Ask how much light a skylight gives and the honest answer starts with a question: compared with what? A window in a wall and a window in a roof do the same job in different ways, and the difference is not mostly about size. It is about what each one can see.
A wall window sees whatever stands in front of it. A fence, the house next door, a hedge, the eaves of the extension. The glass may be large and clean, and the room can still feel dim, because most of what the window faces is not sky.
A rooflight faces up. Overhead, nothing blocks the view, which is why we say straight up is clear. This guide explains what that means in practice, what you can change to get more from it, and where it stops helping. If you are still deciding between types, the Vertex Skylights Milton Keynes home page shows the full range of skylights and rooflights.
Why the view decides the light
Daylight in a room is not just the glass. It is the glass multiplied by what the glass looks at. A pane that faces open sky lets in the light of the sky. A pane that faces a brick wall lets in a little of the light bounced from that wall.
Stand at your side window and look out. How much of the view is sky, and how much is something else? In a street of houses, the answer is often a thin strip of sky at the top of the frame. The rest is fence, wall, car, tree or neighbour. That is the light the window has to work with.
Now picture lying on the floor and looking at the ceiling. If there were a window there, nothing would be in the way. The roof has no neighbour above it. A rooflight borrows that unobstructed view, and brings it into a room that has none of its own.
Why the sky overhead gives good light
The sky is not equally bright all over. Under a cloudy sky, the part overhead is brighter than the part near the horizon, so a window that faces up receives more of the brighter part. A window facing sideways takes in a band of sky nearer the horizon, and often part of that is hidden anyway.
There is a second effect. Light from above arrives at a steep angle and travels deep into the room before it meets a surface. Light from a side window arrives at a shallow angle and is spent on the wall nearest the glass. That is why a deep room can have a bright window end and a dark far end, and why a rooflight over the dark end can balance it.
We do not put a number on any of this, because it depends on your sky, your roof and your room. What we can say is the direction of the effect: the more of the window's view that is open sky, the more light it brings.
What sets the amount of light in your room
Six things decide how much daylight a rooflight delivers. Some you choose, some you inherit.
- Size of the opening. A larger opening admits more. It also takes more of the roof structure, so it has to be checked against the rafters or the flat roof build-up.
- Number of units. Two small units spread light along a room better than one larger unit at one end.
- Position. Over the work surface, table or stairwell, the light lands where it is used.
- Orientation. A roof slope facing south receives direct sun for more of the day. A north slope gives an even, cool light with little glare.
- Pitch. A steep roof window looks outwards as well as up. A flat rooflight looks straight up.
- Glass and finish. The type of glazing changes how much light passes and how much heat comes with it.
There is also the surface of the room. Pale ceilings and walls pass light on. Dark ones absorb it. A light shaft that finishes in a good pale reveal is part of the design, not an afterthought.
Which type lights a room in which way
The kinds of skylight and rooflight each distribute light in a different pattern. This table compares them side by side, so you can match the type to the room.
| Type | Light pattern | Suits |
|---|---|---|
| Roof window | A pool of light below the slope | Lofts, bedrooms, landings |
| Flat rooflight | Light straight down on one area | Kitchens, hallways, bathrooms |
| Roof lantern | Light from above and all four sides | Extensions, open-plan rooms |
| Sun tunnel | A small bright spot at the ceiling | Windowless bathrooms and landings |
| Walk-on rooflight | Light down through a floor | Basements and extensions |
Size and number: more is not always better
It is tempting to treat the biggest rooflight as the best one. A room that is evenly lit usually has the light spread through it, not stacked in a single shaft.
Long, narrow rooms, such as a galley kitchen or a hallway, often work better with a row of smaller units than with one big one. Wide rooms, such as a rear extension, suit a roof lantern or a large flat unit over the middle, because it floods the centre and lets the walls bounce the light outwards.
The roof itself limits what is possible. Rafters, trimmers and the bearing points of the structure all affect the size and position of the opening. If a position is wrong for the roof, we say so at the survey and show the nearest one that works. For the practical side of that, see which roofs can take a skylight.
Orientation: sunlight, glare and heat
The direction the glass faces changes the character of the light as much as the quantity.
A south-facing slope catches direct sun, which brightens a room and warms it. Used well that is a pleasure, in a cold kitchen on a bright winter morning. In summer it can be too much, and the answer is a blind, a solar control coating or a different position. A north-facing slope gives steady, soft light all day, which suits a studio or a home office, and brings little direct heat.
