Every skylight is the same idea: a piece of roof replaced with something that lets light through. Almost none of them are the same building. A polycarbonate dome over a warehouse bay and a glazed roof over a five-storey atrium share a name and very little else — not the structure, not the drainage, not the price, and not the questions you need to ask before you buy one.
This guide covers all of them, including the ones we do not build, because choosing well starts with knowing the whole range. It explains what a skylight is for, the ten types in common use and how each one fails, which type suits which span, how to choose between them, and what decides whether the finished roof works. The photographs are from CORTEX projects unless they are marked as reference images.
What a skylight actually is
A skylight is a glazed opening in a roof. That is the whole definition, and everything difficult about skylights follows from its two nouns.
A roof exists to keep water, snow and heat out. Cutting a hole in it and filling the hole with glass reverses that priority at one point: the roof must now let light through while still doing every other job the rest of it does — on a surface where water does not run off by itself and snow does not blow away.
That is why a skylight is harder than a window. A window sits in a wall, and a wall is vertical, so rain falls off it. A skylight sits in something close to horizontal, and horizontal is where water stands, snow collects and dirt stays.
Skylight, rooflight, roof window, lantern — the same thing?
Nearly. The differences are worth knowing because specifications use the words loosely.
- Skylight — the general international term for any glazed opening in a roof.
- Rooflight — the British word for the same thing, most often a unit on a flat or shallow roof. In UK practice “rooflight” tends to mean the product and “skylight” the opening.
- Roof window — a unit set into a pitched roof, in the plane of the roof, usually openable and sized for someone to look out of. It is a window that happens to be in a roof: frame, sash, and designed to be reached from inside.
- Roof lantern — a raised, framed structure on a kerb above the roof line, with sloping sides meeting a ridge or a flat cap.
- Atrium roof — a glazed roof over a multi-storey void. At that scale it stops being a unit from a catalogue and becomes a structure engineered for one building; more on that in free-form skylights.
Is a skylight a window? For thermal performance, fire and safety it is usually assessed as one — it is a glazed opening in the building envelope. For detailing it is not, because everything about how it is built comes from the fact that it is lying down rather than standing up.
Why light from above beats light from the side
This is the part most people have never been told, and it is the reason skylights exist at all.
A vertical window can only see half the sky. Draw a line out level from the glass: everything above it is sky, everything below it is ground. So a window faces a hemisphere of which only the upper half is sky — and the lower half, the ground and the street, is dark.
A horizontal opening in a roof faces the entire sky dome. Nothing stands in front of it, and none of its view is ground.
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A window sees half the sky Draw a line out level from the glass: everything above it is sky, everything below is ground — and ground is dark. Half of what a window faces sends back little light.
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A roof opening sees all of it Nothing stands in front of it and none of its view is ground, so the same area of glass collects several times the daylight — and puts it straight down onto the floor.
Section, with nothing opposite. A neighbouring building takes more of a window’s sky away; it rarely touches a roof’s.
The result is that a roof opening delivers several times the daylight of a vertical window of the same area — around three times is the figure usually quoted. Not marginally better: several times better, for the same hole in the building.
The second effect matters more on a large plan. Side light does not travel. A window lights a strip of floor beside it, and the useful depth is roughly twice the height of the window head above the floor. A head at 3 m lights about 6 m into the room; beyond that, the lights are on at midday. Doubling the glass in the wall does not move the line, because the light still does not reach.
Light from the roof has no such limit, because it comes down onto the plan rather than across it. A roof opening lights whatever is beneath it, wherever that is. On a deep floor — a shopping mall, a factory, a terminal, a warehouse, a research building — the middle of the plan cannot be daylit from the walls at any cost. It can only be daylit from above.
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Side light stops; roof light lands A window lights a strip about twice its head height deep, and adding glass to the wall does not push that line further in. An opening in the roof lights whatever is under it — including the middle of a plan no wall can reach.
Rule of thumb: the side-lit zone is about twice the window head height deep. Section, not to scale.
Ava Center is the same argument as a built case: a car park that became a shopping mall after its frame was standing, where the open void in the middle of the plan turned out to be the only route daylight had to the retail floor.
That is the whole argument for a skylight. Everything else in this guide is a benefit or a consequence. This is the reason.
The six jobs a skylight does
Worth listing, because a skylight specified for one of these and detailed for none of the others is where most of the problems start.
1 · Daylight. The primary job, measured as a daylight factor or as useful daylight across the year — and modelled rather than guessed, as below.
2 · Cutting the lighting load. Artificial lighting is a large share of a commercial building’s electricity, and it runs in exactly the hours daylight is available. A roof that daylights the plan lets the lights go off, and on a mall, a factory or a warehouse the saving is large and it lasts as long as the building does.
