The short answer
There is no single maximum, because the limit depends on the tube's diameter, how reflective its inner surface is and how many bends it has to take. As a rule, a straight run of a few metres is where a sun tunnel works well, and past the length the maker publishes for that diameter it stops being worth fitting. Wider tubes can go further than narrow ones, and every elbow shortens the usable run.
The figure that matters is the one on the maker's data sheet for the exact model, and we check it against your roof before we quote.
Why length matters so much
A sun tunnel is a mirror-finished tube. Daylight enters at the dome, bounces down the inside of the tube and leaves through a diffuser in the ceiling. Each bounce loses a little of the light, because no reflective surface is perfect. A short tube means few bounces and most of the light arrives. A long one means many bounces, and a growing share is absorbed on the way.
The loss is gradual rather than sudden, so a tube does not stop working at some magic length. It simply gets dimmer. Past the maker's stated limit the diffuser glows rather than lights the room, and at that point a small roof window or a rooflight would give you more for the money.
What sets the limit for your house
Three things decide how far a tube can run, and they work together.
- Diameter
- A wide tube carries more light and loses a smaller share per metre, so it can run further. The catch is that it needs a larger round hole in the ceiling and a clear path between the rafters or joists.
- Bends
- A straight drop from roof to ceiling is the best case. A tube that has to angle round a chimney breast, a water tank or a purlin loses light at each change of direction. Makers usually count an elbow as the equivalent of some length of straight tube, so a route with two bends has a shorter allowance than a straight one.
- Tube type
- Rigid tubes have a smooth polished finish, and they keep more light over a given distance than flexible ones, which have a ridged, concertina wall. Flexible tubes are easier to route round obstacles, but they should be used for the shorter runs. Our guide to rigid or flexible sun tunnels sets the two out side by side.
Where long runs come from in Devon homes
The length of a tunnel is really the depth of the roof build plus any space between the roof and the ceiling. That varies a great deal around Exeter.
- A 1930s semi in Heavitree usually has a shallow pitched roof over a modest loft, so the tube from roof to landing or bathroom ceiling is short.
- A Georgian or Victorian terrace has a taller loft void and sometimes a second floor below it, so a tube to the top-floor landing is longer, though still workable.
- A cob cottage in the Culm or Exe valley may have thick walls, a deep thatch or heavy slate roof and irregular rafters, which affect where a tube can go as much as how far.
- A converted barn or a house with a high vaulted section can need a tube much longer than any maker recommends, and the sensible answer there is a different product.
A loft room changes things too. If the space between roof and ceiling has been converted, the tube may be replaced by a roof window, and our page on whether a sun tunnel can go through a loft deals with the cases where it can't.
What to do if your route is too long
If the distance is beyond the safe length for the diameter you want, there are three sensible routes.
The first is to move the position. A tube a metre or two to one side may give a shorter, straighter drop between the same two points, and the ceiling position can often move more easily than you would think.
The second is to step up the diameter, if the roof space and the ceiling will take it. That buys extra range without changing the route.
The third is to choose a different product. A small flat rooflight or a roof window delivers far more light over a deep roof build than a long tube. We lay out that trade in sun tunnel or rooflight: which brings more light. And if the room is a stairwell or landing and the ceiling is deep, the lining of a rooflight can be splayed to bring the light into the room, which a tube cannot do.
Insulation and length
A long tube passes through a longer stretch of cold roof void, and that has a second effect beyond light. Cold air around a tube can chill its walls, and warm moist air from the room then condenses on the cold surface. Tubes that run through an unheated loft are normally wrapped in insulation to prevent it. We cover the cause and the fix in do sun tunnels cause condensation.
The finish at the ceiling matters at any length. The diffuser sits in a ceiling that you look at every day, and where the plaster meets its ring, the edge should be clean and the paint line straight. Our finish schedule records how that edge is formed and decorated.
How we check the length at survey
We measure it on site, because plans of an old house are rarely accurate to the metre. At survey we look at:
- the position of the dome on the roof and the position of the diffuser in the ceiling;
- the true distance between them, measured along the line the tube will take;
- any rafters, ceiling joists, trusses, pipework, wiring or a cold water tank in the way;
- the roof covering, which decides the flashing and the way the dome is weathered;
- how much of the route is insulated loft and whether it is boarded for storage.
Then we compare the route with the maker's limit for the diameter you are considering. If it sits comfortably inside it, we say so. If it is marginal, we tell you what the diffuser will look like and offer the alternatives. A tube that is at the edge of its range will work, but it will give the room less light than you probably expect.
Fitting a tunnel is still building work in a roof, so it needs a roof opening cut and weathered correctly, and where Building Control is involved, we handle the notification. The planning position for the dome is covered in our sun tunnel installation page.
Next step
If you have a dark landing, bathroom or hallway and you want to know whether a tunnel can reach it, send us a photo of the room and the roof and ask for a quote. We will measure the route at survey and tell you the length we think it will run, and what the room will gain.