Roof Lantern Calculator — Opening, Kerb & Glazing Bars

The structural opening (the hole cut in the roof) is SMALLER than the lantern, not larger. The lantern sits on a timber or insulated upstand built inside the opening and overhangs that upstand by about 6mm per side for weatherproofing. The industry rule is: lantern order size = structural opening + (2 × external upstand width) + 12mm overhang. With a typical 70mm upstand, a 2000 × 1000mm lantern needs roughly an 1848 × 848mm opening (about 76mm smaller per side). Always confirm the exact reduction against your lantern and upstand data sheet.

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.

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.

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.

Many roof lanterns fall under permitted development in England, but the test depends on the roof type. On a PITCHED roof a rooflight must not project more than 150mm from the plane of the existing roof slope. On a FLAT roof the key limit is different: the lantern (kerb plus glazing) must not exceed the highest part of the existing roof — so a normal kerbed flat-roof lantern can sit well over 150mm above the flat plane and still be permitted development, as long as it stays below the ridge. Permitted development does not apply to flats, listed buildings, or where an Article 4 direction or conservation-area restriction is in force, so always check with your local planning authority. (Source: Planning Portal — Roof, Town and Country Planning (GPDO) 2015, Schedule 2, Part 1, Class C.)