How much light does a skylight let in compared with a window?

A window in a wall and a window in a roof are doing the same job, letting daylight into a room, but they are pointed at completely different parts of the sky. Stand under a rooflight and look up: you see a wide, open patch of sky with nothing between your eye and it but glass. Stand at a wall window and look out: you see a narrower band of sky above the horizon, and below that, ground, gardens, other roofs and whatever else happens to be opposite.

Daylight is not spread evenly across the sky. The overhead sky, the part directly above a room, is almost always the brightest part of it, on a clear day or a grey one. A rooflight faces that bright patch directly. A wall window faces sideways into it, catching only the lower, often dimmer, part of the same dome.

Two panes of the same glass, cut to the same size, let almost identical amounts of light straight through them. What changes the total daylight reaching the room below is not the glass but the angle it sits at against the sky.

• A horizontal or near-horizontal rooflight sees the whole upper half of the sky at once, the brightest zone there is.
• A vertical wall window sees roughly the lower half of the sky, and loses much of that to the horizon line, where buildings, fences and trees tend to sit.
• A pitched roof window sits between the two, seeing more sky than a wall window but usually less than a flat rooflight of the same area, depending on the pitch.

This is why architects and daylight designers treat roof glazing as doing far more work per square metre than wall glazing, and it holds true whether the sky outside is bright blue or flat grey cloud, because it is about which part of the dome the glass can see, not how sunny the day is.

A wall window has to contend with everything standing between it and the sky at eye level: the roof overhang above it, a fence or hedge, a neighbouring house, even a parked van on a narrow street. On the flat horizons of the Fens to the east and north of Peterborough there is often little in the way at ground level, yet a wall window still only ever sees a slice of sky bounded by its own frame and whatever is opposite it. A rooflight set into a loft conversion or an extension roof, by contrast, usually has open sky above it and nothing between the glass and that sky at all.

Curtains, blinds and net curtains cut a wall window's daylight further, for privacy reasons that a roof rarely needs to answer to. A rooflight over a landing, a kitchen sink or a bed can often be left uncovered by day without anyone looking in, which is one more reason the same size of glass does more work overhead than it would in a wall.

Daylight designers use rules of thumb rather than a single fixed number, because every roof, room and outlook is different. Even so, the pattern is consistent enough to be useful when you are weighing up a rooflight against enlarging a wall window instead.

The practical upshot is that a modest rooflight can often do the daylight job of a much bigger wall window, which matters in a loft room where wall space is short and most of it is sloped ceiling rather than upright wall in the first place.

Because a rooflight is so much more effective per square metre, it is easy to go too big. A generously sized rooflight over a small room can swing from a welcome lift in daylight to noticeable glare and summer heat gain, especially on a south-facing slope, long before it swings the room dark. Sizing a rooflight is as much about controlling that surplus as it is about chasing more light: the glass specification, any blind, and the exact position in the ceiling all play a part, which our guide to orientation for north, south, east and west facing rooflights sets out in more detail.

The same logic works the other way when a room already has a reasonable wall window and just needs lifting rather than transforming. A single, well-placed rooflight, sized to the room rather than to the biggest opening the roof structure allows, usually does that with less glare and less heat than doubling up on a wall window would. Our page on upsizing a rooflight opening covers what changing the size of an existing unit involves, including what the roof structure has to give.

More daylight is not the only thing a rooflight brings to a room that a wall window cannot. Because it looks up rather than out, it changes the sense of the room itself, opening the ceiling to sky rather than adding another view of the garden. Our page on whether skylights make rooms feel bigger looks at that effect on its own terms, separate from the daylight numbers here.

The type of glass in the rooflight also has a say in how much of that daylight actually reaches the room. Clear double glazing lets through most of what falls on it; a heavier solar control coating, chosen to cut summer glare on a south slope, trims some light back in exchange for comfort. Our guide to the best glass for a skylight weighs those trade-offs against clarity and the view. Where the priority is the room below staying comfortable to look up into, not just brighter, the position of the glass relative to where you actually sit or lie matters as much as its size; see our guide to placing a rooflight over the bed for how that works in practice.

Flat rooflights, pitched roof windows, lanterns and tubular sun tunnels all sit at different angles to the sky and suit different rooms, roofs and budgets. Our full comparison of every type of rooflight and skylight sets the daylight, the view and the practical differences side by side, so you can weigh a rooflight against a bigger wall window, or against a different roof product entirely, with the same information in front of you. Every quote from us also comes with a sightline sketch, a drawing of the view from where you actually use the room, so daylight and the view it opens up can be judged together rather than as separate decisions.

If you are weighing a rooflight against other ways of lifting a dark room, our page on what affects the cost of a rooflight sets typical UK market ranges against the daylight and the view different products give, and our Peterborough rooflight and skylight installation page covers the full range we plan and fit. Every installation is carried out to current Building Regulations, with a 10-year workmanship guarantee, and where the work is notifiable we handle the Building Control notification.

Does a bigger rooflight always mean more daylight?

Up to a point. Doubling the glass area roughly doubles the daylight it lets in, but a rooflight that already lights a room well can be made too large before it lights it noticeably better, adding glare and heat instead. Matching the size to the room, not to the largest opening the roof allows, usually gives the better result.

Do north-facing rooflights let in less light than south-facing ones?

A north-facing rooflight sees less direct sun but still looks at a wide stretch of open sky, so the daylight it lets in stays fairly even through the day without the glare a south-facing slope can bring. It is a different kind of light rather than simply less of it, and our page on orientation sets out how each direction behaves.

Would two smaller rooflights work better than one large one?

Sometimes, particularly in a long or awkwardly shaped room, because two smaller openings spread the daylight and the view of the sky across more of the ceiling rather than concentrating both in one spot. Our guide on how many skylights a room needs works through when that split makes sense.

Does tinted or coated glass cut down how much light comes in?

Yes, by design in most cases. A solar control coating is there to reduce glare and summer heat, and it takes a small amount of visible light along with it. The trade-off is usually worthwhile on a bright, unshaded south-facing slope, and our guide to the best glass for a skylight sets out how much difference the common coatings make.