Plenty of houses have a room that never sees daylight: a landing in the middle of the plan, a bathroom on an inside wall, a hallway that runs between two doors. A roof window would help, but it needs a wide opening, a clear run of ceiling beneath it and a room directly under the slope. A sun tunnel, sometimes called a tubular skylight or light tube, solves the same problem in a different way. A small dome on the roof collects daylight, a reflective tube carries it down through the loft, and a diffuser in the ceiling spreads it into the room. The opening in the roof is far smaller than any window, which is why sun tunnels suit places where a conventional rooflight will not fit. They are not a window, though. They give light, not a view, and what they deliver depends on the length and shape of the tube. They work in a simple way, do well in some rooms and badly in others, and ask a few specific things of the roof they pass through.
How a sun tunnel works, part by part
A sun tunnel has three visible parts and one hidden one. On the roof sits a clear dome, usually acrylic, which gathers daylight from the sky and the sun. Beneath it is a base that fixes the dome to the roof and seals the opening. Between roof and ceiling runs the tube, its inner surface coated or lined with a highly reflective finish, so that light bounces down it with little loss. At the bottom, in the ceiling of the room, a diffuser panel spreads the light across the space and hides the tube.
The hidden part is the weatherproofing. On a tiled or slated roof, the base has to be fitted into the covering so that water runs around it and not under it. That is done with a flashing, a collar of lead or purpose-made metal that overlaps the tiles or slates on the upslope side and tucks underneath on the downslope side. On a flat roof, the base sits on a short raised kerb with the roof membrane turned up its sides, just as it is around any flat-roof glazing.
Because the dome is small, the roof opening is modest and the work stays compact. The work inside is mostly cutting a round hole in the ceiling, sleeving the tube through the loft and fitting the diffuser. There is no need to alter rafters in most cases, because the opening is small enough to sit between them. If the line of the tube does cross a rafter, the rafter can sometimes be avoided by shifting the position a little, and on a bigger opening a trimmer is added. Our guide to roof window sizes covers the rafter question in more detail.
Many models offer extras. A light kit puts an electric lamp in the diffuser so the room can be lit at night with the same fitting. A ventilation or extractor kit can be added to some bathroom versions. Check what is compatible before you buy, because the choice of kit affects the diffuser and the tube.
Rigid or flexible tube, and why the route matters
Tubes come in two broad kinds. A rigid tube is made of straight reflective sections, sometimes with angled elbows. It carries light well, because the surface is smooth and the path is simple. A flexible tube is a ribbed, concertina-like pipe that can bend around obstacles. It is quicker to fit in a cluttered loft, though its folds scatter more of the light and it works best when it is kept as short and taut as it can be.
Every metre of tube and every bend takes a little away from the daylight that arrives at the diffuser. That is why the route is the most important design choice. A straight, short drop from dome to ceiling gives the brightest result. A long tube, or one with several bends, gives a softer one. We therefore look at the loft first: where the joists and rafters run, whether a water tank or a chimney breast is in the way, and how much insulation lies between roof and ceiling.
Where the loft is full of obstacles, the usual answer is to move the dome rather than to bend the tube. A gentle offset is acceptable, but a tube that wanders down from one side of the roof to the other tends to disappoint. Some manufacturers publish performance tables for different tube lengths and diameters. Ask for them, and ask the supplier which diameter suits the size of the room. A wider tube carries more light, and a hall that is long and narrow may need a different arrangement from a square bathroom.
One further point is the finish at the ceiling. A diffuser that is slightly domed or prismatic spreads light more evenly than a flat one, and a white ceiling around it helps the room feel bright. A dark ceiling around the opening will swallow part of the effect.
Where sun tunnels do well, and where they do not
Sun tunnels earn their place in rooms where a window is impossible. A landing between two bedrooms, a stairwell in the middle of the house, a bathroom or cloakroom with no outside wall, a walk-in wardrobe or a utility room can all be lifted by one. The light is bright in the daytime and changes with the weather, which makes the room feel connected to the outdoors even without a view.
They work on pitched roofs and on flat ones. On a pitched roof the dome sits proud of the covering, and on a house where the loft is used only for storage, the route down is straightforward. On a flat roof, a short tube through a shallow roof build-up can be just as simple.
Their limits are plain. A sun tunnel does not give you a view, it does not open to let in fresh air unless a ventilation kit has been added, and it does not brighten a room as much as a large window. In a big kitchen or living room, one tube is rarely enough. A roof window, a lantern or several tunnels are the answer there, and the larger options are set out in our page on lanterns and flat rooflights.
