Ridge orientation, and why it changes the light in the room
Two lanterns of the same size on the same extension, one with the ridge running north to south and one running east to west, do not light the room the same way. The difference is not subtle once you have lived under both, and it is decided at the ordering stage and never afterwards.
This page explains what ridge orientation actually changes, and how the decision gets made on a real roof.
A lantern is not one rooflight, it is four slopes
A flat rooflight has one plane, facing straight up, taking light from the whole sky more or less evenly. A lantern has four sloping planes plus a ridge, and each slope faces a different direction.
That is the whole of the mechanism. Each slope collects light from the part of the sky it faces, so the direction the ridge runs decides which slopes are large and which are small, and therefore which parts of the sky the room is drawing from.
The two long slopes do most of the work
On a rectangular lantern the two slopes either side of the ridge are the large ones. They face at right angles to the ridge line. If the ridge runs north to south, the big slopes face east and west. If the ridge runs east to west, the big slopes face north and south.
The two hipped ends are smaller
The ends contribute, and on a nearly square lantern they contribute a lot, but on a long narrow one they are a minor part of the total.
What each orientation gives you
| Ridge runs | Large slopes face | Character of the light | Peak |
|---|---|---|---|
| North to south | East and west | Two distinct events: morning on one side, evening on the other | Early and late |
| East to west | North and south | One steady north slope plus one strong south slope | Middle of the day |
| North east to south west | North west and south east | A long soft build through the day | Mid morning and mid afternoon |
Ridge north to south
The big slopes face east and west. The room gets direct morning sun through the east slope and direct evening sun through the west one, with a quieter middle of the day. In a kitchen used at breakfast and again in the evening, that timing suits how the room is actually occupied.
The catch is the west slope in summer. Low evening sun comes in at a shallow angle to a west facing slope and it is the hardest light of the day to shade, which is why a west facing lantern slope is specified as though it were south facing.
Ridge east to west
The big slopes face north and south. The north slope gives steady, even, indirect light all day, which is the light people mean when they talk about north light in a studio. The south slope gives the strong direct component, concentrated around the middle of the day.
This orientation gives the most even overall daylight and the strongest midday peak, and it puts the largest solar load onto a single slope, which makes the solar control specification more important rather than less.
Where the light lands in the room
Orientation changes the direction light arrives from, and that decides where the bright patch sits on the floor.
Sun through an east facing slope comes in at a low angle in the morning and throws a patch of light onto the west side of the room. Sun through a south facing slope at midday drops it almost directly below the lantern. Sun through a west facing slope in the evening throws it onto the east side, often right across a dining table or a run of worktop.
This is worth thinking about against the plan of the kitchen. A screen, a hob, a worktop where somebody chops, or a desk in a corner of the room are all places where a predictable patch of direct sun in the late afternoon is either welcome or a nuisance, and the answer is different in every household.
What actually decides it on a real roof
Most of the time this is not a free choice, and it is worth saying so plainly.
- The shape of the extension. A narrow back addition takes a lantern with the ridge running along its length, and that settles the orientation before anything else is considered. This is the usual case in the Victorian terraces here.
- The structure below. The joists run one way. An opening long in the direction of the span is very different work from one long across it.
- The fall of the roof and the outlet position. The lantern must not dam the flow.
- Planning and visibility. In a conservation area or inside the National Park, the profile seen from outside can decide the ridge direction on its own.
- Daylight. Where the first four leave a genuine choice, this is where the decision is made.
On a square or nearly square extension the choice is usually open, and that is where this conversation matters most.
Pitch interacts with orientation
A steeper slope faces more toward the horizon and less toward the top of the sky. That has two effects worth knowing.
A steeper south facing slope takes more low winter sun, which is useful, and less high summer sun, which is also useful. A shallower slope does the opposite: it behaves more like a flat rooflight, collecting from the whole sky fairly evenly and taking the summer sun at close to its most efficient angle.
So a steeper pitch on an east to west ridge gives a more seasonal room, brighter in winter relative to summer. A shallow pitch gives a flatter response through the year. Neither is right in the abstract.
