Roof lantern vs skylight: which is better for an extension?

Both put glass in a flat roof. The difference is shape. A flat skylight, also sold as a flat rooflight, is a single sealed pane sitting on a low kerb, almost flush with the roof. A roof lantern is a small glazed pitched roof, with a ridge and sloping panes, standing proud of the flat roof on a taller kerb.

Light

Daylight depends mainly on how much glass there is and where it sits, not on its shape. A 2 m by 1 m skylight and a lantern with a 2 m by 1 m base let in much the same light at midday.

The lantern’s sloping sides do catch low sun earlier and later in the day, and they throw light further across the ceiling. A skylight gives a more defined pool of light beneath it. In a north-facing room either one is worth having; in a south-facing room, think harder about heat than about light.

Position beats type. Glass placed near the back of the room, close to the house wall, lights the part of the plan that the new extension would otherwise leave dark.

Height and how the room feels

This is where the lantern earns its price. From inside, the ceiling lifts by half a metre or more over the middle of the room and you look up into a glazed void. In a dining room or an orangery that is the whole idea.

It needs space to work. As a rough rule the lantern should be no more than a third of the ceiling area, with at least 600 mm of flat ceiling all round. In a room under about 12 m² a lantern crowds the ceiling and a skylight looks calmer.

From outside, a skylight is invisible from the garden. A lantern is seen from upstairs windows, yours and the neighbours’.

Planning height

In England a single-storey extension built under permitted development may be no more than 4 m high, and the eaves no more than 3 m where the room is within 2 m of a boundary. A lantern’s ridge is part of the roof, so its height counts. On a flat roof at 3 m that is rarely a problem, but check the drawing shows the lantern’s top, not only the roof deck. A flat skylight adds almost nothing.

Heat in and heat out

Roof glass counts towards the limit on glazing in an extension, which is about a quarter of the floor area plus any openings covered over. Rooflights must also meet a maximum U-value, 2.2 W/m²K when measured flat, and good units do far better.

A lantern has more glass surface and more frame for the same hole in the roof, so it loses a little more heat in winter and gains more in summer. Ask for solar-control glass in either if the roof faces south or west. If the room has a kitchen in it, an opening section is worth having to let hot air out; both types are sold with electric vents.

Cleaning and leaks

A flat skylight should be set at a slight fall, usually a few degrees, so rain runs off and does not sit in a puddle that dries to a ring. A lantern’s pitch sheds water and dirt better, though its ridge and corner cappings are more joints to maintain.

Leaks are nearly always a matter of how the kerb was built and waterproofed, not the unit. The roofer should dress the roof membrane up the kerb and under the frame. Ask who is responsible for that junction, because it sits between two trades.

Cost

For the same daylight, a lantern costs more: more glass, more frame, a taller kerb and more plastering inside. We have not found a published trade-body figure we trust enough to print, so get both priced on the same quote and compare.

Which to choose

A garden room extension with a solid roof can take either. Decide before the roof structure is designed, since the joists are trimmed around the opening, and use the cost calculator to see how the room itself prices up.