Why overhead glass still works when the sun is low
A rooflight looks straight up at the open sky, and in winter that open sky, not the sun, does most of the lighting work.
A wall window sees, at best, half the sky, and in a Cambridge terrace much of that half is taken up by the house across the garden, the neighbour's back addition and the fence. A skylight in the roof sees the whole dome above it. On a grey December day, when there is no sun to speak of, the sky directly overhead is brighter than the sky near the horizon, which is why daylight designers model overcast conditions that way. So the glass pointing up collects the best of what the winter sky offers, while the wall window is looking at the dimmest band of it and at other people's brickwork.
That is also why a rear room that feels gloomy by November, for example a kitchen at the back of a gault brick terrace off Mill Road, changes so much once a rooflight goes in. It is not borrowing sunshine. It is borrowing sky. For the wider picture of how a room gets its daylight and what it needs, see how much daylight a room needs.
What a Cambridge December actually gives you
Cambridge sits at roughly 52 degrees north, so around the winter solstice the noon sun climbs to only about 14 degrees and sets before 4pm.
The land here is flat, with no hills to cut the day short, so the sun you get is the sun the latitude allows. In late December that means sunrise after 8am, sunset before 4pm and a day a little under eight hours long. The sun rises well round to the south east, skims low across the southern sky and sets in the south west. It never gets high enough to pour straight down through a flat roof the way it does in June.
In practice a winter rooflight gives you two kinds of light. On grey days it brings in soft, even sky light that fills the room without shadows. On clear days, for a few hours around the middle of the day, it may also let in a shaft of low, warm sun that lands on a wall rather than the floor. How much of that direct sun you get depends almost entirely on which way the glass faces.
What each slope gives a room in December
With the sun at 14 degrees, the slope and pitch of the roof decide whether any direct winter sun reaches the glass at all.
The geometry is simple enough to picture. A roof window on a south slope pitched at 40 degrees tilts its glass towards the low sun, so the midday rays meet it at a generous angle and come through strongly. The same window on a north slope of that pitch has its glass tilted away from the sun, which stays below the plane of the glass all day, so it gets no direct sun in midwinter at all, only sky light. A flat rooflight sits in between: the sun reaches it, but at a shallow, glancing angle, so more of it bounces off the surface.
| Glass faces | December sun | What the room gets |
|---|---|---|
| South slope | Midday, strong | Warm sun patch high on walls |
| East or west | Brief, very low | Short morning or afternoon glow |
| North slope | None direct | Steady, cool sky light |
| Flat roof | Glancing | Even light, faint sun |
None of these leaves a room dark. A north-facing roof window in a loft bedroom still brings in far more light than a small gable window would; it simply brings it without the sun. The full comparison across all four seasons is in which way a skylight should face, and the way the sun patch travels across the floor and walls is covered in whether the sunlight from a skylight moves through the day.
- Winter solstice, about 14°. The lowest noon sun of the year travels furthest: it crosses the whole room and lands high on the back wall.
- Spring and autumn equinox, about 38°. The light lands on the floor, deep in the room.
- Summer solstice, about 61°. The highest, strongest sun drops steeply and lands close under the window.
- South-facing roof window. The same opening lets in a different patch of light every month.
- Deepest reach. In December the sun patch can be on the wall opposite, which is why we plan where it falls, not just how big the glass is.
Getting the most from a rooflight in winter
Winter light is the weakest light of the year, so small losses from glass, dirt, shafts and blinds matter more in December than they do in June.
- Keep the glass clean. Autumn leaves, grime and moss on a low-pitch or flat rooflight cut winter light noticeably. Routine care is set out in rooflight cleaning and care.
- Splay the shaft. In a loft or a thick roof, angling the lining outwards spreads the low light over more of the room instead of trapping it in a deep box.
- Choose glass for the whole year. Solar control coatings that tame a south slope in July also trim a little winter light, so the balance is set by orientation, not by habit. See energy efficient rooflights.
- Let the blind go right back. A blind that parks fully clear of the glass gives back every bit of the winter sky. Whether you need one at all is covered in does a skylight need a blind.
Position matters too. Where the rooflight sits decides where the light falls: over the worktop, the table or the sofa, rather than onto a corridor or a blank wall. For a room that is dark all year, skylights for dark rooms walks through finding the cause before choosing the unit, and where a full rooflight cannot go, a sun tunnel carries sky light down to a hallway or bathroom.
How we plan for the winter months
We look at the light in both seasons before we quote, because the rooflight that is right in July also has to earn its place in January.
At the survey we note which way each roof slope faces, its pitch and what stands to the south of it, then work out where the sun patch will land in midwinter as well as midsummer. The written light plan that comes with every quote sets out the position, size, glass and shading, with the reason for each, so you can see what the room will feel like on a short December afternoon before the roof is cut. You can read more about the method in how we plan the light, or go back to the overview of skylight and rooflight installation in Cambridge. When you are ready, book a light survey.
Questions people also ask
Do skylights work on cloudy days?
Yes. On an overcast day the light comes from the whole sky rather than the sun, and a rooflight faces the brightest part of that sky, directly overhead. The light is soft and even, with no hard shadows, which suits kitchens and workrooms.
Is a north-facing rooflight worth having in winter?
It is. It receives no direct sun in December, but it gives steady, cool sky light for the whole of the short day, with no glare. For a studio, a study or a loft bedroom that is often exactly what you want.
Do rooflights make a room cold in winter?
A modern double or triple glazed rooflight fitted to current Building Regulations limits heat loss, and insulating the reveal around the frame stops cold spots. On a clear, low-sun day, a south-facing unit can add a little warmth as well as light.
Does snow on a skylight block the light?
Thick snow on a flat or shallow rooflight will dim it until it slides off or melts, while steeper roof windows tend to shed it sooner. Snow lying on the surrounding roofs and gardens can brighten the room a little by reflecting more light back up to the sky.
