Polycarbonate dome vs glass roof light

Polycarbonate domes and glass roof lights are both used to bring daylight through flat or shallow-pitched roofs, but they represent very different specification choices. Understanding the material differences helps you decide what belongs over a habitable room versus a utility space or commercial plant area.

Polycarbonate dome roof lights

A polycarbonate dome is a moulded plastic skylight, typically double-skin with an air cavity for modest insulation. The unit sits on a kerb or upstand and is secured with a clamping frame. Domes are lightweight, impact-resistant and relatively inexpensive to manufacture.

They are common on warehouses, corridors, bathrooms and older domestic flat roofs where cost and simplicity outweigh optical quality.

Glass flat roof lights

A glass roof light uses a factory-sealed insulated glass unit — toughened outer pane, laminated inner pane, low-E coatings and argon fill in a typical residential build. The unit is set into a timber upstand with glazing tape and structural silicone.

Glass flat roof lights are the standard choice for extensions, kitchen diners and loft conversions where clarity, longevity and thermal performance matter.

Material and appearance

• Optical clarity — glass maintains a clear, neutral view of the sky; polycarbonate can appear slightly diffused even when new and may yellow with UV ageing.
• Surface durability — glass resists scratching and crazing; polycarbonate is softer and more prone to surface marking from cleaning or debris.
• UV stability — modern glass coatings are stable for decades; polycarbonate relies on UV inhibitors that degrade over time.
• Weight — polycarbonate domes are lighter, which can simplify handling on site; glass units are heavier and require a structurally adequate upstand.

Thermal and acoustic performance

• Insulation — a double- or triple-glazed glass roof light achieves significantly lower U-values than a typical double-skin polycarbonate dome.
• Condensation — poorer thermal resistance in polycarbonate increases the risk of cold-surface condensation in heated rooms below.
• Sound — laminated glass units provide better rain-noise attenuation than thin plastic domes.

Cost and lifecycle

• Upfront cost — polycarbonate domes are generally cheaper to buy and install.
• Whole-life cost — glass units cost more initially but retain appearance and performance for longer; polycarbonate may need replacement when clarity or seals fail.
• Maintenance — both need periodic cleaning; polycarbonate requires gentler cleaning agents to avoid surface damage.

Where each type fits

• Polycarbonate dome — utility rooms, commercial plant areas, budget refurbishments where appearance is secondary.
• Glass flat roof light — habitable rooms, design-led extensions, anywhere building regulations or insurers expect overhead safety glass.

If you are refurbishing a flat roof over a habitable room, replacing an ageing polycarbonate dome with a glass unit is one of the most effective upgrades for comfort and appearance. The kerb may need adjustment to suit the flat unit’s bearing width — confirm against the manufacturer’s setting-out drawing before ordering.

For living spaces, specify custom frameless or custom framed glass units. Unusual kerb sizes or linked openings may need a bespoke roof light review.

Summary

Polycarbonate domes trade optical quality and thermal performance for lower cost and impact resistance. Glass roof lights deliver the clarity, durability and insulated build that residential extensions expect. For rooms you live in every day, glass is the specification that ages well.

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.