3 · Wellbeing and orientation. The daily cycle of daylight regulates the body clock, and people in daylit space consistently report feeling better in it. In retail and hospitality a daylit volume simply reads as a better place to be. A skylight also tells you the time of day and what the weather is doing, which a lit ceiling never will.
4 · Ventilation. Warm air rises. An opening at the highest point of a volume is the most effective ventilation opening available, because it is where the warm air has already gathered. It is the same physics that makes ridge vents the main climate control in a glass greenhouse.
5 · Smoke ventilation — the one nobody expects. In many jurisdictions, automatic opening vents in the roof are a fire-safety requirement in atria, malls, warehouses and stairwells, to clear smoke and keep escape routes usable. Smoke vent skylights — AOVs, automatic opening vents — are a product category of their own, and on a large building the smoke strategy often decides how many roof openings there are and where, before daylight is considered at all. If the building has an atrium, ask the fire engineer before you ask the architect.
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Ventilation: the top does the work Heated air climbs and pools beneath the roof. Open the highest point and it escapes, while cooler air is pulled in near the floor to take its place — no fan in the loop.
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Smoke: the same opening, a fire-safety job Smoke rises the same way. Automatic vents at the top hold it in a layer under the roof, keeping the floors and stairs beneath it clear long enough to get out.
Section. Vent sizes and the depth of the smoke layer come from the fire strategy, not from a drawing.
6 · Architecture. A shaft of daylight is the cheapest dramatic effect a building can have. It is also the one most often ruined by specifying it as a product and discovering the structure, the drainage and the solar control afterwards.
The ten types, and where each one works
Skylights divide less by appearance than by what holds them up. Each entry below says what the type is, where it works, and how it fails — because how a thing fails is more useful to know than what it looks like.
Two of the ten, the flat rooflight and the dome, are products bought from a catalogue. Two, the atrium roof and the free-form or cable-supported roof, are structures engineered for one building. The six between them are framed systems, and that middle band is where most commercial skylight projects sit and most of the decisions are made.
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1 · Flat rooflight small openings, flat roofs
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2 · Dome warehouses, light not view
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3 · Pyramid 2 – 8 m, seen from below
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4 · Ridge long, narrow apertures
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5 · Barrel vault concourses, arcades
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6 · Monopitch against a higher wall
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7 · Roof lantern raised on a kerb
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8 · Walk-on daylight to a basement
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9 · Atrium roof over a multi-storey void
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10 · Free-form / cable large spans, slim structure
Silhouettes, not to scale. The word under each is what you are buying.
1 · Flat fixed rooflight
A flat or nearly flat glazed unit on an upstand kerb in a flat roof. The simplest skylight there is.
Works for small openings, up to about two metres, in flat roofs — offices, houses, extensions, plant rooms.
Fails at drainage. A truly level rooflight holds water, and water left on glass dries into mineral deposits, grows algae and finds every imperfection in a seal. Even a “flat” rooflight is normally laid a few degrees out of level so that rain runs off it. A flat rooflight with no fall is a maintenance contract.
2 · Dome skylight
A moulded polycarbonate or acrylic dome on a kerb. The cheapest skylight per square metre, and by a wide margin the most common one in the world.
Works for warehouses, factories, workshops, plant enclosures and back-of-house areas — anywhere the requirement is light in rather than a view out, and nobody looks at the roof.
Why it works. The dome sheds water and snow by its own shape, it spans its opening without framing, and it is light, cheap and hard to break.
How it fails. Polycarbonate yellows under ultraviolet light and passes less light every year — an old dome passes noticeably less than it did when new. It scratches, so it dirties and cannot be cleaned back to clear. And it diffuses light rather than transmitting it, so you get brightness without a view of the sky.
Dome or flat? A dome drains itself; a flat unit has to be laid to a fall. A dome diffuses; glass transmits. A dome is cheaper; glass lasts. For a warehouse, take the dome. For anywhere people look up, do not.
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Flat, no fall Water stays on the glass, dries into mineral rings, grows algae and finds every weakness in the seal. A maintenance contract, not a rooflight.
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Flat, laid to a fall Even a "flat" rooflight is set a few degrees out of level, so rain runs to one edge and off the roof instead of standing on the glass.
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Dome Water and snow leave in every direction by themselves, and the curve carries its own span. What it cannot do is stay clear: polycarbonate yellows and scratches.
Falls exaggerated. A few degrees is enough for water to leave the glass.
In Iran the polycarbonate bubble dome is the default this whole article is quietly compared against. It is the right answer for an industrial shed. It is the wrong answer for a lobby.
