The gap between rafters sets the ceiling
A sun tunnel is sold by the width of its tube, and the sizes are a short list of round numbers. Makers publish their own ranges, so the exact choices depend on the make. The roof then narrows that list for you before you have expressed a preference.
The tube has to pass between two neighbouring rafters. The measurement that matters is the clear gap, which is the centre spacing minus the thickness of the timber. Take an example reading: rafters at 600 mm centres, each 50 mm thick, leave a clear gap of 550 mm. A 350 mm tube sits inside that with room either side, and the maker's instructions give the clearance the flashing and fixings need. Rafters at 400 mm centres, with the same timber, leave 350 mm. A tube of the same width would touch both sides, so it would be a tight fit at best.
We read the gap in the loft with a tape, at the exact spot where the tube will pass. Spacing varies across one roof, and older roofs can have timbers at irregular centres, so a single reading taken at the eaves tells you little about the gap higher up the slope.
| Rafter centres | Clear gap | Tube that sits inside |
|---|---|---|
| 400 mm | 350 mm | 250 mm |
| 450 mm | 400 mm | 250 mm, 350 mm with care |
| 600 mm | 550 mm | 350 mm |
Wider tubes pass more light
The diffuser in the ceiling is sized to the tube above it, so a wider tunnel gives a larger area of light in the room. The area of a circle grows with the square of its diameter, so a 350 mm tube has almost twice the cross-section of a 250 mm one: it collects from a larger dome and delivers through a larger opening.
That gain is real, but it is only part of the picture. The tube length and the number of bends take light away from every size. A short, straight run in a wide tube is the brightest arrangement. A long run with an offset in a narrow tube is the dimmest. The maker's data sheet for the model gives the performance figures, and we do not quote any that the sheet does not state. Our answer on tube length covers what a long run costs in light.
Same room, same roof: moving from a 250 mm to a 350 mm tube gives a tube with almost twice the cross-section.
Match the size to the room
The room under the diffuser matters as much as the roof above it. A narrow landing, a small cloakroom or a utility room has little floor to light, and a smaller tube can do the job. A deep hallway, a kitchen with no window or a bathroom with an internal wall suits the wider tube where the roof allows it.
- Landings and cloakrooms. A smaller diameter is often enough, and it fits between closely spaced rafters.
- Internal bathrooms. The wider size is worth asking for where the gap allows. Some ranges offer a ventilation kit at the diffuser, which can help ventilate the room alongside the extractor.
- Long hallways. Two smaller tunnels along the length can light the space more evenly than one large one, provided the loft has two clear gaps.
- Kitchens with a darker corner. The position of the diffuser matters here as much as its width, so we mark the ceiling point from below before the tube is set.
Where each room sits in the house, and which ones gain most, is covered in our guide to where sun tunnels work best.
What a larger tube adds to the job
The dome is larger on a wider tunnel, so the opening in the roof covering is larger as well. On a pitched roof that means more tiles or slates lifted and refitted around the flashing. On a flat roof it means a bigger base on the upstand, with the roof covering turned up around it.
Inside, the ceiling hole grows with the diffuser, and the loft insulation has to be laid back around a wider tube. None of this is complicated, but it is more work than a small tunnel, and the quote reflects it.
If the wider size does not fit the gap, there are two choices. A smaller tube goes in without any structural change. Or a rafter is trimmed to make room, which is a structural alteration to the roof timbers and needs Building Regulations approval. We would raise that at the survey so you can weigh it before anything is ordered. On a flat roof the question is the same: our answer on flat roof sun tunnels looks at the joists instead of the rafters.
What we record at the survey
Choosing a diameter rests on readings taken in the loft and on the roof: the clear gap between rafters, the length and bends of the tube run, and the pitch under the dome. Each goes on your survey sheet, with the room it will light.
From those, the quote names one diameter, one flashing for your roof covering and one diffuser. If the roof only allows the smaller size, the sheet shows the gap that decided it, so you can see the reason for yourself. Our sun tunnel installation page sets out the full process from survey to fitting.
Tubular skylight size is quoted in inches by some makers and in millimetres by others; a 10 inch tube is about 250 mm and a 14 inch tube about 350 mm. The quote uses millimetres throughout, to match the tape in the loft.
faq
diameter questions
Can I fit a wide tube if my rafters are close together?
Only if a rafter is trimmed, which is structural work needing Building Regulations approval. The alternative is a narrower tube, or two small tunnels in separate gaps.
Is a 350 mm tunnel twice as bright as a 250 mm one?
Its tube has almost twice the cross-section, but the light in the room also depends on tube length, bends and the dome. The model's data sheet gives the maker's figures.
Does the diameter change the sun tunnel's heat loss?
A wider tube has a larger dome and diffuser, so more surface to lose heat through. The dome skins and the diffuser lens matter more than the width alone.