A lucarne is a window projecting from the roof slope with its own small roof and side cheeks — in principle the same thing as a dormer, but in the French building tradition usually narrower, taller and more richly detailed. The word derives from the Latin lucerna, a lamp, referring to the daylight it brings into the roof space.
Where the ordinary dormer is a wide, often flat-roofed box intended to deliver daylight and headroom, the lucarne is typically a single, vertically proportioned window with a gabled or hipped roof of its own. In the classic French version it also stands in line with the facade, with masonry or stone surrounds that appear to continue up past the eaves.
Types
- Lucarne à fronton — with a triangular or segmental pediment over the window
- Lucarne à croupe — with a small hipped roof on the structure
- Lucarne pendante — whose sill runs down into the facade itself
- Œil-de-bœuf — the round or oval version, literally an ox eye
- Chien-assis — the low, wide version with a shallow-pitched roof
Construction points
The same rules apply as for any dormer:
- Structure — rafters or purlins are cut to form the opening. A trimmed opening is needed to carry the load to the remaining rafters; that is a job for a structural engineer
- Junctions — the critical points are the valley along the cheeks and the junction of the lucarne roof with the main roof; sound lead or lead-replacement work belongs there
- Insulation — the cheeks and small roof are modest in area but large per square metre of room gained. Insulate them to the same standard as the main roof and keep the airtight layer continuous
- Permission — forming a dormer generally requires approval. Depending on the country and location it may fall under permitted development, but in a conservation area or on a listed building it very rarely does. Check with the local authority before you build
Related terms
- Dormer
- Roof window
- Roof pitch
- Purlin
- Fascia board
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