The part of a rooflight nobody sees in the brochure
Every skylight has two openings, one in the roof covering and one in the ceiling, and the shaft is the short tunnel between them.
The trade has several names for it. On a pitched roof window it is usually called the lining or the reveal. On a flat roof it may be called a light well or the upstand and reveal. Where a rooflight sits over a first floor room with an unconverted loft between, it becomes a proper shaft that can be a metre or more deep, framed in timber and boarded on four sides.
Depth is set by the building, not the product. A flat roof over a new extension carries joists, insulation, a deck and the upstand the rooflight sits on, so even a modest roof gives a reveal several hundred millimetres deep once the plasterboard is on. Current Part L insulation levels have made those build ups thicker than they were on older extensions. In a pitched roof the lining runs the depth of the rafters plus any insulation below them. The deeper the shaft, the more it behaves like a tube, and the more its shape matters. The difference between a skylight, a rooflight and a roof window is set out in our comparison guide; the shaft question applies to all three.
What a straight shaft does to the sun in Cambridge
A straight shaft has vertical sides, and when the sun is low its beam strikes those sides before it can reach the ceiling opening.
Cambridge sits at about 52 degrees north. At midday the sun stands roughly 14 degrees above the horizon at the December solstice, 38 degrees at the March and September equinoxes and 61 degrees at the June solstice. As a beam drops down a shaft it also travels sideways, and the lower the sun, the further it travels for each millimetre of depth. Simple trigonometry gives the sideways travel for a beam meeting the opening square on at noon:
| Noon sun | Altitude | 400mm deep shaft | 1m deep shaft |
|---|---|---|---|
| December | 14 degrees | about 1.6m | about 4m |
| March, September | 38 degrees | about 0.5m | about 1.3m |
| June | 61 degrees | about 0.2m | about 0.55m |
Read that against the width of the glass. Over a 600mm wide opening, a 400mm straight shaft lets a band of direct sun reach the room from spring to autumn, but in December the whole beam lands on the shaft wall. With a 1m shaft over the same opening, direct noon sun reaches the room only around midsummer, and only as a narrow strip. The rest of the year the room gets sky light and whatever the walls bounce down. That is not a fault; it is geometry, and it is why the shaft is part of the light plan rather than an afterthought. Our answer on winter light covers the low sun in more detail.
Straight or splayed: how splaying spreads the light
A splayed shaft widens as it comes down, so the ceiling opening is larger than the rooflight and the light fans out instead of dropping in a column.
The geometry is easy to picture. Take a 600mm rooflight over a 400mm deep reveal. Straight, the ceiling hole is 600mm across. Splay both sides at 45 degrees and each side gains 400mm, so the ceiling hole becomes 1.4m across while the glass stays the same size. Three things follow. The sloping walls face the room, so light that strikes them is reflected down and outwards rather than across to the opposite wall. More of the floor and walls can see the glass directly. And the bright opening sits in a wider, graded frame, so the step from glass to ceiling is softer on the eye, which helps with glare.
On a pitched roof window, the usual guidance from manufacturers is to set the upper lining level and the lower lining plumb. The level top lets the high sky reach deeper into the room, and the plumb bottom stops the lower lining shading the floor directly beneath. Where a shaft rises through a loft, the whole shaft can be flared, though the width at the ceiling is limited by the joists and anything else in the loft.
Splaying is not always possible or wanted. It needs extra trimming at ceiling level, and on a flat roof extension the reveal depth is often too shallow to gain much. Where the ceiling is only a short way below the roof, a straight shaft with a good finish may be all the room needs. Where the loft is deep and cluttered with services, a sun tunnel can be the better way to carry light down, since its reflective tube is built for exactly that job.
Finishes: what the shaft is lined with
The lining decides how the light that hits the walls is passed on, so its colour and sheen are as much a light choice as a decorating one.
- Matt white plaster or paint. Reflects light evenly in all directions, keeps the colour of daylight neutral and hides the joins. The default for most rooflights and roof windows.
- Satin or gloss paint. Mirrors more of the beam in one direction, which can throw a hard bright line onto a wall or ceiling. Better kept for small areas.
- Timber boarding. Warm and handsome in an older cottage or a barn style extension, but a darker surface returns less light and tints it, so the room reads warmer and slightly dimmer.
- Manufacturer lining kits. Pre-finished white linings for roof windows that fit quickly and give a clean edge where they meet the frame.
Behind the finish, the shaft has to be insulated and sealed. A lining boarded straight onto cold timber can form condensation, and a gap at the frame junction becomes a cold spot. Our answer on insulating around a skylight explains what goes behind the plasterboard.
Where the shaft matters most in Cambridge homes
The shaft is most important where the roof is deep or the room below needs light spread across its whole width.
A flat roof over a side-return extension on a Romsey or Mill Road terrace usually gives a shallow reveal, so the rooflight position and the finish do most of the work; see flat roof rooflight installation. A loft room in a 1930s semi has a lining through deep rafters, where a level head and plumb sill make a clear difference to the roof window. A landing or bathroom under an unconverted loft needs a true shaft, and the choice between a flared shaft and a sun tunnel depends on the depth and what sits in the loft, as our answer on skylights in unconverted lofts explains. At the survey we measure the depth, sketch the shaft and show where the light will fall before anything is cut. The home page sets out how we plan skylight and rooflight installation across Cambridge, or you can book a light survey.
Questions people also ask
Does a deeper light shaft mean less light?
A deeper shaft lets less direct sun reach the room, especially in winter, because more of the beam lands on the walls. Sky light still comes through, and a light, matt, splayed lining returns a good share of what hits it.
Can an existing skylight shaft be splayed later?
Often, yes, when the rooflight is replaced or the ceiling is opened up. It means trimming the ceiling joists wider and relining the shaft, so it is simplest to decide at the same time as the new unit.
Should the light shaft be painted the same white as the ceiling?
Usually. Matching matt white keeps the shaft and ceiling reading as one surface and gives the most even spread of light. A contrasting colour draws the eye to the opening and makes the edge look sharper.
Is a light shaft the same as a sun tunnel?
No. A light shaft is a boxed, lined opening under a rooflight or roof window. A sun tunnel is a sealed reflective tube with a dome on the roof and a diffuser on the ceiling, designed for long runs where a lined shaft would be impractical.