3 · Pyramid skylight
Four triangular glazed faces rising from a square or rectangular kerb to a point. A framed structure rather than a moulded unit.
Works for openings of roughly two to eight metres across, in flat roofs, where the skylight is seen and has to look deliberate — hotel lobbies, entrance halls, courtyards, villas, shops.
Why it works. Every face slopes, so every face drains and sheds snow. The geometry is stiff by nature, so the members can be slim. And it reads as an object rather than a hole.
How it fails. At the apex and along the hips, where four faces and their gaskets meet. A pyramid is four easy slopes and one difficult junction, and the junction is where it leaks if the detail was drawn rather than engineered.
4 · Ridge skylight — continuous, gable or hip
Two sloping glazed planes meeting at a ridge. A continuous ridge skylight runs the length of a roof; a gable or hip ridge skylight closes at its ends.
Works for long, narrow openings — a corridor, a concourse, a circulation spine, a factory bay, an arcade. The classic industrial daylighting arrangement, and still the best one for a long plan.
Why it works. It is a linear system, so it repeats, so it is economical per metre. Both planes drain to gutters at the eaves. It admits light along a line rather than at a point, so it daylights a route instead of a spot.
How it fails. At the ends and at the gutters. A continuous ridge light has a long gutter on each side, and a long gutter is a long opportunity for a blockage. It also takes light from two directions only, so a ridge running north–south and one running east–west behave quite differently through the day.
A structural ridge skylight is the version in which the ridge member is part of the building’s structure rather than a glazing member sitting on it. Worth settling at design stage, because it changes who designs it.
5 · Barrel vault skylight
A single curved surface, semicircular or a shallower segment, spanning between two supports. In effect a ridge light with the ridge rounded off.
Works for spans from a few metres to tens of metres — concourses, arcades, station platforms, atria, covered streets.
Why it works. Curvature is structurally efficient: an arch carries load along its line in compression rather than in bending, so its members are smaller than a flat roof of the same span would need. It sheds water away from the crown in both directions. And it curves in one direction only, so it can be glazed with flat panes in a curved frame — far cheaper than glass that curves two ways.
How it fails. At the springing, where the arch meets its supports. A barrel vault pushes outward at its base, and whatever is underneath has to take that thrust or be tied against it. Designing the vault and discovering the thrust later is how a barrel vault becomes an argument.
6 · Monopitch or lean-to skylight
A single sloping plane, usually running up against a wall or a higher roof.
Works for extensions, courtyards against a taller building, lean-to spaces and covered walkways.
Fails at the abutment. A monopitch light meets a wall along its head, and that one junction has to take the building’s movement, the water running down the wall above and the thermal movement of the glazing — at the point where access for repair is worst.
7 · Roof lantern
A raised framed structure on a kerb, with sloping sides meeting a ridge or a flat cap — in effect a pyramid or a small hipped ridge light, lifted up.
Works for domestic and small commercial roofs, where the raised profile is wanted for its look as much as for its light.
Fails at thermal performance, historically. A lantern is a lot of frame for its area of glass, and the frame is where the heat goes. Thermally broken lanterns are much better than older ones, but the ratio of frame to glass is still the number to check.
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Pyramid: the apex and hips Every face drains, so the planes are easy. The risk is where four planes and their gaskets meet at a single point.
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Ridge: the gutters and the ends Both planes drain to eaves gutters that run the full length — a long gutter is a long chance of a blockage. The closed ends are the other weak point.
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Barrel vault: the springing An arch carries load efficiently, in compression — and pushes outward at its base. The walls take that thrust, or a tie does. Designed for at the start, not discovered.
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Monopitch: the abutment The head of the glass meets a wall that moves differently, collects water running down from above, and is the hardest place to reach for repair.
Schematic. In each case the risk sits where two things meet, which is why these are detailed and tested rather than assumed.
8 · Walk-on rooflight and glass floor light
A glazed unit rated to be walked on, bringing daylight into a basement or a lower floor through a terrace, courtyard or pavement above.
Works for basements, lower ground floors, and rooms beneath terraces — the one situation where there is no roof to open, because there is a floor there instead.
Can you walk on a skylight? Only one rated for it, and the difference is structural rather than cosmetic. A walk-on unit is designed for the concentrated load of a foot or a wheel, usually with a laminated build-up of several plies so that one broken ply does not leave a hole in the floor. A standard rooflight is not walk-on and should be marked as fragile. Falls through fragile rooflights are a recognised cause of serious injury in roof work, and that is why the marking exists.
9 · Atrium roof — the atrium skylight
A glazed roof over a multi-storey void inside a building. The largest thing on this list, and the point at which a skylight stops being a product.
