Does a Roof Lantern Let in More Light Than a Flat Skylight?

A flat rooflight has one face, angled just enough to shed rainwater, so almost every ray that reaches it during the day arrives close to square on. A roof lantern has four or more faces: the shallow slopes plus, on most shapes, upright glazing in the sides below the ridge. That upright glass is doing something the flat pane cannot. It faces the sky sideways as well as up, so it picks up sun sitting low over the roofline in the morning and again in the evening, hours when a flat pane mostly sees its own shadow.

The gain is not about the lantern letting in "more sky" in some abstract sense. It is that a lantern samples the sky from several angles at once, while a flat rooflight samples it from one. Over a winter afternoon, when the sun barely climbs above the ridge line of a Peterborough terrace, that difference between one angle and several is most of the story.

Picture the sky as a dome over the roof. A flat rooflight is a single window cut in the top of that dome, looking straight up. A lantern adds windows around the sides too, low down near the kerb, angled out towards the horizon.

• Early and late in the day, the sun sits near the horizon, and it is the lantern's upright sides that catch it, not the sloped top.
• Around midday, both products are working, but the flat rooflight is closest to its best angle, so the two come nearer to level.
• On an overcast day, the effect is gentler, because diffuse sky light arrives from every direction anyway, but a lantern still gathers a wider slice of that grey sky than one flat pane does.
• In deep winter, when the sun barely clears the roofline, the lantern's sides are doing more of the work than its sloped top.

This is also why orientation matters more with a lantern than with a flat rooflight. A lantern with clear sides facing south west can carry a good spread of afternoon and evening light on into a kitchen used at teatime, in a way a single flat pane over the same spot cannot match. Our guide on whether a lantern is wasted on a north-facing extension goes further into what orientation changes and what it does not.

None of this is free. A lantern's extra glass comes with more frame and more bars than a flat rooflight of the same footprint, because the roof shape needs rafters at the hips and ridge to hold it up. Every bar is a strip of ceiling that stays plasterboard rather than sky, so a wider or more heavily divided lantern can give back some of the light it gains from its shape.

Slimmer rafters claw a good deal of that back. Aluminium sections can be built noticeably narrower than the timber ones used on older lanterns, so less of the sky is lost to frame at each division. That is worth understanding before you compare products on paper, and our guide on aluminium roof lanterns and slim rafters sets out what to look for in the profile. The number and spacing of the bars themselves is a separate decision again, covered in roof lantern glazing bars and the view up, and it affects the view up from a chair just as much as it affects the light.

Solar control coatings cut down the heat and glare a lantern's larger glass area can bring on a sunny south-facing extension, and they do this by reducing some of the light along with the heat. Fitted well, the loss is modest and the room stays comfortable rather than turning into a greenhouse by mid-afternoon. Our page on solar control glass for roof lanterns explains how that trade-off is set, and it applies just as much to a flat rooflight's single pane.

A flat rooflight is a simpler shape, and simple has its own advantages. It sits low and close to horizontal, tipped just enough for rainwater to run off, often at a fall of around 1:80, so from below it reads as one uninterrupted pane rather than a stepped roof shape. In a narrow extension, or a room with a low ceiling, that single clean rectangle can suit the space better than a lantern that has to rise well above the flat roof to fit its sides in.

A flat rooflight is also the calmer choice where the priority is the view straight up rather than the extra light. Our comparison of a roof lantern or flat rooflight for the view works through that room by room, and it is worth reading alongside this page if the sightline matters as much to you as the daylight. And where the room already gets plenty of light from a large run of bifold or sliding doors, a flat rooflight can add sky overhead without competing for attention with the glass in the wall.

Across the new-town townships, Bretton, Orton and Werrington among them, a great many rear extensions sit under a flat roof, and the choice between a lantern and a flat rooflight comes up on almost every one. A kitchen facing south or west over a garden usually gets the clearest case for a lantern, because the low evening sun through its sides carries the light on into the time of day the room is actually used. A narrower side-return extension against a Fletton brick terrace, where the flat roof is only a metre or so wide, often suits a flat rooflight better simply because there is not the width to raise a lantern's sides without it looking oversized from the garden.

Either way, the deciding factor is the room, not a fixed rule. That is also true in the stone villages around Stamford, where a lantern's height can sit awkwardly against a low-eaved roofline, and a slim flush rooflight often reads as the tidier fit.

Size, pitch and orientation all move this comparison before glass choice even comes into it, and none of them can be judged from a brochure photo of someone else's kitchen. A lantern angled towards a walled garden in Werrington will behave quite differently from the same product on an open plot at the edge of the Fens, where there is nothing to the west to block the evening sun. Building Control also has a view on the glass and structure either way, set out in roof lantern Building Regulations, and we handle that notification where the work is notifiable.

What decides it in practice is a drawing rather than a rule of thumb. Every quote comes with a sightline sketch, a drawing of the view and the light from where you actually sit, showing how a lantern's sides or a flat pane's single face behave from your own chair, table or worktop. You can see how that drawing is put together on the sightline sketch page, and the wider factors that set your budget either way are covered on what affects the cost of a rooflight. Both products are installed to current Building Regulations with a 10-year workmanship guarantee. Book a home visit and we will measure the room and show you both options side by side, or see the full service on roof lantern installation in Peterborough.

Does a bigger lantern always mean more light?

Not in a straight line. Width and length matter, but so do the number of bars a larger frame needs to hold the glass up, and the pitch of the sides. A slightly smaller lantern with slim aluminium bars can outperform a wider one carrying a heavier, more divided frame.

Is the difference worth it if the extension already has bifold doors?

Often less so. A room that already takes in a wide view of sky and garden through the doors has less to gain from a lantern's extra angle of light, and a flat rooflight can add sky overhead without crowding the ceiling above an already glazed wall.

Do lanterns need more cleaning to keep that extra light?

The upright sides catch more airborne dirt and rain streaking than a flat pane simply because there is more glass and more edges. Self-cleaning coatings help, and our page on whether self-cleaning glass is worth it on a roof lantern sets out where they work best.

Can I compare the two properly before committing to either?

Yes. Bring us the room, the aspect it faces and how it is used, and we will talk through what you would actually see through each product before drawing the sightline sketch for both.