Every house has a room that never gets daylight. It might be the landing at the top of the stairs, a bathroom in the middle of the plan, a hall with a front door that has no glass, or a utility room tucked between other rooms. A roof window needs a clear route to the roof and a wall of rafters it can sit between. When there is a loft or a floor above, or a roof too far away, it cannot go in. A sun tunnel can. It is a slim tube, rigid or flexible, with a mirrored interior, that carries light from a small dome on the roof to a diffuser in the ceiling. This guide explains how that works, where it makes sense and where it does not, and what the roofer has to do to fit one without causing a leak.
How a sun tunnel works
The parts are simple. On the roof is a clear dome or flat glazed collector. Under it is a flashing and a base, which join the unit to the roof covering. Beneath that runs the tube, with a highly reflective inner surface that bounces light along its length by repeated reflection. At the bottom, in the ceiling of the room, a diffuser spreads the light, usually a frosted or prismatic panel that looks like a ceiling light fitting. Some models include a dimmer or shutter, and others include an electric light kit so the same fitting gives light after dark.
The light arrives as a soft glow, not a view. You cannot look out of a sun tunnel, and it does not provide ventilation, apart from variants with a fan unit. It is a daylight source and nothing else. In a room with no window at all, even that is a big improvement.
Rigid and flexible tubes
The tube comes in two forms. A rigid tube is a metal pipe, usually aluminium, with a polished or mirrored coating on the inside. It transmits more light because the surface is smooth and the straight run causes the least loss. A flexible tube is a ribbed, foil-lined duct, easier to route around joists and pipes but less efficient, since the ribs scatter light and bends cost more. For a short, straight run through a loft, rigid is the better choice. Where the route has to bend, a rigid elbow section or a flexible link may be needed. Every bend loses light, and a tube that snakes around the loft may disappoint. Most makers specify a maximum length and a maximum number of bends for each tube diameter.
Diameters range from small, suitable for a cupboard or a toilet, to large, enough for a hall or landing. A bigger tube carries more light, but needs more room in the loft and a bigger hole in the roof.
Where a sun tunnel suits
The best places are rooms below a pitched roof with a loft in between and no easy route for a window. Typical examples:
- A landing or stair: the dark space at the top of the stairs is a classic, since the tube can run through the loft above it.
- An internal bathroom or WC: a sun tunnel with a diffuser gives daylight to a windowless room. A model with a fan kit helps with extraction, though it does not replace the regulation extract needs.
- A hall or utility room: daylight where a door is solid and there are no side windows.
- A walk-in wardrobe or dressing room: it gives colour-true light and no glare.
- A corridor in an extension: where a flat roof lies directly overhead, a shorter tube is possible.
It suits less well where the roof above is a flat roof with a thick insulation build-up, or a roof facing north, where the light is weak. If the room is directly below a roof with no loft, a roof window or a flat rooflight is usually a better answer, as in flat roof rooflights. If you want a view and ventilation as well as light, choose a roof window, using roof window types as a guide.
How much light to expect
A sun tunnel is not a lamp, and its output varies with the weather and the time of day. On a bright day it will light a small room well. On a dull winter afternoon the glow is much dimmer, and in the evening it gives nothing, which is why many models offer an integrated light fitting. Light drops with the length of the tube, the number of bends and the diameter. A short, wide, straight tube gives the most. A long, narrow, bent one gives the least. Orientation helps too: a south-facing roof gives more than a north-facing one. A tunnel is best seen as a way to take the gloom off a room, not as a way to make it bright all day.
Glare and heat are rarely a problem, because the tube carries only light and a little heat. Some makers offer an insulated tube or a ceiling-level insulated collar to limit heat loss, because the tube passes through the cold loft and can feel cold on the room side. Condensation can form on the diffuser in a bathroom if warm moist air meets the cold tube, and a sealed diffuser and an insulated tube reduce that.
