A sun tunnel carries useful daylight a surprisingly long way down a straight run, but every extra metre and every bend takes some of the light with it. There is no single honest figure, because the limit comes from the data sheet of the exact tube, the size of the dome and how much sky it sees. We work out the run at survey, measure the actual route from the roof to the ceiling, and tell you what the chosen product can manage for that room.
Where the light is lost
A tubular skylight collects light in a dome on the roof, sends it down a reflective tube and spreads it through a diffuser in the ceiling. Light is lost in three places. The dome and the diffuser each cut a little. The tube walls are mirrored but not perfect, so each bounce costs a small share. And the longer the tube, the more bounces the light makes before it arrives.
A short, straight tube between a roof and a landing ceiling behaves very differently from a long one that crosses a deep loft. In the first case you get a bright circle of daylight that reads almost like a ceiling lamp. In the second the room still gets daylight, but it is softer, and on a dull Sheffield day it will look dim.
Straight beats bent
Bends do more harm than length. A rigid tube with a smooth mirror finish loses little along a straight run. A flexible tube has corrugated walls, so light scatters more, and if you pull it round an elbow the loss climbs quickly. Our page on rigid or flexible sun tunnels sets out which to pick for which route.
So we start from the straightest path through the roof space. Where a rafter, a purlin or a water tank blocks the line, we would rather shift the dome position along the slope than add a kink. Two angled rigid elbows can work, but each one costs light, and we say so before we fit them.
| Factor | Effect on light |
|---|---|
| Wider tube | More light collected and delivered |
| Longer run | Less light arrives at the ceiling |
| Each bend | A noticeable drop, more with flexible tube |
| Roof facing | South and west slopes feed the dome best |
| Shaded dome | Trees or a taller neighbouring roof cut the supply |
Width counts as much as length
A wider tube carries more light than a narrow one over the same distance, which is why a long run on a narrow tube feels so much weaker. If the loft is deep, a wider tube is often the better answer than a longer or more angled one. The limit then becomes space: the tube must fit between the rafters, or the rafters must be trimmed. Trimming a rafter is a structural change that Part A covers and approval must precede, so we flag it at the survey rather than discover it in the loft.
Rooms where length is the real question
Typical places for a tunnel in a Sheffield house are windowless landings, inner hallways in 1930s semis and bathrooms in terraces where an extension sealed off the old window. In a two-storey house the tube may have to pass through a bedroom or roof void on its way down, which is where length starts to bite. A top-floor landing or a bathroom under a shallow roof is easier. If you are comparing it with a window, read our page on choosing between a tunnel and a window on a landing.
Insulation, condensation and a cold loft
The tube passes through the loft, which is cold. Warm air from the room can rise up an uninsulated tube, meet its cold wall and condense, and the water then runs down inside the tube or stains the ceiling. A longer tube has more cold surface, so length makes this worse. We insulate the tube through the loft space and seal the ceiling end against air leaks. Ask whether an insulated version suits your loft before you pick the product.
Roof type and collar
On a pitched roof, the dome sits in a flashing that is shaped to your tile or slate. On a flat roof, it sits on a short upstand collar. Either way the dome position is part of the reach calculation, because a dome low on a shaded slope will gather less light than one higher on a clear south face. We check the orientation and the nearby trees on the day.
The life plan for a sun tunnel
The dome is the part that weathers, and the diffuser seal is what lets dust into the room if it ages. Our life plan names both and says how each is replaced without lifting the roof: the dome comes out from above and the diffuser from the ceiling. The tube itself stays in place. Year 5 is when we check the dome and the ceiling seal.
Paperwork and guarantee
Where the opening is notifiable, for example because a rafter is trimmed, scheme members can self-certify; otherwise Building Control must be told, and we take care of that. That certificate is what a later buyer expects to find in the file. The guarantee runs for 10 years and is tied to the work that hides behind the ceiling: flashing, collar and tube joints.
Planning is rarely a problem for a low dome, and our page on the planning position for a sun tunnel explains why. If a bathroom is the target, see fitting one in a bathroom.
The free survey ends with a fixed written quote that names the tube, its route and the light to expect. Read more on our sun tunnels page.