Light
A roof light delivers far more daylight than a window of the same size
The amount of sky a window can see, not its area, is what determines how much light gets into a room.
Both approaches to daylight in a room work. What differs is what they cost you, and the cost is what this sets out.
The difference in one place
- A horizontal roof light sees the whole sky; a vertical window sees roughly half.
- Daylight falls off sharply with distance from the opening.
- Deep plans and dark rooms are usually a geometry problem.
It is about how much sky the opening sees
Daylight comes from the whole dome of the sky, not only from the sun, so an opening facing straight up receives from every direction. A vertical window sees at most half the sky, and less once buildings, fences and trees block the lower portion. This is why a modest roof light can transform a room that a much larger window would only brighten near the glass.
It is also why a ground floor window overlooked by a neighbouring wall performs so much worse than its size suggests.
Light falls off fast from the opening
Daylight levels drop sharply with distance into a room, which is why the back of a deep room feels gloomy even on a bright day. A useful rule of thumb is that a side-lit room gets genuinely usable daylight to roughly two to two and a half times the height of the window head. Raising the head of a window therefore does more for the depth of the room than widening it does.
This is the single most useful piece of daylight arithmetic for anyone extending or altering a house.
Extensions often make the original rooms darker
Building out from the back of a house puts a new roof over what used to be an external wall, so the original room now looks into a covered space. This is the most common unintended consequence of a rear extension and it is entirely predictable at design stage. Roof lights over the junction, or a glazed link, restore daylight to the original room and are much easier to include in the design than to add later.
Asking specifically what happens to the daylight in the existing rooms is a question many extension designs never get asked.
Surfaces multiply what comes in
Once daylight is in the room, it bounces, and the reflectance of the walls, ceiling and floor determines how far it travels. A white ceiling is the single most valuable reflective surface, because light arriving from a window hits it first and is redistributed deep into the room. A deep-coloured room is not a mistake, but it needs to be understood as a room that will absorb daylight rather than spread it.
A pale window reveal, in particular, reduces the harsh contrast between the bright opening and the darker wall around it.
Heat comes with the light
A horizontal roof light gains a great deal of solar heat in summer, and a room under one can become uncomfortably hot. Solar control glass, external blinds and openable roof lights all mitigate it, and external shading works far better than internal. Conversely, large glazed areas lose heat at night and in winter, so the glass specification matters in both directions.
The quieter point is that building regulations in many countries limit the proportion of glazing or require compensating performance, so check the rules locally before designing.
Where more glass is not the answer
A room that is dark because a neighbouring building blocks the sky will not be fixed by a bigger window facing that building. Sun tunnels and light pipes bring daylight into internal spaces such as landings and bathrooms with no external wall at all. Borrowing light through internal glazing — a glazed door, a fanlight, a screen — moves daylight between rooms without an external change.
Underneath it, for a genuinely landlocked room, good artificial lighting designed for the whole day is a more honest solution than a token opening.
Side by side
| Consideration | What it means in practice |
|---|---|
| It is about how much sky the opening sees | A horizontal roof light sees the whole sky; a vertical window sees roughly half. |
| Light falls off fast from the opening | Daylight falls off sharply with distance from the opening. |
| Extensions often make the original rooms darker | Deep plans and dark rooms are usually a geometry problem. |
The takeaway
Raise the head of the window, look at how much sky the opening sees, and check what an extension does to the rooms behind it.
The best rooms look like a decision, not a purchase.
Questions readers ask
How far into a room does daylight reach?
Usefully, to around twice the height of the top of the window, though this varies with orientation, obstructions and internal surfaces. Raising the window head is the most effective single change.
Do I need permission for a roof light?
It depends entirely on your country, your property type and whether it is protected or in a designated area. Check with your local authority before ordering anything.