Works for malls, offices, hotels, terminals, hospitals and civic buildings — anywhere a deep plan needs daylight all the way down to the ground floor.
Why it is different. Everything scales badly at once. The structure has to span; the drainage has to collect a large area and get it off the roof; the solar gain lands on a volume that has to be cooled; the maintenance cannot be done from a ladder; and the fire strategy almost certainly involves it. An atrium roof is engineered for its building, not selected from a range — atrium roofs in detail.
10 · Free-form and cable-supported skylights
The two ways to cover a large opening without the structure taking over the view.
Free-form means the surface curves in two directions and cannot be described by a single radius. The curvature does the structural work, which lets the members be slim — but every panel is different and every node meets at a different angle, so the geometry has to be rationalised into something that can be fabricated before it can be priced. See free-form skylights for the roof, and free-form structures for shells that turn from roof into wall.
Cable-supported means the roof stays flat or shallow while pre-tensioned cables reinforce a steel lattice, so the steel works in tension and compression instead of bending. The members shrink back to slim sections and the roof reads as a light lattice instead of a heavy frame, across spans of 10 to 45 m — see cable-supported skylight.
Choose between them by shape, not by preference. If the roof curves in two directions, curvature is already doing the work and cables add nothing. If it has to stay flat across a long span and the members have grown too deep to see past, that is what cables are for.
- Auto Mall roof glazing, installed above a live city block
- 2,400 m²
- turned components fabricated for the Fanap cable system
- ≈ 11,000
- span range, cable-supported skylight
- 10 – 45 m
- where free-form skylights begin, with no upper limit
- 8 m +
Which type for which span
The most useful way to sort skylights is not by shape but by the size of the opening, because span decides whether you are buying a product or commissioning a structure. The bands below are approximate, and every project moves the boundaries — the point is the shape of the table, not the exact numbers.
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Span decides what you are buying Below about eight metres a skylight is a unit chosen from a range and installed. Above it the roof needs a structure of its own, and the question stops being which unit and becomes how the load gets to the ground.
Bands drawn at equal widths, not to scale.
| Opening | Types that work | Why |
|---|---|---|
| Under 2 m | Dome · flat rooflight · roof window | Self-supporting, or a simple framed unit. No structure required. |
| 2 – 8 m | Pyramid · ridge · lantern · monopitch | Framed systems. The frame spans; the building does not have to. |
| 8 – 20 m | Barrel vault · continuous ridge · framed atrium roof | The roof needs a structure of its own, and curvature starts to pay. |
| 20 – 45 m | Cable-supported · space frame · free-form | Bending becomes the enemy; tension, triangulation or curvature replaces it. Built: Fanap Campus, three cable-supported roofs of 20 × 20 m and 26 × 17 m. |
| Over 45 m | Space frame · cable net · free-form gridshell | The surface itself has to become the structure. |
The answer changes at about eight metres. Below it you buy a unit and install it. Above it you buy a structure, and the question changes from which unit to how the load reaches the ground. For the largest roofs, the structures that answer it are space structures and cable net structures.
Fixed, opening or retractable
Three levels of movement, and three different buildings.
Fixed. Sealed, no moving parts, the most weathertight and the cheapest to maintain. If ventilation is handled elsewhere and there is no smoke requirement, fixed is the right default.
Opening, or vented. A powered or manual opening light, for air, for smoke, or for both. At the top of a volume it is the most effective vent available. It is also a moving seal, which is the part of any skylight most likely to leak in ten years — so the gasket detail, the drive and the access for maintenance are all specification items, not afterthoughts.
Retractable. The roof slides, folds or pivots away so the space is open to the sky. Genuinely spectacular, and genuinely a machine on a roof: rails, drives, seals that must work in both positions, drainage that works while it moves, and a wind and rain sensor that closes it on its own. Popular in hospitality across Iran and the Gulf, it is the type most often bought for the effect and regretted for the upkeep.
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Fixed No moving parts: the most weathertight and the cheapest to keep. The right default wherever ventilation and smoke are dealt with elsewhere.
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Opening A vent for air, for smoke or for both. The seal that has to close tight after every opening is the part most likely to leak in ten years.
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Retractable A machine on a roof: rails, drives, seals that work open and closed, drainage that works while it moves, and a wind sensor that closes it on its own.
Sections, schematic. Accent rings mark the seals that move.
The rule of thumb: every moving part in a roof is a future maintenance item in the wettest, hottest and least accessible part of the building. Specify movement where it earns its place, and fix everything else.
