How We Plan the Light: Rooflight Light Planning

Why we plan the light before the roof is cut

Rooflight light planning means deciding what the room should feel like at different hours and seasons, then choosing the position, size, glass and shading that produce it. We do that work first, on paper, before a single tile or membrane comes off.

A skylight is a hole in the roof that the sun will shine through for the next twenty or thirty years. Once the rafters are trimmed or the flat roof deck is cut, moving it is a second building job. The size can be argued about, the frame colour can be changed on a drawing in seconds, but the opening itself is fixed the moment the saw goes in. That is why the planning belongs at the start.

The disappointments people describe after a rooflight goes in are rarely about the fitting. They are about the light. A loft bedroom on a south slope that is too warm to sleep in by June. A roof lantern that sits over the dining table while the worktop stays dim. A roof window that lights the ceiling and the top of a wardrobe while the desk underneath is in shade. A bright stripe that crosses the television every afternoon in March. None of these are faults in the product, and every one of them could have been seen coming with a sun path, a tape measure and an hour in the room.

Cambridge makes the planning more important, not less. The land is flat, the sky is wide and there are very few hills or tall ridges to shade a roof. A rooflight here sees almost the whole sky and the whole arc of the sun, so orientation decides more than it would in a valley town. The terraces of Romsey, Petersfield, Mill Road and Chesterton have long, deep rear rooms whose best daylight can only come from above. The new streets at Eddington, Trumpington Meadows and Great Kneighton already have generous windows, so any extra roof glazing has to add light without adding a summer heat problem.

How the sun moves over a Cambridge roof

At Cambridge's latitude of about 52 degrees north, the midday sun climbs to roughly 61 degrees above the horizon at midsummer and only about 14 degrees at midwinter. That single swing explains almost everything a rooflight does through the year.

1. 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.
2. Spring and autumn equinox, about 38°. The light lands on the floor, deep in the room.
3. Summer solstice, about 61°. The highest, strongest sun drops steeply and lands close under the window.
4. South-facing roof window. The same opening lets in a different patch of light every month.
5. 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.

In June the sun rises well to the north of east, arcs high across the south and sets well to the north of west. Days run to more than sixteen hours, and a roof of any orientation catches sun at some point. In December the sun rises south of east, stays low across the southern sky and sets south of west after less than eight hours. At the equinoxes in March and September it rises due east, sets due west and reaches about 38 degrees at noon.

Two practical consequences follow. First, a flat rooflight faces straight up at the sky, so in summer it receives high, strong sun for most of the day, and in winter it receives very little direct sun at all, just the soft light of the sky. Second, a pitched roof window tilts toward one part of the sky. A south slope pitched at 40 degrees is almost square on to the midday sun in spring and autumn, which is why it can feel so warm on a clear April afternoon when the air outside is still cool.

Clouds matter too. East Anglia is one of the drier and sunnier parts of England, but a grey sky is still the normal condition for much of the winter. On an overcast day the brightest part of the sky is overhead, so a flat rooflight or a steep roof window facing the zenith gathers more light than a vertical window of the same glass area. That overhead advantage is the whole reason to light a deep room from the roof.

What each roof slope gives the room below

The direction a slope faces sets the character of the light: its warmth, its timing and how much control it will need. No direction is wrong, but each one asks for different glass and a different plan.

1. North slope. Soft, steady light all day with no direct sun: even, glare-free and good for studios, kitchens and home offices.
2. East slope. Bright morning sun that has moved off by lunchtime: bedrooms and breakfast rooms.
3. South slope. The strongest light, peaking at midday. Worth pairing with solar-control glass or a blind to keep summer heat down.
4. West slope. Warm, low evening sun: living rooms and kitchens used after work.
5. The sun’s path. It rises in the east, is due south at noon and sets in the west, higher in summer and lower in winter.

North. A north slope rarely sees direct sun except early and late on summer days. The light is even, cool in tone and free of glare, which is why painters and architects have always liked it. It suits a home office, a studio, a loft room used for work, or a kitchen where you want steady light on the worktop without a hot patch. The trade off is quantity: the room will be less bright than under a south slope, so the opening often needs to be a little larger, or there need to be two.

East. An east slope takes direct morning sun, low and warm in colour, and then settles into sky light from late morning onward. It is a good choice over a bedroom for anyone who wakes with the day, and over a breakfast area. The low morning sun can reach deep into a room, so think about where it will land at 7am in May before placing a bed or a mirror.

South. A south slope gives the most light and the most heat. In the middle of the day the sun is near square on to the glass. That is welcome in February and can be too much in July. A south skylight nearly always needs solar control glass, a blind, and a way of letting warm air out.

