Glass vs polycarbonate for roof lights

Glass and polycarbonate both transmit daylight through a roof, but they are not interchangeable materials. Glass — in a sealed insulated unit with safety panes — is what UK residential extensions specify. Polycarbonate — a moulded or extruded plastic — appears in budget domes, canopies and industrial skylights where optical quality and thermal performance are secondary.

Glass roof lights

Modern glass roof lights use factory-assembled insulated glass units: toughened outer pane, laminated inner pane, low-E coatings and argon-filled cavities in a typical residential build. The unit is permanently sealed and set into a timber upstand.

Glass delivers optical clarity, predictable thermal performance, compliance with overhead safety glass requirements and a service life measured in decades.

Polycarbonate roof glazing

Polycarbonate is a thermoplastic used in single- or double-skin domes and flat panels. It is lightweight, impact-resistant and inexpensive relative to glass IGUs.

It is common on warehouses, canopies and older domestic flat roofs. In habitable rooms it is increasingly rare because of yellowing, poorer insulation and regulatory preference for safety glass.

Material comparison

• Clarity — glass: neutral and stable for the life of the unit; polycarbonate: can appear hazy when new and yellows with UV ageing.
• Surface hardness — glass: resists scratching; polycarbonate: soft, marks from cleaning and debris.
• Thermal insulation — glass IGU: low U-values with double or triple glazing; polycarbonate: poor unless part of a much heavier multi-wall system.
• Weight — glass: heavier, needs structural upstand; polycarbonate: light, easier to lift.
• Safety overhead — glass: laminated inner pane retains glass if broken; polycarbonate: impact-resistant but not classified as safety glass to BS EN standards in the same way.
• Lifespan — glass: decades with minimal degradation; polycarbonate: may need replacement when clarity or seals fail.

Cost and whole-life value

• Purchase price — polycarbonate domes cost less per unit upfront.
• Replacement cycle — polycarbonate may be replaced every 15–20 years; glass roof lights are a long-term installation.
• Energy — poorer insulation in polycarbonate increases heating cost in rooms below over the building’s life.
• Property value — extensions with quality glass roof lights present better than visible yellowed plastic domes.

Regulatory and insurance context

Dwelling extensions in England must meet Approved Document L thermal standards and Part K safety requirements for glazing. Overhead glass in critical locations should be safety glass — typically laminated. Polycarbonate does not automatically satisfy these requirements in habitable spaces.

Check with your architect and insurer before specifying plastic overhead glazing in a residential project.

Where to specify each

• Glass — all habitable extensions, loft conversions, kitchen diners, anywhere appearance and thermal performance matter. Configure frameless or framed units online.
• Polycarbonate — unheated utility spaces, commercial plant areas, temporary structures — where confirmed as acceptable.
• Specialist projects — acoustic, security or oversized builds may need a bespoke roof light review with an enhanced glass build.

Summary

Glass is the correct material for roof lights in heated, habitable UK buildings. Polycarbonate suits budget and utility applications where regulations permit. If you are investing in an extension you will live with for decades, specify an insulated glass unit — not a plastic dome.

Every Vant Glass roof light is made to order in Britain, backed by a 20-year guarantee and free UK mainland delivery. Configure frameless or framed sizes in the online calculators or call 03330 902 592.