Choosing the material: glass, polycarbonate or ETFE
Three materials for three different kinds of roof. The table sets them side by side; the drawing shows the difference that matters most, which is how each one spans.
| Measure | Glass | Polycarbonate | ETFE |
|---|---|---|---|
| Light transmission | Highest, and stable for the life of the building | Good when new; falls as it yellows | Very high, and stable |
| Clarity | Transmits — you see the sky | Diffuses — brightness, not a view | Transmits, slightly hazy |
| Weight | Heaviest — it sizes the structure beneath | Light | A fraction of glass |
| How it spans | Pane by pane, supported at its edges or points | Self-supporting as multiwall sheet or a dome | Inflated cushions between frames |
| Insulation | Ug 1.0 – 1.6 double, 0.6 – 0.9 triple | Good in multiwall — air in the flutes | Good as a multi-layer cushion |
| Life | Decades, unchanged | Degrades under UV | Long, and repairable |
| When it breaks | Can break — which is why the build-up matters | Impact-resistant | Punctures rather than shatters |
| Use it for | Anywhere people look up | Warehouses, factories, plant screens | Very large, very light roofs |
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Glass Clear, stable for the life of the building, and the heaviest of the three — its weight is what the structure beneath is sized for.
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Polycarbonate Light and impact-resistant, self-supporting as multiwall sheet or a dome — but it diffuses rather than transmits, and it yellows under UV.
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ETFE Foil layers inflated into cushions and held at low pressure. A fraction of the weight of glass, which makes it the answer when the roof's own weight is the problem.
Sections, thicknesses exaggerated.
The decision rule. Transmission lost is daylight lost every day for the life of the building, and daylight is why the opening exists. A material that transmits less each year is buying a discount on the one thing you came for. So for any space people occupy and look up in, the answer is glass. For a warehouse, where the need is light on a floor, polycarbonate is honest and correct. And for a roof so large that its own weight is the problem, ETFE is the only one of the three that solves it.
Reading a skylight glass specification
Overhead glass carries three numbers and one non-negotiable requirement. Most specifications quote the numbers without saying what they trade against each other. The ranges below are the ones published for the free-form skylights we build.
- Light transmission (LT)
- Solar factor (g), with solar-control coating
LT decides how bright the space feels and the quality of its daylight. The solar factor sets how much of the sun’s heat gets in — the cooling load.
- Triple glazed
- Double glazed
Lower loses less heat
The trade you cannot avoid: light transmission and solar factor move together. Glass that lets in more daylight lets in more heat. A solar-control coating separates them to a degree — it turns back more of the infrared than of the visible light — but it cannot separate them completely. Asking for maximum daylight and minimum solar gain is asking for two different pieces of glass.
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Clear Daylight and solar heat arrive together and pass together. More light means more heat, one for one.
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Solar-control coating A coating reflects more of the invisible infrared than of the visible light, so it separates the two — partly. Some light is always lost with the heat.
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The trade you cannot avoid Asking for the most daylight and the least solar gain is asking for two different glasses. Choose which one the climate says matters more.
Arrow weights illustrative, not measured.
Which way to settle it depends on the climate, and this is where a project in Iran or the Gulf differs from one in northern Europe. In a hot, high-sun climate the cooling load dominates, and the sensible specification sits at the lower end of light transmission with a strong solar-control coating. In a cold, dull climate heat loss dominates and daylight is scarce, so transmission goes up and the argument moves to Ug.
Ug is about comfort, not only energy. A cold inner glass surface radiates cold onto the people beneath it and collects condensation. Under an atrium roof, the inner surface temperature is what decides whether the top floor is comfortable in winter.
The requirement nobody should negotiate
Overhead glass must be specified so that it cannot fall on people.
In practice that means a laminated inner leaf: two or more plies bonded to an interlayer, so that if the glass breaks the fragments stay stuck to the interlayer and the pane stays in the opening instead of arriving on the floor.
Where the glass is toughened and drilled, as it is in point-fixed and spider-glazed roofs, it should also be heat-soak tested. Toughening leaves a small chance that a nickel sulphide inclusion in the glass breaks the pane on its own, years later. Heat soaking takes each pane through a controlled temperature cycle so that panes carrying a suspect inclusion break in the factory rather than over a lobby three years later. Over occupied space this is not a refinement; it is the reason the test exists.
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Toughened alone Toughened glass breaks into small pieces — which is safe on a wall and unacceptable over people, because the whole pane comes down.
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Laminated inner Two or more plies bonded to an interlayer. If it breaks, the fragments stay stuck to the interlayer and the pane stays in the frame.
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Heat-soaked, where drilled Toughened glass can hide a nickel sulphide inclusion that breaks it years later. Holding the pane at temperature makes suspect panes fail at the works instead.
Sections, fragments stylised.
How thick does skylight glass need to be?