West. A west slope brings afternoon and evening sun, which arrives when the house has already warmed up and the sun is low enough to shine straight into eyes and screens. West light is lovely in a sitting room on a winter afternoon and demanding in a bedroom on a summer evening. Glare control deserves as much attention here as heat.

A flat roof has no orientation of its own, but the room below does. A flat rooflight or a roof lantern still lets in low morning and evening sun at an angle, and the position of the opening decides which walls and surfaces that sun will reach.

Where the light lands, not just where it enters

The glass is on the roof, but you live with the patch of sunlight on the floor, the wall and the worktop. Predicting that patch is simple geometry, and it changes the position of the opening more often than any other part of the plan.

Take a flat rooflight in a kitchen extension with a 2.4 metre ceiling. At midsummer noon the sun is high enough that the bright patch lands about 1.3 metres to the north of the point directly beneath the opening. At the equinox it moves to about 3 metres away. In midwinter the direct sun, when it appears, reaches the floor nearly 10 metres away, which in practice means it lands high on the north wall or not at all. The same opening lights the floor in summer and the far wall in winter. If that far wall is a run of glossy units, a painting or a television, you need to know now.

The depth of the opening shapes the light as well. A flat rooflight sits on an upstand of at least 150 millimetres above the finished roof, and the ceiling, insulation and joists add more depth underneath. A deep, straight sided shaft trims off low sun and makes the light look like it is coming down a tube. Splaying the reveals, so the ceiling opening is wider than the glass, spreads the light across the room and makes the edge of the opening less harsh against a bright sky. Painting the reveals a pale, matt colour helps the light bounce down instead of being soaked up.

For a pitched roof window, height matters. A window placed higher on the slope throws light deeper into the room and onto the far wall. A window placed lower gives a view out and more light near the eaves, where a bed or desk often sits in a loft. Two smaller windows side by side usually spread light more evenly than one large one. In a terrace loft with a steep rear dormer and a shallow front slope, the plan often has to balance the light from a dormer window against the light from a conservation rooflight on the front.

Choosing the glass for the slope

Glass is where the light plan turns into a specification. The same opening can feel cool and calm or bright and hot depending on the glass in it, so we choose the unit for the orientation and the room, not from a default list.

Three numbers do most of the work. The U-value measures heat lost through the unit in winter. Under Part L of the Building Regulations, a new or replacement rooflight in an existing home needs a U-value of 2.2 W/m²K or better, now measured with the unit lying flat, which is a harder test than the old vertical rating. The g-value measures how much of the sun's energy passes through the glass. The light transmittance measures how much visible light gets through. Good solar control glass cuts the g-value much further than it cuts the light, and that gap is what you are paying for.

• Low emissivity coatings keep winter heat in the room and are standard on any decent unit.
• Solar control coatings reflect a large share of the sun's heat. They make the biggest difference on south and west slopes and on flat rooflights under the high summer sun.
• Triple glazing lowers heat loss and reduces cold downdraughts under large lanterns, at the cost of weight and a little light.
• A laminated inner pane holds together if the glass is broken, which matters for any glazing above where people stand, and is needed under Part K for overhead glass.
• Tints such as neutral, grey or blue change the colour of the light as well as the heat. A blue tint that looks crisp on a showroom sample can make a warm kitchen feel cold.

Our page on energy efficient rooflights goes into the specification in more detail. As a working principle, a north slope can usually take clear, high light glass, an east slope benefits from moderate solar control, and a south or west slope or a large flat lantern should start from strong solar control and work back only if the room needs more light.

Controlling glare and summer heat

Heat and glare are separate problems with separate answers. Heat is the sun's energy building up in the room. Glare is a bright source against a darker background in your line of sight. A plan has to deal with both.

Solar gain works in two steps. Sunlight passes through the glass, lands on the floor, the worktop and the furniture, and warms them. Those surfaces then warm the air. The most effective defence stops the sun before it reaches the glass. An external awning blind on a roof window intercepts much of the heat outside, and on a south facing loft bedroom it can be the difference between a room you can sleep in and one you cannot. Internal blinds work well for glare and privacy, and they help with heat when paired with low g-value glass, but on their own they leave the heat inside the room.

Ventilation is the other half. Warm air rises and pools at the ceiling, which is exactly where a rooflight sits. An opening rooflight lets that air out at the highest point, and Part F allows an opening unit to count toward purge ventilation when its free area is declared. For flat roofs and high ceilings, where a handle is out of reach, an electric opening rooflight with a rain sensor closes itself when the first drops land, so it can be left open on a warm day with the house empty.

