The short answer
There is no single maximum length. Light loses a little at every reflection inside the tube, so each extra metre and each elbow leaves the diffuser dimmer, and the real limit is the point where the ceiling is no longer bright enough for the room's job. A short, straight drop over a landing or hall gives a bright disc. A long, bent run over a stairwell gives a faint one. We measure the route in your loft before we put a figure on it.
- Free survey, fixed written quote
- 10-year workmanship guarantee
What takes the light away along the tube
Inside the tube, a ray of daylight zigzags from wall to wall. The mirrored lining returns it at each bounce, but never all of it, so the more bounces a ray makes, the less arrives. Length adds bounces. So does a narrow tube, because the ray meets the wall more often.
Elbows cost more than straight metres. At a bend the light hits the lining at a steeper angle and loses a bigger share in a single reflection. A route with two elbows can leave the room darker than a longer route with none, which is why we count bends before we count metres.
The lining finish matters as well. A smooth, polished interior holds its brightness better than a ribbed one. If you want to compare the two types, our guide on rigid or flexible sun tunnels sets them side by side.
Tube width and dome size sit on the other side of the sum. A wider tube and a larger dome collect and carry more light for the same run, and they ask for a bigger opening in the roof and a wider gap between joists.
| Route over the room | Effect at the ceiling | What we usually do |
|---|---|---|
| Short and straight, roof directly above | Brightest disc the product gives | Fit as drawn |
| Long and straight | Softer light, still clearly daylight | Pick a wider tube if the loft allows |
| One gentle offset | Slight loss, usually fine for a landing or wardrobe | Keep the offset to one |
| Several elbows or a long flexible length | Dim, uneven light that disappoints in a hall | Move the diffuser or propose another product |
Where you measure the run from
The run is the path the light travels, from the underside of the dome to the face of the diffuser, following every offset. It is longer than the straight line between the two points, and it is not the same as the height of the house.
On a bungalow with a shallow loft over the hall, the run can be little more than the depth of the roof structure and the ceiling void. On a tall house with a steep roof, the same landing needs a tube that drops through a deep roof space and then the floor-to-ceiling gap of an attic or box room. Houses with a high ridge produce long runs without any bends at all.
A loft that has been floored for storage matters too. Boards that sit over the joists lift the walking level, and the tube has to pass up through them before it reaches the roof. We take that into account when we mark the route.
When the run is longer than one tube should carry
A stairwell under a high roof is the usual place this arises. The landing is a long way below the tiles, and a single tube from the dome would deliver a faint glow by the time it arrives.
- Move the diffuser towards the roof, so the tube is shorter. A landing light that sits nearer the stair head can work as well as one in the middle.
- Use a wider tube and a larger dome, if the loft has the room for them.
- Fit two tubes, each with a shorter run, over a long corridor.
- Choose a different product. A roof window over the stairwell, or a rooflight on a flat section of roof, may suit a very deep route better than any tube.
A rooms-in-the-roof layout, a dormer or a chalet bedroom for instance, is another case where we steer away from a tube. The route would cross living space, and a roof window gives a view as well as light.
Worth knowing
The loft hatch tells us little. The deciding measurements are taken between the dome position and the ceiling, so we climb up and trace the route.
What we measure at the survey
We start under the proposed diffuser position and work upwards with a torch and a tape. The things we write down are the joist spacing, any tank, pipe, cable or flue near the line, the thickness of the roof structure, and the covering on the slope. From those we draw the shortest route with the fewest bends, and we compare it with the second choice if the first fails.
Where a roof is shaded by a tall neighbouring gable or a tree, the dome collects less, so a long run under a shaded slope is the combination to avoid. We note which way the slope faces and what stands in front of it.
What you receive is a free survey and then a fixed written quote naming the tube type, the number of elbows and the diffuser position. If a tube will not give the brightness you want, we suggest what will. The sun tunnel cost guide lists what moves the price, and our short answer on whether sun tunnels work covers how bright to expect a typical room to feel. The main sun tunnels page has the wider picture, and you can book a survey or send photos of the loft to us on WhatsApp 07880 284133.
