Sun Tunnel Sizes and Diameters

Most ranges sit at the narrow end around 250mm to 300mm across, move up to a mid-size band around 300mm to 350mm, and top out at a wide tunnel of around 450mm to 550mm. A narrow tunnel through a short, open loft can outperform a wide one dragged through a long, obstructed run, so diameter on its own never tells the whole story. This guide works through how the size is actually chosen. For how the tube itself is built and lined, see our guide to dome, tube and diffuser, and for the full range of fittings we cover, start at sun tunnel installation.

That is why the difference between the smallest and largest standard sizes feels much bigger in a finished room than the numbers on a spec sheet suggest. A narrow tunnel in a short, straight run can still throw a strong, usable light into a small space, while the widest tunnels are chosen less because a room needs quite that much daylight and more because the room is large enough, or far enough from the dome, that the extra collecting area is needed just to keep the light from thinning out along the way. Our answer on how bright a sun tunnel will make your room goes further into how diameter and run length combine.

A wide tunnel is not automatically the right choice for a large room, and a narrow one is not automatically wrong for a small one. What sits between the dome and the ceiling matters as much as what the room is used for, which the next few sections work through in turn.

A wardrobe or a small store room is used briefly, in passing, and mostly for finding things on a shelf, so a narrow tunnel that lifts the gloom without flooding the space is usually enough. A bedroom or a standard bathroom is occupied for longer and wants a more even wash of daylight across the whole ceiling, which is where the mid-size band earns its place. Our guide to sun tunnels in bathrooms and en-suites covers how that plays out against tiling, steam and fittings already on the ceiling, and our answer on whether a small sun tunnel will light a walk-in wardrobe covers the smallest end of the scale directly.

A hallway or a landing is a different case again. It is a room people pass through rather than sit in, but it is also often the darkest part of a house, lit at one end by a window and nowhere else, and a run of daylight along its length changes how the whole floor reads rather than just one spot on the ceiling. That is usually where the widest standard tunnels are put to work. Our guide to sun tunnels for hallways and landings covers how placement along the corridor is judged alongside size.

Two narrower runs also give more flexibility over where each dome sits on the roof, which matters where the loft above only offers a clear route in two separate places rather than one continuous line. Deciding between one wide tunnel and a pair of narrower ones is a judgement made on survey, weighing the shape of the room against what the roof and loft actually allow, and our answer on how many sun tunnels a room needs sets out how that call gets made.

A rigid section of the widest diameter needs more clearance to swing into position past a truss web, a water tank or a run of pipework than the same length in a narrower size, and where the loft is already tight, that clearance can be the detail that rules a size out rather than the room's own needs. A flexible section of the same wide diameter bends less easily than a narrow one, since more material has to curve around the same obstruction, so the trade between rigid and flexible tube shifts as diameter goes up. Our guide to rigid tube or flexible tube covers that trade in full, and our guide to sun tunnel routes through lofts covers how a route is planned around what a particular loft actually contains.

Insulation depth plays its own part. A wider tube needs a wider gap boxed clear through the loft insulation along its whole length, so heat does not bridge across a larger surface than a narrow tunnel would present. None of this rules a wide tunnel out where the room genuinely needs one; it simply means the size and the route are chosen together on survey, not the size first and the route worked out afterwards.

On a plain ceiling with open joist spacing, that is rarely a problem. It becomes a genuine planning question where the ceiling already carries downlights, a smoke detector or a loft hatch nearby, since a wide diffuser needs more clear plasterboard around it than a narrow one to avoid crowding an existing fixture. Getting the ceiling finish right around the opening, so the flange sits flush rather than proud and the edge is made good cleanly, is as much a plastering detail as a glazing one. Our guide to sun tunnel diffusers and ceiling finishes covers the finish options for each size.

On a concrete or clay tile roof, a larger flashing plate usually sits within the existing lap of the tiles without much extra disruption, since the courses above and below simply need to clear a bigger footprint. On the stone and slate roofs found around Stamford and the villages to the west, where Collyweston stone slate is common on older cottages, a wider flashing has to be set against thinner, more brittle slates that space differently to a concrete tile, and the widest domes are not always the easiest fit on that kind of covering. Our guide to sun tunnel brands and flashings covers how flashing kits differ by size and by roof covering, and a flat roof brings its own separate upstand detail regardless of the diameter chosen above it.

A home visit walks the room that needs daylight, walks the loft above it with a torch and a tape rather than guessing from below, and checks the roof covering the dome will sit on, before any size is confirmed. That is the same order of work behind every fitting we cover, whether the job in hand is a sun tunnel or one of the other nine rooflight and skylight services we install and replace across Peterborough and 30 miles around; our guide to what affects the cost of a rooflight sets out the general factors that apply across all of them. Whichever diameter ends up being right for your room, it carries the same 10-year workmanship guarantee and an install to current Building Regulations as everything else we fit.

Should I just choose the widest tunnel available, to be safe?

Not as a rule. A wider tunnel means a bigger ceiling cut, a bigger flashing footprint on the roof, and more clearance needed through the loft, none of which helps a small room that only needed a modest lift in daylight. The right size is the one that matches the room and the route, not the largest one on the price list.

Does a wider tunnel always need a longer survey to plan?

Not longer, but more careful, since a wide rigid section has less room to manoeuvre past an obstruction than a narrow one. Where the loft is tight, that can point the job towards a flexible section, a different route, or in some cases two narrower tunnels instead of one wide one.

Can I fit a narrow tunnel now and upgrade to a wider one later?

Only by treating it as a new job. The dome, flashing, tube and ceiling opening are all sized to one diameter, so moving up a size later means reopening the roof and the ceiling rather than simply swapping a part.

Is there a practical largest size for an ordinary house roof?

In practice yes, set less by the roof structure than by how much light a domestic room can actually use and how large a flashing plate a given roof covering can take cleanly. Beyond the widest standard band, a second tunnel usually serves a room better than a single oversized one.