Glare depends on where people look. A west roof window above a sofa facing a television will create glare every sunny evening between spring and autumn. Moving the window along the slope by a rafter bay, or turning the furniture, often solves it without changing the glass. Over a desk, the aim is light from the side or behind, never a bright pane directly in front of the screen.

Planning the light room by room

The use of the room decides what good light means. A kitchen, a loft bedroom and a windowless landing need three different answers, even on the same house.

Kitchens and side returns. In a terrace side return, the long, narrow extension along the back of the house, the dark end is usually the part joined to the old back room. We position the glass to reach that end, often with a run of flat rooflights or a slim lantern along the party wall side, rather than putting all the glass over the garden end, which already has the doors. A common rule of thumb sizes a roof lantern at 15 to 20 percent of the flat roof area, but the position counts for more than the percentage. Over the worktop you want even light with no hard sun patch cutting across the chopping board.

Loft bedrooms. Light for a bedroom is about the morning and the night. An east slope welcomes the sunrise. A south slope needs blackout and heat control. Any bedroom roof window benefits from a blackout blind, and a high placed window gives light without a view straight in from neighbours. Our roof window installation page covers opening types, including escape windows.

Bathrooms. Top light gives privacy that a wall window cannot. The glass needs to handle steam, so an opening unit or good mechanical extraction is part of the plan.

Home offices. Steady light matters more than bright light. North or east light, or a solar control unit on a south slope with a blind, keeps the screen readable from nine to five.

Landings, hallways and inner rooms. Where there is no direct run from roof to ceiling, a sun tunnel carries daylight down a reflective tube to a diffuser. The light is soft and even, with no sun patch at all, which suits a stairwell or a windowless bathroom.

Light, planning rules and Cambridge houses

The best place for the light and the easiest place to get permission do not always match. A good plan finds the position that satisfies both, before anyone draws the final scheme.

Adding a rooflight to a house is usually permitted development under Class C, provided it projects no more than 150 millimetres from the roof plane and sits no higher than the highest part of the roof. That still applies in a conservation area unless an Article 4 direction has removed it, and flats and listed buildings follow stricter routes. Greater Cambridge Shared Planning handles applications for both Cambridge City and South Cambridgeshire, and the city alone has 18 conservation areas. Where the front slope of a terrace is in view, a flush fitting conservation rooflight with a slim profile is the usual accepted pattern.

That can create a genuine trade off. The rear slope of a Mill Road terrace may face north, and the front slope south onto the street. The planning answer and the light answer can point in opposite directions. In those cases we show the options side by side: what a larger rear opening with clear glass gives, and what a smaller front conservation unit with solar control gives, so the choice is made with both in view. Building Regulations approval is normally needed for new rooflights, and we handle the Building Control notification where the work is notifiable.

What goes into your written light plan

Every quote we give comes with a short written light plan. It records what we intend to install and why, so you can check the thinking before agreeing to anything.

The plan starts at the survey. We look at the slope or flat roof, take the orientation with a compass, note anything that will shade the glass, such as a neighbouring gable, a chimney or a tall tree, and measure the room and the structure. We ask how you use the room and at what times. Then we write it down:

1. Position. Where the opening goes, and where the light will land at key times of the day and year.
2. Size. The unit size and the ceiling opening, including any splayed reveals.
3. Glass. The glazing specification for that slope, with the reason for it.
4. Shading and ventilation. Blinds, opening method and any sensors.
5. Price. The written quote for the work, agreed before the roof is opened.

Once the plan is agreed we fit to current Building Regulations, make good inside and back the installation work with our 10-year workmanship guarantee. If you are replacing a tired or misted unit, the same plan applies: a replacement is a chance to upgrade the glass and shading for the slope it sits on. For what drives the budget, see our costs guide, and for more detail by topic, our rooflight and skylight guides.

When you are ready, book a light survey and we will stand in the room with you and show you where the light will land.

Questions people ask

Is a north facing rooflight worth having?

Yes, if you want steady, glare free light. A north skylight gives soft light all day with very little heat gain, which suits kitchens, offices and studios. It gives less light than a south unit of the same size, so it is often sized up or paired.

Does solar control glass make the room darker?

A little. Good solar control glass blocks much more of the sun's heat than its visible light, so the room stays bright while running cooler. Strong tints reduce light more, which is why we match the glass to the slope.

Can you plan the light for an extension that is not built yet?

Yes. We can work from the drawings and the site orientation, mark where the light will fall, and suggest where the rooflight or lantern should sit before the roof structure is finalised. That is the cheapest time to get it right. Book a light survey or message us on WhatsApp.