It is one of the most searched questions in this subject, and the honest answer is that there is no standard figure. Thickness comes out of four things: the size of the pane, how it is supported, the wind and snow loads on that roof, and the access class. Anyone quoting a thickness without those four is quoting a guess.
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1 · Pane size A larger pane bends more under the same load, so it needs more glass.
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2 · How it is held All four edges, two edges, or drilled and point-fixed — each gives the same pane a different stress.
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3 · Loads on that roof The snow and wind at that site, on that shape — including drifts and the local peaks at edges.
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4 · Access class Non-fragile, cleaning access, or walk-on. The heavier the requirement, the thicker the build-up.
How to choose the right skylight type
Choosing a skylight is a short run of questions, and the order matters: every answer removes options, and by the third most roofs are down to one or two. The first four settle the type. The last three settle how it is built.
- A floor above it comes first Under a terrace, a courtyard or a pavement, only a walk-on rooflight is rated to be stood on, whatever the size of the opening.
- Under 8 m: size, then plan Below about 2 m, a dome or a flat rooflight. Up to about 8 m, the plan picks the framed type — a square opening, a long one, or one that runs along a wall.
- Over 8 m: the shape of the roof Curvature in two directions makes it free-form. A roof that must stay flat takes cables. Anything else is a barrel vault or a framed atrium roof — and past about 45 m, a space frame or a cable net.
Thresholds approximate, as in the span table. Whether the skylight opens and what it is glazed with come after the type is known, not before.
- Is there a floor above the opening? If the daylight has to come down through a terrace, a courtyard or a pavement, the only candidate is a walk-on rooflight rated for that load, and the question is settled. If not, whatever you fit cannot be stood on, and it should be marked as fragile.
- How wide is the opening? Under about two metres it is a unit you buy — a dome where the need is light on a floor, a flat glazed rooflight laid to a fall where people look up. Between two and eight metres it is a framed system. Past eight metres the skylight needs a structure of its own, and the rest of the choice is about that structure.
- What shape is the opening in plan? Up to eight metres, the plan picks the framed type. A square or round opening takes a pyramid or a lantern; a long, narrow one takes a ridge skylight, or a barrel vault as it widens; one that runs along a taller wall takes a monopitch.
- Over eight metres, does the roof curve or stay flat? A surface that curves in two directions is a free-form skylight: its curvature is already carrying the load. A roof that must stay flat across 10 to 45 m is the case for a cable-supported skylight, which keeps the members slim. And where the glazed shell carries on down to become the wall, it belongs with the free-form structures.
- Does it have to open? For air, for smoke, or for both. If smoke, the fire strategy sets how many vents there are and where. If neither, fix it — a fixed skylight is the tightest and the cheapest to keep.
- What is underneath it? Where people stand or look up: glass, with a laminated inner leaf. Where the only need is light on a warehouse floor: polycarbonate. Where the roof is so large that its own weight is the problem: ETFE.
- Which sun, and which winter? In Iran and the Gulf, lower light transmission with a strong solar-control coating; where winters are long and grey, more light and a better Ug. Then check the snow and wind on that shape, and how the outer face will be reached for cleaning.
| Type | Span | Structure | Glazing | Application |
|---|---|---|---|---|
| Flat fixed rooflight | Under 2 m | Self-contained unit on an upstand kerb, laid a few degrees out of level | Insulated glass with a laminated inner leaf | Offices, houses, extensions, plant rooms |
| Dome skylight | Under 2 m | Moulded dome on a kerb, spanning without a frame | Polycarbonate or acrylic, which diffuses | Warehouses, factories, workshops, back-of-house |
| Pyramid skylight | 2 – 8 m | Framed: four sloping faces meeting at an apex | Flat triangular glass panes | Hotel lobbies, entrance halls, courtyards, villas, shops |
| Ridge skylight | 2 – 20 m wide, any length | Framed: two sloping planes to a ridge, with a gutter along each side | Glass; polycarbonate on industrial roofs | Corridors, concourses, factory bays, arcades |
| Barrel vault skylight | A few metres to tens of metres | Arched frame curved one way; its outward thrust taken by the supports or a tie | Flat panes in a curved frame | Concourses, arcades, station platforms, covered streets |
| Monopitch or lean-to | 2 – 8 m | One sloping plane, its head against a wall or a higher roof | Insulated glass | Extensions, courtyards against taller buildings, covered walkways |
| Roof lantern | 2 – 8 m | Raised frame on a kerb, sloping sides to a ridge or a flat cap | Glass in a thermally broken frame | Houses and small commercial roofs |
| Walk-on rooflight | Small, pane by pane | Rated unit carried by the floor structure | Multi-ply laminated glass rated for foot or wheel load | Basements, and rooms under terraces, courtyards and pavements |
| Atrium roof | 8 m and over | A structure engineered for its building | Laminated insulated glass, usually solar-control | Malls, offices, hotels, terminals, hospitals |
| Free-form skylight | 8 m and over, no upper limit | Doubly curved grid; the curvature carries the load | Faceted flat panes, or cold-bent or hot-bent glass | Landmark atria, domes and shells |
| Cable-supported skylight | 10 – 45 m | Flat steel lattice reinforced by pre-tensioned cables | Flat laminated insulated panes | Courtyards and atria that must stay flat |
The last three rows are the ones CORTEX designs and builds, and each can be seen finished in our projects — from the free-form atrium at Ava Center to the three cable-supported courtyard roofs at Fanap.
