Flat rooflights vs roof lanterns.

A roof lantern is a four-sided pitched glazed structure that sits on top of an extension roof, formed from glazed panels around a hipped or ridged frame. The form is read from below as the volume of the lantern; visible glazing bars define the geometry. Lanterns work in plan as a square or rectangle, occasionally as a hexagon for projects that want the period reference more explicitly.

A flat rooflight is a single plane of glazing at 0–5° pitch, framed by a thin aluminium perimeter and drained at the perimeter rather than via a pitched run-off. From below it reads as a flat plane of sky; the glazing bars that define a lantern are absent. The room below reads as having a hole in the ceiling rather than a structure above it.

On daylight performance, both deliver overhead light into a deep-plan space. A lantern often delivers slightly less daylight in absolute terms because the glazing bars block some of the area; a flat rooflight delivers more uninterrupted glass per square metre of opening. The difference is noticeable on bright days and barely visible on overcast ones.

On thermal performance, flat rooflights typically hit lower U-values than equivalent lanterns because the framing is simpler and the surface area-to-volume ratio is better. Both can be specified to Part L 2025; Future Homes Standard projects favour flat rooflights for the easier U-value compliance.

On weathertightness, lanterns drain via the pitch — water runs down the slopes and off the eaves. Flat rooflights drain via a perimeter channel, which has to be detailed correctly into the roof build-up but is robust over a 25-year life when it is.

When a flat rooflight makes sense over a lantern. The architectural reading wants the overhead daylight to be a flat plane of glass rather than a pitched form; the project register is contemporary; the U-value brief sits at Approved Document L 2025 limits or below; the structural opening is rectilinear and does not call for the volumetric depth of a lantern.

When a lantern makes sense over a flat rooflight. The architectural register is traditional or transitional; the elevation reads with a pitched-form vocabulary that a flat rooflight would not match; the brief specifically calls for the lantern as a feature element on the roof. Roof lanterns are not a Maxlight product — we recommend manufacturers we rate where the brief specifies a lantern.

Detail considerations. Flat rooflights drain via a 1:80 minimum fall on the outer kerb; lanterns drain via the angled glazing into a perimeter gutter. The flat rooflight integrates more cleanly with a single-ply or membrane roof build-up; the lantern integrates more cleanly with a tiled roof. Whole-rooflight U-values sit lower on a flat aluminium build than on a pitched lantern with multiple framing intersections, because every framing junction is a thermal path.

Regulatory context. Both routes meet BS EN 14351-1 product standard certification; whole-rooflight U-value is rated against the BFRC test conditions and assessed against Approved Document L 2025 limiting U-values. the project U-value target is confirmed with the architect so the specification matches the regulatory case at design stage.

The five factors that separate the two systems. The decision is design-led — the project tells you which is right.