What thickness should a rooflight be?

“How thick should a rooflight be?” is one of the most common specification questions — and one of the most misunderstood. Thickness is not a single number you choose from a menu. It is the result of span, glazing build, load requirements and frame detail, confirmed by the manufacturer for your specific opening.

Overall thickness vs pane thickness

Two measurements are often confused:

• Pane thickness — the depth of an individual glass sheet (e.g. 6 mm toughened outer pane, 6.4 mm laminated inner pane).
• Unit thickness — the overall depth of the factory-sealed glass assembly, including all panes, cavities, interlayers and edge seal.

The unit thickness is what matters for upstand height, insulation build-up and internal ceiling detail. Individual pane thicknesses are determined by structural and safety engineering within the unit — you do not specify them independently on a standard roof light.

What determines unit thickness?

Several factors drive the sealed unit depth:

Glazing type. Double-glazed builds are shallower than triple-glazed. A triple-glazed unit adds a third pane and a second cavity, increasing depth but improving thermal performance. Frameless roof lights are available in both configurations.

Span. Larger glass areas require thicker panes to resist wind load and deflection. A small kitchen roof light and a large extension panel of the same glazing type will have different pane thicknesses within the unit.

Inner pane construction. A laminated inner pane is slightly thicker than a monolithic toughened pane of the same nominal gauge, because the interlayer adds depth. Laminated inner glass is standard on Vant Glass overhead units.

Safety and load requirements. Non-fragility and impact-resistance classifications influence minimum pane thicknesses. These are resolved at manufacture, not by the specifier.

Double vs triple glazing depth

Double glazing is the default for most residential roof light applications. It balances thermal performance, weight and unit depth.

Triple glazing adds a third pane and a second argon-filled cavity. The unit is deeper and heavier, but centre-pane U-values improve — worthwhile where Part L compliance or passive-house targets demand it.

The depth difference between double and triple is meaningful for upstand design. Confirm the unit depth for your chosen glazing type before the upstand is built.

Frameless border width adds to overall depth

On frameless roof lights, a decorative border sits at the external face of the glass. Border width — typically 50 mm, 82 mm, 100 mm or 150 mm — is additional to the sealed unit depth. It affects the external appearance and the maximum clear glass size, not the thermal performance of the glazing itself.

Framed roof lights use an aluminium frame profile that adds its own depth at the internal and external edge. Frame depth is separate from glass unit depth.

Upstand height and installation depth

The timber upstand must be tall enough to support the glass unit and receive weathering detail. If the upstand is too shallow, the unit cannot be properly seated and weathered.

Provide your intended upstand height when ordering. The manufacturer’s setting-out drawing will confirm the required upstand dimension for your specific unit.

Thickness and walk-on glazing

Standard flat roof lights — however thick the sealed unit — are not rated for foot traffic. Walk-on glazing uses a multi-pane toughened laminated build typically around 33 mm for the glass assembly alone, with additional panes for thermally broken external applications. This is a fundamentally different product. See walk-on roof lights.

Do not assume that specifying a thicker standard unit makes it walk-on safe. It does not.

What to confirm at quotation stage

Before cutting the roof deck or building the upstand, confirm from the manufacturer:

1. Overall sealed unit depth for your opening size and glazing type.
2. Required upstand height.
3. Internal and external dimensions including border or frame.
4. Weight of the unit (relevant for crane access and manual handling on site).

For bespoke roof lights with unusual spans, the glass build-up is project-specific. Do not extrapolate from standard calculator dimensions.

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.