Six things that decide whether it works
A skylight that fails almost never fails because the glass was wrong. It fails on one of these.
1 · Daylight: sized by model, not by eye
The instinct is to put glass wherever the plan looks dark and see what happens. What happens is either too little light, or too much in one place with glare — and both are permanent.
Daylight is modelled: the sky conditions for that latitude across the year, the geometry of the opening, how much light the surfaces it lands on reflect, and the depth of the space below. The model says how much opening is needed and where, and it very often shows that less glass in the right place beats more glass in the wrong one — a saving as well as a better building.
2 · Solar gain: the same opening, working against you
Every skylight is a solar collector pointed at the sky. In a hot climate that is a cooling load for the life of the building, delivered to the top of a volume where heat is already collecting.
It is managed in a fixed order, because each step is cheaper than the next: the shape and orientation of the opening first — one that faces north, or is shaded by its own geometry, collects a fraction of what a flat one does; then the glass; then shading, outside the glass wherever possible, because external shading stops the heat before it enters and an internal blind stops it after; and only then the mechanical system, which is the most expensive way to solve it and the one most projects reach for first.
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1 · Shape and orientation first An opening that faces the sky but not the sun — north-facing, or shaded by its own geometry — collects a fraction of the heat of a flat one. It costs nothing to draw.
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2 · Shade outside, not inside External shading stops the heat before it passes the glass. A blind under the glass stops the glare, but the heat is already in the building.
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3 · Then glass, then shading, then plant Each step is cheaper than the next, and the mechanical system — the one most projects reach for first — is the most expensive way to solve it.
Northern hemisphere. Sun angles illustrative.
3 · Water: every drop needs a route
Rain does not run off a level surface; it sits there. So every skylight is detailed around a fall, however slight, and around where the water goes when it gets to the edge.
Two things catch people out. Condensation is not a defect. Warm, humid air inside meets cold glass on a still winter night and forms water on the inner face — reliably, by physics. A well-detailed skylight has a condensation channel in the glazing bars that collects it and carries it to the perimeter. One without drips into the room, and the occupants are told it is a leak.
And the gutter is the system. On a ridge light, a barrel vault or an atrium roof, the perimeter gutter collects the whole roof. A blocked gutter on a skylight is not a nuisance: it backs water up under the glazing line, which is the one direction the detail was never designed to resist.
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Condensation is not a defect Warm, humid air meets cold glass on a still winter night and forms water on the inside — by physics, reliably. A channel in the glazing bar collects it and carries it to the edge.
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The gutter is the system Blocked, the perimeter gutter fills until water backs up under the glazing line — the one direction the detail was never designed to resist.
Sections, exaggerated.
4 · Snow and wind: the loads that govern
Snow governs more skylights than wind does, and not in the way people expect. It does not lie evenly on a sloped or curved roof: it slips off the steeper parts and piles up wherever the roof flattens — in valleys and where the slope changes — so the local load in a trough can be several times the uniform figure. The case that usually governs is the unbalanced one — snow on one side of a vault and not the other — because an arch carries an even load efficiently and an uneven one in bending, which is what it is worst at.
Wind on a roof is mostly suction. It lifts. A skylight is held down by its fixings, not by its weight, and the peak suctions on a roof are local — at edges, corners and changes of shape — and can be several times the average. A skylight sized on the average wind load is under-designed exactly where it matters.
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Snow, unbalanced An arch carries an even load efficiently. Snow blown or slid off one side leaves an uneven one, which it carries in bending — and that case usually governs.
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Wind on a roof lifts it On a roof the governing wind load is suction. The skylight is held down by its fixings, not by its weight — and the pull is strongest where the flow breaks away.
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Peaks at edges and corners Local suction at a roof's edges and corners can be several times the average. Sized on the average, a skylight is under-designed exactly where the wind pulls hardest.
Magnitudes illustrative. Design figures come from EN 1991 or the local code.
