How much light does a rooflight actually give?

It sees the whole sky

A wall window sees roughly half the sky dome and half the ground. A rooflight sees the sky directly, and the sky is far brighter than the ground.

Nothing obstructs it

A window looks at a fence, a hedge, a house opposite. A rooflight has clear view unless there are overhanging trees.

It lights the middle of the room

Light from a window falls away rapidly with distance from it. Light from a rooflight lands where the rooflight is, which can be the centre of a deep room.

This is why one modest rooflight transforms a room that two windows leave dim.

How far the light reaches

Roughly under the unit and about a metre past its edge in each direction, then it falls away.

Which is why two smaller rooflights spread along a long room beat one large one in the middle. Distribution beats total area.

What reduces it

Deep reveals that are not splayed, which shade the edges of the opening.

Dark reveal colours, which absorb rather than reflect. Paint them white regardless of the walls.

Solar control glass on a north facing unit, where there is no gain to control and it simply reduces light.

Overhanging trees.

A working rule

Glazing of around 20 per cent of floor area is a common target for a habitable room, and a rooflight counts for more than its area toward that.

Two to three times a wall window of the same size

The usual figure quoted is that a rooflight admits around twice as much daylight as a vertical window of equal area, and in the right conditions rather more.

The reason is geometry. A wall window sees a hemisphere of which roughly half is ground and buildings, and it looks at the sky at a shallow angle. A rooflight faces the sky directly and sees the brightest part of it, the zenith, which on an overcast day is several times brighter than the horizon.

That last point matters in this country, because overcast is the normal condition. The advantage of a rooflight is greatest on exactly the days when you most want the light.

What the reveal takes back

The reveal. A rooflight in a thick roof sits at the end of a tunnel. A square boxed reveal throws a narrow beam straight down and blocks a surprising amount of what the window collects. A splayed reveal, cut back at an angle at head and sill, opens the beam out and can make a modest window feel considerably larger. It costs a little more in plastering and it is the best value decision available on any installation.

Pitch. A shallow roof gives a longer reveal and less headroom gain, so the same unit delivers less usable light than it would in a steep slate roof.

Glass specification. Solar control glass reduces light transmission as well as heat. Triple glazing reduces it noticeably. Obscure glazing reduces it further. Each is a deliberate trade rather than a fault.

Dirt. A rooflight faces the sky and collects everything that falls out of it. On a pitched unit rain does most of the cleaning; on a near flat one it does very little.

How much you need

Glazed area around 15 to 20 percent of floor area gives a bright room without turning it into a greenhouse. Below about 10 percent a room reads as adequately lit rather than bright; above 25 percent you are into overheating territory on any south or west facing slope.

Two rather than one

Consistently better in a room of any size. One large unit gives a bright pool with the corners dark by contrast; two spaced across the slope read as ambient daylight.

They also fail independently, and on a cut roof with a purlin two narrower units are frequently easier to fit than one wide one because each can sit clear of the timber without trimming.

What it feels like

Different from a wall window rather than simply brighter. Overhead light is even, it reaches the back of a room that a wall window leaves dark, and it changes through the day without the low glare that makes an east or west facing wall window difficult.

North facing rooflights give the best working light in any building: even, cool and glare free all day, which is why studios have always used them.