Roof lantern installation in flat-roof extensions

Sizing and positioning a lantern

A lantern works best centred over the space that most needs light, often the dining area or the middle of an open-plan kitchen. As a rough principle, it should be in proportion to the room: too small looks lost, while too large can make the room hot in summer and cold in winter.

Position also depends on the roof structure. Joists run in one direction, and the opening is formed between trimmers that carry the cut joists. Keeping the lantern away from walls, steel beams and the edge of the roof simplifies the structure and leaves room for the flat roof to drain around it.

The structural opening and upstand

The lantern sits on an upstand, also called a kerb, built up from the roof deck. The upstand lifts the glazing above the finished roof level so that water can flow past it, and it is insulated to avoid a cold bridge around the opening. Manufacturers specify the upstand height and the internal dimensions, which must be followed closely.

Larger lanterns, or openings that interrupt several joists, may need a structural engineer to design trimmers or a timber or steel frame. Planning this at the drawing stage avoids redesign on site.

Weathering: where installations succeed or fail

Most lantern leaks come from the junction between the roof covering and the upstand, not the lantern itself. The flat roof membrane, whether felt, EPDM or GRP, is turned up the upstand and sealed, with the lantern frame then fixed and capped over it. Drainage falls should carry water around and away from the upstand rather than letting it pond against it.

Coordination matters here. If the roofer and the lantern installer are different trades, someone needs to own the junction detail. On a builder-led extension, it is sensible for the main contractor to manage that sequence.

For coverings and roof build-ups, see our flat roofing service.

Glass, ventilation and comfort

Glass specification affects comfort more than people expect. Solar-control coatings reduce overheating on south-facing lanterns, while good insulating units limit heat loss and condensation in winter. Opening vents, manual or electric, help release warm air from the highest point of the room.

Condensation can form on the frame or glass if the room is humid and poorly ventilated, particularly in kitchens. Extract ventilation, an insulated upstand and a well-sealed internal finish all reduce the risk.

Building regulations and planning

A lantern is part of the extension design, so it is covered by the extension's building regulations approval. Glazing area counts towards the energy performance of the room, and large lanterns combined with big doors may require calculations to show compliance. Safety glass is also required.

For planning, the height of the lantern above the roof is included in the overall extension height. Where an extension relies on permitted development, that height needs checking against the limits.

Installing a lantern in an existing flat roof

Adding a lantern to an existing extension is possible, but it means cutting into the roof structure and covering. The joists need trimming, an upstand is built and the roof membrane around it must be properly joined to the existing covering. If the roof covering is old, it can be more sensible to replace it at the same time so the new junction is not relying on worn material.

Common questions