5 · Safety: what breaks, and what lands on it
The glass side is covered above: a laminated inner leaf, heat-soaked toughened glass where it is drilled, and fragile rooflights marked as fragile so that nobody walks on one that was never rated for it.
One more: what falls onto a skylight matters as much as what falls out of it. On a building with rooftop plant, a terrace or taller neighbours, the glass has to take whatever can arrive on it — a dropped tool, a branch, hail. That is a specification input, and it is usually discovered late.
6 · Access: how it gets cleaned and repaired
A skylight is the dirtiest glass in the building, because it faces up. Dust, bird droppings and pollution land on it and stay, and a roof specified at 60% light transmission that has not been cleaned for five years is no longer at 60%.
So: how is it reached — from inside or outside? Does the roof have anchor points, a walkway, a gantry? Can one broken pane be replaced without dismantling its neighbours, and from which side?
These questions have to be answered at design stage, because the answers change the design. A roof with no way to maintain it is a roof that will not be maintained — and a skylight that is not maintained stops being a skylight and becomes a ceiling.
What a skylight costs, and why
Skylights are priced per square metre of glazing, and that figure hides almost everything that decides it. In order of leverage:
1 · Span, and what carries it. Below about eight metres you buy a framed unit; above it you buy a structure, and the cost steps up rather than rising smoothly. This is the largest single factor, and it is set by the architecture, not by the specification.
2 · Flat, singly curved or doubly curved. A flat or singly curved surface takes flat panes — the cheapest glass there is. A doubly curved surface needs panes that are faceted into flat approximations, cold-bent on site or hot-bent in a mould, in rising order of cost. That choice is made on the geometry at concept stage, and it decides more of the budget than the glass specification does.
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Span: a step, not a slope Past about eight metres the price per square metre jumps, because you are now paying for a structure that carries the glass, not a frame that holds it.
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Curvature: the glass follows the shape Flat and singly curved roofs take flat glass, the cheapest there is. Double curvature needs faceting, cold-bending or hot-bending — in that order of cost.
Shapes only — no figures are implied.
3 · How many panels are identical. The same argument as every other envelope system. A roof built from a dozen panel types costs a fraction of one built from ninety, and rationalising the panel set is free if it is done before the geometry is fixed — and impossible afterwards.
4 · The glass build-up. Laminated, insulated, coated, heat-soaked, switchable. Each step is justified by something specific, and each is worth nothing where that something is not required.
5 · Opening lights and smoke vents. Each one is a mechanism, a seal, a control and a maintenance item. Count them, and know what each costs.
6 · Access and installation. A skylight goes in at the highest point of the building, often over an occupied space and often after the building below is finished. At the Auto Mall in Tehran, 2,400 m² of free-form roof glazing went in above a live city block — a programme and access problem before it was a glazing one.
7 · Drainage and the perimeter. The junction between the skylight and the roof around it is where cost and risk concentrate. A large open area of glazing is the cheap part.
When a skylight is the wrong answer
Worth saying, because the honest cases are the useful ones.
- When side windows would do. If the plan is shallow enough to be daylit from the walls, use the walls. They cost less, they drain themselves, and they give a view.
- When the cooling cannot be carried. In an extreme climate, on a building with no capacity to cool the extra gain, a skylight can cost more in cooling than it saves in lighting. Model it before committing.
- When nothing can ever reach it. A skylight with no maintenance access will dirty, leak and stay that way.
- When the roof above is in use. If the roof is a terrace, a plant deck or a route, a standard rooflight is a hazard, and a walk-on unit is a different and much more expensive product.
- When the brief is a view rather than light. People look out sideways, not up. A skylight gives you sky; if the brief is a view of something, it is a window.
- When it is added after the structure is fixed. The biggest decisions about a skylight — span, geometry, where the loads go, where the water goes — belong at concept stage. Added later, a roof opening is an alteration to someone’s finished design, and it is priced like one.
Where to go from here
Four routes, depending on the roof you have.
- A small opening in a flat roof — a flat rooflight, a dome or a pyramid is what you need, and it is a product decision rather than an engineering one. Most commercial skylight suppliers carry all three; the questions that matter are the fall, the glass build-up and the access.
- A roof that curves in two directions, or a geometry no single radius describes — the curvature is doing structural work, and the problem is panelisation: free-form skylights and free-form structures.
- A roof that must stay flat or shallow over a long span, where the members have grown deep enough to hide the sky the skylight was meant to show — that is what cable reinforcement is for: cable-supported skylight, 10 to 45 m.
- A very large, very light roof — the surface itself may need to become the structure: cable net structures and space structures.
Whichever it is, the drawings that let us answer quickly are the roof, the plan underneath it and the section through both.