How much light does a rooflight actually give?
Roughly two to three times as much as a vertical window of the same glazed area, and in a room with obstruction outside the wall it can be more. That figure surprises people, and it is the single most useful number in the whole subject, because it explains why a modest rooflight fixes a room that two large windows never did.
The reason is geometry rather than glass. It is not that rooflight glass is better. It is that a horizontal opening is pointed at a far larger area of sky, and the sky is where daylight actually comes from.
Why the sky does the work, not the sun
Daylight design in this country is based on the overcast sky, not on sunshine. Under cloud the whole dome glows, and the brightest part of it is directly overhead, roughly three times brighter at the zenith than at the horizon.
A vertical window faces sideways. It sees half the sky at best, and the half it sees is the dim half near the horizon. A rooflight in a flat roof faces straight up at the brightest part. So even before any obstruction is taken into account, the same area of glass laid flat is collecting from a better part of the sky than the same glass stood upright.
Obstruction, and what a rooflight removes
Then the real world intervenes. A ground floor window in an Essex terrace looks at a fence at the end of a short garden, the neighbouring house, or a mature tree. Each of those takes a further bite out of the sky the window can see.
A window facing a boundary six metres away with a two metre fence has lost a substantial part of its already limited view of the sky. A rooflight in the same room has lost nothing, because nothing stands above the roof. This is why the multiple is not a fixed number: in an open aspect it might be two, and in a shaded side return between two terraces it can be four or five.
Daylight factor, and the numbers used in design
The measure used in practice is the daylight factor, which is the illuminance at a point inside expressed as a percentage of the unobstructed illuminance outside at the same moment.
| Average daylight factor | How the room reads |
|---|---|
| Under 2% | Electric light needed for most of the day |
| 2% to 5% | Daylit, though supplementary light is used at times |
| Over 5% | Genuinely bright, electric light rarely wanted by day |
The useful thing about the figure is that it is a ratio, so it holds in January as well as in June. A room at 5% is at 5% on a dull February morning. What changes with the season is the absolute number of lux outside, which ranges from around five thousand under heavy winter cloud to well over twenty thousand on a bright summer day.
Glazed area against floor area
The rule of thumb is glazing area of around fifteen to twenty per cent of the floor area of the room below. For a four by five metre kitchen extension that is three to four square metres of rooflight, which typically produces something comfortably in the daylit band.
It breaks in two directions. In a deep room with one external wall it is too little, because the far corner is still in shadow and the light needs to travel further into the plan. In a small south facing room it is too much, and you have built a greenhouse. Which is why orientation belongs in the conversation before area does.
What the roof build-up takes back
The number on the data sheet is for the glass. What reaches the room is less, and the difference is the shaft between the glass and the ceiling.
A warm flat roof build-up can be three hundred to four hundred millimetres thick once insulation, deck and finishes are counted. A pitched roof with a lined shaft down to a bungalow ceiling can be a metre. Every millimetre of that depth is a wall the light has to get past, and a deep square shaft acts like a lamp shade turned the wrong way. Two rooflights of the same size in the same roof can differ by a wide margin in delivered light purely on how the opening is lined.
Splayed reveals, and the cheapest gain available
A square reveal, with the lining running vertically straight up to the glass, throws a hard edged rectangle onto the floor and leaves the surrounding ceiling dark.
A splayed reveal, opening outwards towards the room, spreads light across the ceiling and roughly doubles the apparent size of the opening. On a deep build-up the difference is not subtle. Splaying the sides that face into the room matters most, and a matt white finish on the reveal returns considerably more light than a mid tone. It costs a little plastering and nothing at all in glass. The detail is on the reveals page.
Orientation, and the character of what arrives
Quantity is not the only question. The compass bearing of the plane changes what the light is like.
North gives steady, cool, shadowless light all day with almost no solar gain, and it will take a generous area with neutral glass. East is bright early and calm by afternoon. South collects strong midday and afternoon gain straight down onto the glass and wants solar control with the area disciplined. West gives low hot late sun that is the hardest of all to manage. A flat rooflight sees the whole sky and takes the peak summer gain overhead, which is why solar control glass is almost always the right call on a flat roof.
One large unit against two small ones
For the same total glazed area, two smaller units spaced apart usually light a room better than one large one, because the pools of light overlap and the room reads evenly rather than having a bright zone and two dark corners.
One large unit gives a stronger architectural effect and a bigger view of sky, which in a kitchen with an island underneath is often the point. The compromise depends on the room shape. In a long narrow galley, two or three units in a line is the better lighting answer. In a square room where the glass is the feature, one is. On a pitched slope the choice is frequently made for you by the structure, since a trussed roof caps the width at around five hundred and fifty millimetres between trusses. The wider picture of specifying and installing all of this sits on rooflight and skylight installation in Chelmsford.
What it actually feels like in the room
Numbers only go so far. The change people describe after a rooflight goes in is less about brightness and more about the room having a relationship with the weather again.
The light moves through the day instead of sitting flat. Cloud shadow crosses the ceiling. The colour shifts towards evening. A room lit only from one wall never does any of that, because it is looking at a fence. Sunspire installs roof windows, flat rooflights, lanterns and sun tunnels across Chelmsford and thirty miles of Essex around it, with the Building Control notification handled as part of the job.
Related questions
- Do rooflights need planning permission?
- Do I need building regulations approval for a rooflight?
- Who notifies building control about a rooflight?
- How much does a rooflight cost?
- Do rooflights add value to a house?
- Is a rooflight cheaper than a dormer?
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