How big should the structural opening be?

The structural opening should be approximately 25mm larger than the lantern on each side to allow for the kerb and adjustment. A 2000 × 1000mm lantern needs a 2050 × 1050mm opening.

What kerb depth do I need for a roof lantern?

The kerb (upstand) must be deep enough to accommodate insulation and maintain thermal performance. Minimum 150mm is typical, with 100mm insulation fitted to the inside face. This prevents cold bridging.

What flashing do I need around a roof lantern?

You need continuous flashing around the full perimeter of the kerb, turned up the kerb face and out onto the roof surface. Allow 10% extra for corners and overlaps. Lead code 4 or proprietary flashing tape is typically used.

How are glazing bars spaced on a roof lantern?

Glazing bars run along the length of the lantern, typically at 400–600mm centres depending on the glass weight and wind loading. More bars provide greater structural support but reduce the glass area.

Do roof lanterns need planning permission?

Most roof lanterns on flat roofs fall under permitted development in England, provided they don’t exceed 150mm above the roof plane and the property isn’t listed or in a conservation area. Always check with your local planning authority.

TradeCalculator

Roof Lantern Calculator — Opening, Kerb & Glazing Bars

Calculate structural opening size, kerb depth for insulation, flashing quantities and glazing bar spacing for roof lanterns.

Lantern Length

External length of the roof lantern

Lantern Width

External width of the roof lantern

Kerb Depth

Upstand height (min 150mm recommended)

Glazing Type

Affects weight and thermal performance

Lantern Price (optional)

How We Calculate This

This calculator determines the structural opening dimensions, kerb requirements, flashing lengths and glazing bar count for your roof lantern installation.

The formula

Structural opening = (Lantern length + 50mm) × (Lantern width + 50mm)

Kerb perimeter = 2 × (Length + Width)

Glazing bars = (Length ÷ Bar spacing) − 1

Frequently Asked Questions

Related Calculators

Roof Tile Calculator

Calculate how many roof tiles you need per m² for your roof.

Ridge Tile Calculator

Work out ridge tile quantities and mortar needed.

Felt and Batten Calculator

Calculate roofing felt and batten quantities for your roof.

Soffit and Fascia Calculator

Work out soffit board and fascia lengths needed.

Gutter Sizing Calculator

Calculate gutter size and downpipe requirements for your roof area.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.