How much daylight a lantern actually delivers
Orientation decides the character of the light. Area decides how much of it there is, and the two get conflated.
Overhead glazing is far more efficient than a window of the same area, because it faces the whole sky rather than a slice of it and because nothing outside is shading it. A modest lantern over a kitchen does more for the daylight in that room than a much larger window would.
That efficiency is why a lantern transforms an internal or single aspect room and why it is worth the structural work to install one. It is also why the solar control conversation exists at all, because the same efficiency operates in July.
The ends of the lantern
The hipped ends are the part nobody thinks about, and on a nearly square unit they matter.
On a long narrow lantern the ends are a small fraction of the glazed area and contribute correspondingly little. On a square one they are half the slopes, which means a square lantern draws from all four directions and is far less sensitive to ridge orientation than a long one.
Put the other way round: the longer and narrower the lantern, the more the ridge direction decides. If your extension takes a long unit, this page matters. If it takes a square one, the decision is close to neutral and you should settle it on structure and drainage instead.
Rooflights in the same room
A lantern is frequently not the only glazing in the room, and the others change the answer.
- Where the extension has full width doors facing the garden, the room already has a strong directional light from that side. A lantern with its big slopes at right angles to the doors balances the room rather than reinforcing one side.
- Where a flat rooflight already sits over another part of the plan, the lantern is adding character rather than raw quantity, and orientation is worth more attention.
- Where the room is genuinely single aspect, the lantern is the main source and evenness matters more than timing, which favours the east to west ridge.
Sussex light
There is a local point worth making. The light on this coast is frequently bright overcast rather than clear sun: a high thin cloud layer with the sea reflecting under it. Under those conditions the sky is close to uniformly bright and orientation matters far less, because every slope is collecting from much the same source.
That means the orientation decision is really a decision about the clear days, and about the two or three months either side of midsummer when the direct component dominates. It is a real decision, and it is not the only thing that decides whether the room feels bright.
Winter and summer are different rooms
Orientation changes with the season, because the path of the sun does.
In December the sun stays low and never gets far round to the north. A south facing slope takes almost all of the direct light there is, and east and west slopes take very little because the sun rises and sets well south of due east and west. An east to west ridge therefore gives a noticeably brighter room in midwinter.
In June the sun rises well north of east and sets well north of west, and it is high overhead in the middle of the day. Now every slope gets a turn, and the north slope that receives nothing direct in December takes early morning and late evening sun in midsummer.
If the room matters most to you in winter, favour the east to west ridge. If the problem you are solving is a dark room in the evenings in summer, the north to south ridge with its west facing slope does that better.
Artificial light under a lantern
A lantern changes how the room needs to be lit after dark, and it is worth planning at the same time rather than afterwards.
- The ceiling around the lantern is now the only ceiling there is, so downlights have less area to sit in and they end up close to the kerb.
- Light thrown up at the glass at night reflects back into the room and shows every mark on the inside of the panes.
- A pendant hung under the ridge reads well and gives the room a centre after dark, which a lantern removes during the day.
- Lighting inside the reveal of the kerb, washing up the sides, keeps the lantern reading as an opening at night rather than as a black hole.
The cable runs for all of that are decided before the ceiling closes, which is the same moment the lantern is being fitted.
Two lanterns instead of one
On a long room, two smaller lanterns spaced along the length give more even light than one large one in the middle, regardless of orientation, because the light source is distributed rather than concentrated. Both units will have the same ridge direction, which is set by the plan.
It costs more perimeter detailing and two openings rather than one. It is worth considering where the room is long enough that the ends read as dark under a single unit.
What we do
At survey we take the orientation of the extension, the position of the outlet, the direction of the joists and the plan of the room. Where the first three leave a genuine choice, we set out what each ridge direction will do to the light and let you decide with the actual room in front of you.
The ridge direction is fixed the day the unit is ordered, and it is the one decision on a lantern that cannot be revisited.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.