They are also not suitable for a room directly under a loft that has been converted. Once a loft is lived in, the tube would run through the loft room itself. For that kind of space, a roof window is the right tool, and the planning and size guides apply.
The quality of the light also depends on the sky. On a heavily overcast day a tunnel delivers a gentle, even glow. On a bright day it can be strong, and some people find a hallway under direct sun a little glaring. A diffuser with a dimmer lamp or a shade can help, and positioning the dome on a slope that does not face due south reduces the peaks.
The roof, the loft and the details that stop trouble
Fitting a sun tunnel is a small job, but it touches three layers of the house: the roof covering, the loft space and the ceiling. Each has a detail that matters.
The roof covering
On slate or tile the flashing has to be integrated with the courses. The covering around the opening is lifted, the base is fixed to the battens, and the lead or metal is dressed over the courses below and beneath the courses above. Cutting a tile or slate to fit is normal, and any broken pieces are replaced. The finish should look as if it belongs, with no gaps for wind-driven rain.
The loft
The tube passes through the insulation layer, and any gap around it is a path for warm air. The tube should be sleeved in insulation, and the junction at the ceiling sealed so that moist air from below cannot reach a cold tube. If it can, condensation forms inside the tube and runs down to the diffuser. In a bathroom, that matters more, because the room supplies the moisture. A sealed diffuser and an insulated tube are the two main defences.
The ceiling
A neat ceiling cut and a firm diffuser frame prevent draughts and keep the room looking tidy. The cut should avoid electrics and pipes above the ceiling, and we check the loft first to be sure.
Thermal performance also counts. The dome is a form of rooflight, and Approved Document L (2021 edition, amended 2023) sets 2.2 W/m²K in Table 4.2 for rooflights fitted to existing dwellings. How building control treats a sun tunnel is a matter for them, so get the supplier's declared U-value for the entire assembly and put the question to your building control body. Where the work is notifiable, that paperwork is ours to handle.
Consent, listed buildings and conservation areas
A sun tunnel looks small, but a dome on a roof slope is still a change to the roof, and whether it needs permission depends on the house and the slope. We do not restate permitted development rules here. Our pages on dormer planning permission and roof window planning explain where to check, and a word with B&NES planning settles the answer for your address.
On a listed building the position is stricter. Any new opening in a historic roof is an alteration, and it needs listed building consent from the council before work starts. Doing it without consent is a criminal offence, and the council's own advice is to talk to it before anything is booked. Because a sun tunnel's dome is small and can be put on a rear slope, it is sometimes a gentler way to bring light into a dark interior than a dormer or a roof window. How the council treats roof alterations is covered in our page on historic dormers.
Inside a conservation area, what counts is how the change reads from public places. A low dome on a rear slope is far less noticeable than a roof window on the front. Some domes are made in a low-profile style, and some come with a flat, glass-like cover instead of a bubble, which sits closer to the slate or tile and is less obtrusive from the street. If your house is in a visible position, the choice of dome shape is worth discussing with the council.
Bats and nesting birds use lofts, and both are protected. Should the loft hold either, we pause and tell you how to proceed.
Cost, fitting and choosing between a tunnel and a roof window
We do not have a sourced typical price for sun tunnels, so we do not quote one. What we can say is what moves the price. The size of the tube, its length, the roof covering, the type of dome, any light or ventilation kit, the loft layout and the access all change the total. A flat roof, a tiled roof and a slate roof each need a different base and flashing. Scaffolding is needed if the roof cannot be reached safely from a ladder or tower.
For comparison, MyBuilder's roof window cost guide (2026) gives a typical UK range of £350 to £600 for installing a standard roof window, with the window itself on top, and about £500 for scaffold. A sun tunnel is generally a smaller job than a roof window, because there is less to cut and less to finish inside. It has no glazing to make good and no interior lining to build. If you are weighing the two, compare like with like by asking each contractor for a written scope that names the product, the tube, the flashing, the ceiling work and the scaffold.
The choice between a tunnel and a window comes down to what the room needs. If you want a view, ventilation or a lot of light in a habitable room, choose a roof window. If you want to light a landing, hall or bathroom that cannot take a window, choose a sun tunnel. If the space is bigger than one tunnel can light, consider two or more, or a larger rooflight.
A site visit settles the practical questions. The visit covers the slope from outside, the loft for tracing the route, and the ceiling below, with photographs of each. You then get a written fixed price, and our own work carries a 10-year written guarantee. For the full range of rooflights and windows, the skylights, roofs and windows hub lists them all.