East and west slopes bring sun at the beginning or the end of the day, with low-angle light that can reach deep into the room. If glare on a screen is a worry, say so at the survey. A rooflight's placement can be chosen to keep the strongest light off the desk, and a blind can take care of the rest.
Rooms where overhead light makes the difference
Some rooms gain more from a view of the sky than others, because the walls around them give the side windows nothing to work with.
A kitchen extension built out towards a boundary often has a fence a metre from the glass. The side doors let in a little, and the middle of the room stays flat. Over the island, a roof lantern, or else a flat rooflight, changes that, because the light arrives from above and the worktop is no longer in shadow.
A loft room has the opposite problem. Its only wall windows may sit in a gable, with the sloping ceiling on either side blocking the view. A roof window in the slope lets the sky in right where the ceiling comes down. A landing or inner hallway with no outside wall at all can borrow daylight from a sun tunnel, which carries it down from the roof through a mirrored tube.
A bathroom is the quietest example. A small unit over the bath gives soft light without a window that faces a neighbour, which is a gain in privacy as well as in brightness. Each of these is a case of the same idea: if the wall cannot see the sky, the roof can.
Glass, coatings and what they do to the light
The glass is where daylight and heat meet. A clear double-glazed or triple-glazed unit passes a lot of light and keeps warmth in. A tinted or solar-control unit passes less heat and slightly less light. A self-cleaning coating makes no difference to the amount of light, and keeps it from dulling.
The regulations set a ceiling on heat loss, not a floor on brightness. Under Part L, Table 4.2 for existing dwellings, a roof window should achieve 1.4 W/m²K or WER band B, and a rooflight 2.2 W/m²K. Those limits are about keeping warmth in. Choose the glass inside them for the comfort of the room.
For a longer look at what the figures mean, see the hub on energy-efficient glazing.
Heat-loss limits that cap the glass
- 1.4 W/m²K
- roof window limit, Part L Table 4.2, or WER band B
- 2.2 W/m²K
- rooflight limit, Part L Table 4.2, existing dwellings
No legal minimum for daylight in a house
There is no Building Regulations daylight requirement for houses in England. Nothing in the regulations says a kitchen must have a certain amount of light, or that a rooflight must reach a threshold. What the regulations do cover is how the unit is built, how it keeps the rain out and how it keeps heat in.
That leaves the brightness of your room to your judgement, which is the right place for it. You decide what the room is for and how bright it should feel. The survey is where we turn that into a position and a size.
If the work needs planning permission, or sits in a conservation area, that is a separate question. The planning guide explains the routes.
Keeping the light you have
A rooflight earns its light over years, so a few habits protect it. Glass on a pitched roof tends to rinse itself in rain. A flat unit holds leaves and dust and benefits from a wipe now and then. A blown or misted double-glazed unit loses clarity as the seal fails and moisture gets between the panes, which is one reason to read up on when a skylight needs replacing.
Trees can grow into the view, too. A rooflight beneath an overhanging branch still lights the room, but the sky it sees is reduced. It is worth knowing before you place one.
Planning the light before the survey
You do not need drawings to start. Walk the room at the time of day it matters most and notice where it is dark. Note which wall is nearest the sky, which furniture will stay and where you stand to work.
At the free survey we measure the roof, check the structure and talk through the options. The written quote you receive carries a fixed figure, and that same figure stands on the day. Fitting a typical rooflight is usually one or two days, with the inside finished tidily. The work carries a 10-year workmanship guarantee in writing, and is installed to current Building Regulations. If you want to start the conversation, WhatsApp 07880 284133 or email info@vertexskylightsmiltonkeynes.co.uk.
Before you book
- Which part of the room is darkest?
- What does the nearest window look at?
- Which way does the roof face?
- Do you work, cook or read under this spot?
- Is glare on a screen a worry?
Light, answered briefly
Does a rooflight give more light than a window?
It gives a different kind. A rooflight faces open sky, so its light is not blocked by a fence or a neighbour. A side window can match it only where its view is clear.
Will a skylight make my room too hot?
It can in a south-facing roof in summer. Solar control glass, a blind or a different position keeps it comfortable. Say what the room is used for at the survey.
Is there a minimum amount of daylight the regulations require?
No. There is no Building Regulations daylight requirement for houses in England. The regulations cover heat loss, structure and weathering.
