance depends on the unit U-value, frame design and installation. Lanterns cast more dramatic light patterns than flat lights.

Vant Glass supplies Brett Martin roof lanterns including pyramid styles — compare against flat frameless or framed lights for cost, height and aesthetics.

Roof pitch influences how water drains across and around a roof light. Very shallow pitches increase ponding risk at upstands and hinges on opening units.

Most flat roof lights suit low-pitch flat roof builds with adequate falls in the surrounding roof finish. Steeper pitches may need different flashing details.

Walk-on terraces need falls away from glazed areas so water does not sit against seals. Confirm minimum pitch with the membrane manufacturer and roof light supplier.

Safety glass is defined by how it behaves when broken. Toughened glass disintegrates into small blunt pieces. Laminated glass holds together on the interlayer. Both are classified under BS EN 12600 impact testing.

Overhead roof lights typically use toughened outer panes and laminated inner panes so failure does not drop shards into the room below.

Critical locations (doors, low-level glazing, bathrooms) have mandatory safety glazing under Part K. When in doubt, specify laminated or toughened safety glass and document compliance.

Self-cleaning glass uses a thin titanium-dioxide-based coating activated by UV light. Organic deposits break down and water spreads in a sheet rather than beading, carrying dirt away.

It works best on external surfaces exposed to rain and daylight. Under overhangs or shallow pitches, performance is reduced and occasional manual cleaning may still be needed.

It is an optional upgrade on roof light orders — weigh cost against access difficulty for maintenance on your specific roof.

Setting blocks transfer the glass self-weight to the upstand at defined points, keeping the pane at the correct position in the rebate for edge cover and movement.

Use the manufacturer's block layout — wrong placement causes point loading, seal distortion and cracked glass.

Blocks are typically neoprene or EPDM, resistant to compression set. They are installed dry; structural silicone and weather seals complete the perimeter.

Spontaneous breakage in toughened glass can occur when nickel sulphide inclusions expand inside the pane, or when edge damage and stress concentrations cause failure without a visible impact.

It is rare but documented on overhead toughened outer panes. A laminated inner pane retains glass if the outer pane fails — a key reason roof lights use laminated inners.

Heat-soak testing can reduce nickel sulphide risk on critical projects. Report sudden failures to the supplier with photos and installation details.

A thermal break interrupts the path for heat to conduct through a frame or glazing assembly. In framed roof lights and walk-on units, a thermally broken detail typically adds an insulated cavity — often with an argon fill — between panes or between frame sections.

Without a thermal break, the perimeter of a roof light can be significantly colder than the centre of the glass, increasing condensation risk and heat loss.

Specify a thermally broken build for external roof terraces, exposed framed units and any glazing separating heated from unheated spaces.

Toughened glass (tempered glass) is heated and quenched to create surface compression. It withstands higher loads and temperature differentials than annealed glass. When it breaks, it forms small granular pieces rather than sharp shards.

It is the standard outer pane on UK roof lights. Important: toughened glass cannot be cut or drilled on site — all processing must happen before toughening.

Toughened glass alone is not walk-on rated; structural floors need a laminated multi-pane build. Standard: BS EN 12150.

Toughened laminated glass bonds two or more toughened sheets with an interlayer. It is stronger than annealed laminated glass and retains fragments if broken — ideal for balustrades and structural floors.

Walk-on units stack multiple toughened laminates into one structural pane. Balustrade panels are typically a single toughened laminate of defined thickness for the span and height.

Do not confuse with standard toughened only — laminating is what keeps the panel together after impact.

The U-value (thermal transmittance) describes how readily heat passes through a roof light or other building element. It is expressed in watts per square metre kelvin (W/m²K). The lower the number, the slower heat escapes in winter — or enters in summer.

For roof lights, always check whether the quoted U-value is for the centre of glass only or the whole unit including frame and edge effects. Whole-unit figures are more useful for Building Regulations and energy modelling.

Roof lights form part of the thermal envelope under Part L. Specifiers often target the best whole-unit U-value that budget and span allow, balanced against daylight requirements.

The upstand is the structural kerb the roof light sits on. It must be square, level and sized to the manufacturer's fixing detail. Glass weight bears on the upstand — sealant weatherproofs but does not carry load.

Typical flat roof details use a timber upstand built above the deck, dressed with roof membrane and lead or membrane skirts.

Form the upstand before ordering glass so dimensions are final. Wrong upstand height causes fixing problems and condensation at cold edges.

Walk-on load rating describes whether a glazed panel can safely carry foot traffic for a stated span, support layout and load case. It is not implied by toughening or thickness alone.

The manufacturer rates the glass unit; the structural engineer designs the opening, upstand and bearings. Concentrated loads from planters or furniture may need separate checks.

Always order purpose-made walk-on glass with documented span limits. Do not use standard roof lights as floor panels.

A walk-on roof light is structural glazing you can walk across. It uses a thick laminated build rated for imposed loads — not a standard insulated roof light.

Applications include roof terraces, glass floors, basement light wells and internal landings. Specify anti-slip finishes on external wet areas and thermally broken builds outdoors.

Standard flat roof lights must never be walked on. Opening size, support and walk-on load rating must align with the manufacturer's span tables.