Are roof lanterns better than flat rooflights?

Two shapes, and what each does to the ceiling above you

A lantern is a framed structure of three or more sloping planes meeting at a ridge or an apex, standing on a kerb, typically rising 300 to 700mm above the roof surface depending on span and pitch. From the room it reads as a raised glazed box in the ceiling, and the reveal below it becomes a deep splayed opening.

A flat rooflight is one plane of glass on a slim aluminium frame, set at around five degrees on a kerb of the same 150mm minimum height. From below it reads as a flush aperture. The ceiling stays a ceiling with a window cut into it, rather than lifting.

Daylight: area of sky against angles of sky

For the same opening in the ceiling, the two collect light differently. A flat unit takes light from directly overhead, which is where the brightest part of an overcast British sky sits. On the grey days that make up most of a Gloucestershire winter, a horizontal aperture is genuinely efficient.

A lantern has planes facing several directions, so it picks up low sun in the morning and evening that a horizontal unit largely misses, and it moves visible bands of light around the room through the day. It also has more glass in total for the same aperture, because the sloping planes have a larger combined area than the rectangle they cover.

Weight and span, and what the deck underneath has to carry

A lantern is the heavier item by a clear margin. It has more glass, more frame, and it carries wind and snow load on planes that are not horizontal. Over a modest span a doubled joist trimming the opening is often enough. Over a wide one the answer is a designed trimmer or a beam.

A flat rooflight of the same aperture puts less load into the structure and puts it more evenly. On a Victorian rear addition in Gloucester with 100 by 50 joists that were never designed with this in mind, that difference is frequently what settles the question.

Heat in and heat out, and the perimeter each one has

Frame is always the weaker part of a glazed unit thermally, and a lantern has a great deal more of it: rafter bars, a ridge, hips and an eaves member around the whole perimeter. A flat unit has four sides and nothing crossing the middle.

So for the same glass specification, a flat rooflight usually returns the better whole-unit U-value, while a lantern needs a well engineered thermal break and a good glass build-up to land in the same territory. Solar gain runs the other way round in summer: a flat unit takes the full midday sun on its face, whereas a lantern spreads the same load across planes at an angle. Both are worth specifying with a solar control coating over a kitchen, and the numbers are on the energy efficient rooflights page.

Rain, falls and how each behaves through a wet winter

Water leaves a lantern quickly. Pitched planes shed rain, and dirt goes with it, so the glass keeps itself reasonably presentable at a height where nobody is going to reach it.

A flat unit at five degrees sheds water more slowly and holds dust along the low edge, where a grey tide line builds up over a season. Neither is a fault. It is simply what the geometry does. On rain noise the pitched planes of a lantern deflect more of the impact and generate less drumming than a shallow single sheet, and a thicker laminated inner pane narrows the gap on both.

Height above the roof plane, and what shows from outside

This is where the two diverge most sharply outside the house. A flat rooflight barely breaks the line of the roof and is close to invisible from ground level. A lantern is a visible object with a ridge, and it can be seen from upstairs windows next door and often from the street.

That has consequences in this county. Behind a Cheltenham parapet a flat unit can sit below the coping and disappear, where a lantern would show above it. On a rear extension in a conservation area the same difference frequently decides which product a planning officer is comfortable with, and it is covered under roof lantern planning permission.

Ceiling height, and the constraint people forget

A lantern needs headroom below it to make sense. In a room with a 2.7 metre ceiling the deep splayed reveal under a lantern is a feature. In a Gloucester terrace rear addition with a 2.2 metre ceiling and a shallow roof, the same reveal eats the ceiling and the lantern reads as a hole in a low box.

The other constraint is the flat roof itself. A lantern needs a margin of roof around its kerb for the covering to be dressed up and over, and for the falls to run past it to the outlet. A small extension roof can be too small for the lantern somebody has in mind while comfortably taking a flat unit of the same aperture.

Which one suits which roof in this county

The pattern across Gloucestershire is fairly consistent. Wide rear extensions on the Quedgeley, Abbeydale and Bishops Cleeve estates, and the larger vale village extensions around Tewkesbury and Newent, usually take a lantern comfortably: new decks, good falls, and ceiling height to spare.

Narrow two-storey rear additions on Gloucester’s Victorian terraces, and shallow roofs behind the Cheltenham parapets, more often suit a flat unit. So do outriggers, side returns and any roof that has to be walked on, where a walk-on rooflight is the only product that answers the question at all. Cotswold garden rooms sit either way depending on what shows above the stone parapet.

Photographs of the roof and the room, a rough size and the address are enough for a first view on which of the two the building will take, followed by a range by email and a fixed price after a survey. That process is on the costs page.