What is the difference between a roof lantern and a skylight?

Picture the two from the garden. A roof lantern rises out of a flat roof as a distinct object, usually rectangular in plan, with glazed sides that lean inwards and meet at a ridge bar running along the top. Older ones were timber and had a lot of glazing bars; a modern aluminium lantern keeps the frame slim and the panes large, but the geometry is the same. It has height, it has a pitch of its own, and you read it as a structure sitting on the roof rather than part of it.

A skylight, or rooflight, is the flat option. It is a single glazed unit dropped into an opening so that the glass sits close to the roofline, either flush or lifted a few centimetres on a low kerb. On a flat roof it might have a gentle fall built in to shed rainwater, but it does not rise into a peak and it has no sides of its own. From below you see one clean rectangle of sky. From outside you often do not notice it at all.

So the honest way to hold the difference in your head is this: a roof lantern is a small glazed roof built on top of your roof, and a skylight is a window let into the roof you already have. The words get used loosely, and a lantern is technically a type of rooflight, but when someone in the trade says lantern they mean the raised, ridged, multi-pane box, and when they say skylight or rooflight they mean the flat single unit.

This is where the choice is usually made for you rather than by you. A roof lantern needs a flat or very low-pitch roof to sit on, because it supplies its own slope. Put one on a flat rear extension and it works exactly as intended. You cannot fit a traditional lantern into a pitched tiled roof, because the roof is already sloping and there is nowhere for the raised box to sit square.

A skylight is the more flexible of the two. It goes into a flat roof happily, and it also goes into a pitched roof, where it becomes the familiar roof window you see in loft conversions. If your project is a pitched-roof skylight installation in a converted loft, a lantern is simply not an option and a skylight is the whole answer. On a flat roof you genuinely get to choose between the two, which is where the rest of this decision lives.

Most flat-roof lanterns and skylights end up over the same kind of room, a single-storey rear extension over a kitchen or dining space. The extension is flat-roofed because a pitch would eat into headroom and sightlines, and the room is deep, so it needs light coming in from above rather than only from the back wall. Both products solve that. They just solve it with a different amount of drama.

A lantern and a skylight of the same footprint do not deliver daylight in the same way. Because a lantern has height and sloped sides, it catches light from more of the sky and pulls it down into the room from several angles at once. It also gives you a sense of volume overhead, so the ceiling feels lifted. A flat skylight of the same size gives you a flatter, more even wash of light through a single plane. Neither is better in the abstract. A lantern makes a room feel taller and more architectural; a flat skylight keeps the ceiling calm and modern and lets the glass do its job without announcing itself.

The area of glass matters more than the shape for how bright the room ends up. A large flat skylight can easily throw more daylight onto the floor than a modest lantern, because raw glazed area is what counts. If daylight is the only thing you are chasing, a big rooflight is often the cheaper way to get it. If you want the feeling of height and a piece of architecture over the table, that is what a lantern buys you.

Here is the part that gets forgotten until the second summer. Both a lantern and a skylight are horizontal or near-horizontal glazing, which means they face the high summer sun almost square on. That is the harshest orientation there is for solar gain, worse than any wall window, and it applies whichever way the house is turned. In Southend and along the estuary this bites harder than it does inland, because the rear of a great many houses faces south across open sky with nothing to shade it, and a flat-roofed kitchen extension with glass overhead becomes a solar collector over the one room that already makes its own heat.

A lantern can make this worse than a flat skylight for a simple reason: it usually carries more square metres of glass. Sloped sides plus a top plane add up to a larger collecting area than a flush unit of the same footprint, so a lantern over a south-facing kitchen can gather a serious amount of heat on a clear day. The shape does not save you. What saves you is the glass specification, and specifically the G-value, which is the share of the sun’s energy the glazing lets through as heat. Clear double glazing sits around 0.6; a solar-control unit is nearer 0.3, which roughly halves the heat coming in while keeping the room bright.

So the rule is the same for both products. On a south-facing or flat installation, ask for solar-control glazing at a low G-value rather than standard clear glass, and treat the height and area of a lantern as a reason to be stricter about it, not a reason to relax. This is the heart of what we do, and our energy-efficient skylight installation work is built around getting the glass specification right for the orientation before anyone talks about the frame. A lantern and a skylight facing the same way need the same conversation about heat and glare.

A roof lantern is generally the more expensive of the two for a given opening. It is a more involved structure, with a frame, a ridge, corner joints and more glass, so it costs more to supply and takes longer to fit. A flat skylight is a single unit and a simpler job, and for the same hole in the roof it will usually come in lower. If budget is the deciding factor, a flat flat-roof skylight installation is the leaner choice, and our skylight installation costs page sets out where each option tends to land.

Both types sit on a builder’s upstand, a raised timber kerb around the opening that lifts the frame clear of standing water and gives the roof covering something to seal against. A lantern generally asks a bit more of that upstand and of the flat roof around it, because it is heavier and taller, and a larger opening may need the structure checking to carry it. In practice both are notifiable work under Building Regulations, with Part L covering the thermal performance of the glazing and Part K covering safety glazing, and both are jobs where the opening and the upstand deserve as much attention as the shiny unit that goes on top.

Start from the roof. If you have a pitched roof, the decision is made and you want a skylight. If you have a flat roof, ask what you are really after: a flat skylight for the most daylight per pound and a clean modern ceiling, or a lantern for the sense of height and a feature over the room. Then, whichever you pick, settle the glass before the frame, because on a south-facing estuary extension the G-value is what decides whether the room is a pleasure in July or a place you avoid.

If a lantern is what you are picturing, our roof lantern installation page walks through the structure, the sizes and the glazing in detail. If you would rather talk it through, tell us the roof type, the rough opening size and which way it faces, and we will come back with a recommendation and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the shape of the glass on top of it.