Fitting a sun tunnel: the roofing work
The roof end is where leaks happen, so it gets the roofer's care. The unit sits on a base, flashed into the covering, similarly to a roof window. The work goes in order. We choose a spot between two rafters, so none need cutting, and mark it from inside the loft. The tiles or slates are lifted around the position, and a hole is cut in the roof boarding or between the battens. The base is fixed to the battens or a small timber frame, and the flashing is dressed into the covering so water runs over it. The underlay is cut and sealed to the base. The dome is fitted and the tiles are laid back.
Flashings come in versions for plain tiles, profiled tiles and slate, and the kit must match the roof, as we explain for roof windows in roof window flashing kits. A flat-roof version has a kerb or a curb base and is waterproofed by the roof membrane turned up around it. On a slate roof, lead is often used for a neat result.
Inside, the tube is run through the loft from the base to the ceiling below. It is supported so it cannot sag, and insulation is placed around it if the loft is cold. At the ceiling a hole is cut in the plasterboard, a collar fitted, and the diffuser fixed in place. Dust sheets protect the room, and the job often takes part of a day. The roof and the loft have to be reachable, and wiring or pipes in the way may need to be moved by the right trade.
Regulations and permissions
A sun tunnel is a small roof opening, but it is still an opening in a roof. Planning rules for roof windows generally concern the size and projection of what sits on the slope. A sun tunnel dome is small and low, and most are modest. Because the position, the roof type and the property can change the answer, the safe step is to check with your council's planning team, and the principles are set out in our guide to planning permission for a skylight. In conservation areas and on listed buildings, ask first. For building control, the cut through the roof structure, the insulation and the fire protection where the tube passes through a ceiling can all matter, so ask your building control body whether the work is notifiable. We handle that side where it is.
Where the tube passes through a ceiling that forms part of a fire-resisting enclosure, such as the ceiling of a protected stair in a loft conversion, a fire-rated version or a fire collar may be needed. That is another reason to mention the project when you ask for a quote.
Cost and value
We have no sourced price for sun tunnels, so we do not quote a typical range. The cost depends on the diameter, the tube type and length, the roof covering and how much work is needed in the loft. Compare it with the alternatives: a roof window needs structure, a lining and more roofing work, while a sun tunnel needs one small opening and a short run of tube. For a dark landing or internal bathroom it is often the lowest-cost route to natural light. Your fixed quote follows the survey, once we have been into the loft and checked the route. For the price of a full window, our roof window cost page gives the published figures.
Choosing between a sun tunnel and a roof window
The decision comes down to what the room needs. If it needs daylight and nothing else, and the roof above is a pitched one with a loft, a sun tunnel does the job with the least disturbance. It needs a hole not much bigger than a dinner plate in the roof, no structural alteration, and little decorating. If the room also needs air, a view or an escape route, only a window will do. A bathroom can have both: a roof window over the bath for ventilation and light, or a tunnel for light with a separate fan for moisture. Where a room is under a loft that you plan to convert, a tunnel is a poor long-term answer, since the conversion will cut across its route. Think about what you plan to do with the loft in five years before you cut the roof.
A tunnel can also complement windows. A landing lit by a tunnel and rooms lit by windows get the benefit of both, and the two do not compete. Where the dark room is on the ground floor beneath a bedroom, a tunnel is not possible at all, because the tube cannot pass through a lived-in room.
Looking after one
Sun tunnels need little. Wipe the dome from a safe place when you can, clear moss and leaves from around the base, and look at the flashing once a year. A cracked dome should be replaced, since the plastic can discolour or craze with age. If the diffuser mists or drips, check for condensation and insulation. If a stain appears on the ceiling around the diffuser after rain, tell us, because a leak at a small flashing is cheap to fix when found early. Our guide on leaking roof windows sets out the usual causes, which apply to domes as well.
If you have a dark room and are wondering whether a sun tunnel, a roof window or nothing would suit, our roof windows page lists what we fit, and a call to 01256 213956 books a visit where we go into the loft and look at the options.
