How to size a roof light opening

Sizing a roof light opening correctly is one of the most important steps in any installation. Get it wrong and the glass may not be fully supported, weathering can fail, or the unit simply will not fit. This guide covers the measurements that matter for frameless and framed flat roof lights.

The three dimensions to understand

Roof light sizing involves three related but distinct measurements:

1. Visible clear glass — the daylight aperture you see from inside the room.
2. Inner pane size — the full dimension of the inner glass sheet, including the area that laps onto the upstand.
3. Structural opening — the cut-out in the roof deck, which must be smaller than the inner pane on every side.

Confusing these three is the most common cause of installation problems.

Frameless units: the upstand carries the load

For frameless roof lights, the sealed glass unit is a heavy assembly. Its weight must bear down onto a timber upstand on all four sides. Glazing tape and structural silicone seal and bond the unit — they are not designed to support its weight.

The structural opening must be cut so the inner pane laps onto timber around the entire perimeter. If the opening is wider than the inner pane, the glass edges cantilever over thin air. Under sustained load the bonded seal slowly creeps, and over time the glass can slip off the upstand.

Rule: the opening is always narrower than the inner pane. Never cut the deck to match the visible glass size.

Measuring the upstand

Take measurements after the upstand is constructed and before the roof light is ordered:

• Measure the internal width and length of the upstand — the timber surface the glass will sit on.
• Check diagonals to confirm the upstand is square.
• Measure at multiple points along each edge; tolerances on site often differ from drawings.
• Note the upstand height and deck build-up — these affect external weathering detail.

Provide these dimensions when configuring your unit. The manufacturer will confirm the structural opening size and inner pane dimension on the setting-out drawing.

Border width and clear glass size

Frameless units include a decorative border (RAL 9005 black as standard) at the external face of the glass. Border width — commonly 50 mm, 82 mm, 100 mm or 150 mm — affects how much of the overall unit is clear glass versus border.

A wider border reduces the maximum clear glass dimension achievable within structural span limits. When configuring in the online calculator, set border width and glass size together to see valid combinations.

Framed units: kerb and frame dimensions

Framed roof lights use an aluminium frame that sits on a builders’ upstand or optional PVC kerb. The frame provides its own fixing zone, but the same principle applies: the structural opening must match the manufacturer’s setting-out dimension, not the visible glass size.

Some framed systems are supplied with a kerb for new-build decks, simplifying the upstand detail. Confirm whether your unit includes a kerb or expects a site-built timber upstand before cutting the deck.

Working from drawings vs site measurement

Architectural drawings are a starting point, not the final dimension. Roof decks, insulation build-ups and upstand construction introduce tolerances that drawings rarely capture.

Best practice:

1. Build the upstand.
2. Measure the internal upstand dimensions on site.
3. Configure the roof light with those dimensions.
4. Receive the manufacturer’s setting-out drawing.
5. Cut the structural opening to match the drawing — not before.

If you are unsure, call the manufacturer before cutting. Cutting an opening incorrectly is far more costly than a phone call.

Common mistakes to avoid

• Cutting to the visible glass size — the opening must be smaller than the inner pane.
• Measuring the plasterboard opening — measure the timber upstand, not the finished ceiling aperture.
• Assuming square — check diagonals and adjust if the upstand is out of tolerance.
• Ignoring border width — a wider border means less clear glass within the same structural limit.
• Relying on silicone to carry load — the upstand supports the glass; the seal only bonds and weatherproofs.

Bespoke and multi-unit openings

For bespoke roof lights — unusual shapes, pitched installations or linked multi-panel layouts — setting-out is project-specific. Do not apply standard rectangular rules without a confirmed drawing.

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.