What a north slope sees of the sun
A north slope faces away from the midday sun, so the question is how far above the roof surface the sun actually climbs through the year. On a typical Cambridge roof pitched at 40 degrees, the answer changes a great deal between June and December.
At noon in Cambridge the sun stands about 61 degrees above the horizon at midsummer, about 38 degrees at the equinoxes in March and September, and only about 14 degrees at midwinter. Tilt the roof 40 degrees away from it and the picture looks like this.
| Time of year | Noon sun height | Sun and a 40° north slope |
|---|---|---|
| Midsummer | About 61° | About 21° above the roof plane |
| Equinox | About 38° | About level with the roof plane |
| Midwinter | About 14° | Below the roof plane |
So in summer the dome still receives direct sun at midday, striking it at a low, glancing angle. Around the equinoxes the midday sun skims along the plane of the tiles. In winter the roof itself stands between the dome and the noon sun. There is a useful twist in summer: the sun rises in the north east and sets in the north west, so early morning and evening light reaches a north slope at a better angle than the noon figure suggests. A north slope is not dark. It is a roof that trades direct sun for sky light for much of the year.
- 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.
Why the dome keeps working without direct sun
A sun tunnel runs on the brightness of the sky as a whole, and a 40 degree slope still looks up at a large part of it. That sky light is what fills the tube on a north roof for most of the year.
Direct sunshine is only one of the two sources reaching a roof. The other is diffuse light scattered across the whole sky, and it arrives from every direction the dome can see. A roof tilted 40 degrees to the north still faces a wide sweep of open sky overhead, and on the flat land around Cambridge there are rarely hills to cut into the lower part of it. The clear dome also sits above the tiles on its flashing, so it is not limited to the view straight out of the roof plane; its curved sides pick up light from above the ridge line and from the east and west.
That is also why the light from a north slope behaves differently in the room. There is less of the bright, moving sun spot that a south facing tube can throw on the diffuser at midday. Instead, the ceiling glows evenly and the level follows the sky through the day. Our page on how sun tunnels behave on cloudy days covers the same diffuse light from another angle, and how sun tunnels work follows it down the tube.
How the dome and kit compensate on a north roof
The parts of the kit that matter most on a north slope are the ones that decide how much light gets in at the top and how much survives the trip down. Getting those right does more than anything else to close the gap with a south slope.
- A raised, clear dome. A dome that stands well above the flashing catches low sun and side light that a flatter lens would miss. Some manufacturers add a reflector or prismatic insert inside the dome to redirect low-angle light down the tube; whether that helps depends on the product and the slope, and we check it against the survey rather than assume it.
- A rigid, highly reflective tube. Every bounce inside the tube loses a little light. On a roof that starts with less, a rigid mirror-finish tube is the stronger choice. See rigid vs flexible sun tunnels for the difference.
- A short, straight run. Fewer and gentler bends keep more of the light. Our guide to sun tunnel length and bends explains why.
- A wider diameter. A larger tube gathers more sky. Where a hall or landing would take a smaller tube on a south slope, stepping up a size is the simplest way to make up some of the difference on a north one. Choosing a sun tunnel diameter sets out the sizes.
When another position on the roof is the better choice
A north slope is a workable position, not always the best one. If the house offers a south, east or west slope, or a flat roof section, within reach of the room, it is worth comparing before the tiles come off.
Plenty of Cambridge homes give a choice. A terrace in Romsey or off Mill Road often has a main roof running across the house with a lower rear outrigger roof at right angles to it, so one of those slopes may face east or west while the other faces north or south. A 1930s semi with a hipped roof has three slopes to consider. A single storey rear extension with a flat roof sees the whole sky. Moving the dome to a sunnier slope can mean a longer run through the loft with a bend or two, and that loss has to be weighed against the gain at the top. That trade is the subject of our guide on where to put a sun tunnel on the roof.
Sometimes the north slope still wins. It may give the shortest, straightest route to the ceiling below, it may keep the dome off a street facing slope in a conservation area, or it may be the only slope above the room at all, as with a bungalow hallway tucked under the back of the roof. In those cases a well chosen kit on the north side can outperform a long, bent run from a sunnier one.
How we decide at the survey
We answer the north slope question on the real roof, not from a rule of thumb. The light survey looks at the slope, the pitch, what the dome would see and the route down to the ceiling.
We check which way each available slope faces and at what pitch, whether chimneys, dormers, trees or taller neighbouring buildings shade any of them, and how long and straight the tube would be from each position. We then look at how the room below is used and when. A landing used mostly in the morning, a bathroom used in the evening and a home office used all day each have different priorities. The written light plan with your quote sets out the position, diameter, tube type and diffuser we recommend, and why. It also says plainly what the light will be like on a north slope in winter, so the result matches what you expect. You can read more about the approach in how we plan the light, or go back to the main sun tunnel installation page. When you are ready, book a light survey.
Questions people also ask
Can a tubular skylight be fitted to a north facing roof with slates?
Yes. The orientation and the roof covering are separate questions. A slate roof takes a flashing made for slate, and the dome performs on a north slope in the same way as on tiles. Our answer on fitting a sun tunnel to a slate roof covers the flashing.
Is a north facing sun tunnel worth fitting in a windowless room?
For a room with no window at all, even the softer light of a north slope is a clear change from relying on a ceiling bulb during the day. The difference from a south slope is in the brightness at noon and in winter, not in whether the room gets daylight.
Will a north facing sun tunnel make the room cold?
No more than one on any other slope. Heat loss depends on the insulation around the tube in the loft and on the diffuser seal, not on which way the dome faces. A north slope does mean almost no summer heat gain through the tube.
Does a steeper north roof make things worse?
Yes. A steeper north slope tilts the dome further away from the sun, so it receives less direct sun at midday. A shallower pitch brings the sun back above the roof plane for more of the year, and a flat roof avoids the problem altogether.
