How many sun tunnels does a room need?

A tube's diameter is matched to floor area first: a smaller tube for a landing or utility room, a mid-size tube for a bedroom or home office, a larger tube for a kitchen or living room. That match is covered in full on our sizes and diameters guide, and it settles whether one tube is enough before shape or layout come into the decision at all.

For a simple square or rectangular room, two tunnels of a smaller diameter rarely beat one tunnel of the right size. Light does fall off toward the edges of a room lit from a single point in the ceiling, but a correctly sized tube still lights the whole floor well enough to work, read or cook by. Splitting an ordinary room into two tunnels means two domes on the roof, two runs through the loft and two ceiling fittings, for a result close to what one properly sized tube gives on its own. See how bright a sun tunnel makes a room for how output scales with diameter.

An open-plan kitchen-diner is a common case for a second tunnel, not because the floor area is unusually large but because the room has two jobs: a counter and hob at one end, a table at the other. One tube placed between the two ends lights neither well from directly overhead. Two tubes, one roughly over each working area, give each end its own patch of daylight rather than a single pool in the middle that neither zone quite sits under.

The same logic applies to a long, narrow landing that also serves as a home office nook, or a galley kitchen that runs the depth of a Fletton-brick terrace. A single tube in the centre of a long run lights the middle and leaves both ends comparatively dim. Two tubes spaced along the run, rather than one oversized tube in the middle, generally reads as more even from the floor.

A room already lit from one side by a window may only need a tunnel over the end furthest from it, not one sized to cover the whole floor. This is the situation in hallways and landings with a small window at one end and a dark stretch beyond it, and in bathrooms and en-suites built into the middle of a house with no external wall of their own. In both cases the question is not how many tunnels the room could carry, but which part of it is actually dark.

The reverse is also true. A room with no window at all, such as a bathroom in the centre of a roof plan or a bedroom created inside a loft conversion, is judged purely on floor area and shape rather than on topping up an existing source. See can a sun tunnel light a windowless room for how that case is worked through.

The count on paper still has to fit the loft above the room. A straight run from a central ceiling point to the nearest clear patch of roof is the easiest tube to fit; a run that has to dog-leg around obstructions loses some light along the way. Two shorter, straighter runs to two separate points on the roof can end up simpler to fit, and brighter in use, than one long run bent around whatever sits in the way.

• Water tanks, rafters, purlins and hip ends narrow where a tube can go
• A valley or a party wall can rule out a run in one direction but not another
• A flat-roof extension ceiling often gives a much shorter, more direct run than a pitched roof above: see taking a sun tunnel up through a flat roof
• A bungalow with no floor above the ceiling gives the most direct routes of all, covered on bringing daylight to the middle of a bungalow

Loft routes are also why the answer to "how many" sometimes changes between the phone call and the survey. A room that looks straightforward from below can have a water tank, a soil stack or a structural beam sitting exactly where a second tube would need to pass.

None of the above replaces a look at the actual room and the loft above it. At a home visit, part of our sun tunnel installation service, we measure the floor, check where a run can go without crossing a tank or a beam, and work out whether one correctly sized tube does the job or two smaller ones serve the room better. The work is carried out to current Building Regulations, with any Building Control notification handled as part of the job, and it carries a 10-year workmanship guarantee whether the room ends up with one tunnel or two.

Book a home visit and we will look at the room, the loft and the roof before settling on a number.

Can one sun tunnel light two separate rooms?

No. Each sun tunnel runs from a single roof terminal to a single ceiling diffuser, so a second room needs its own tube and its own run through the loft, even if the two rooms sit close together.

Does a longer or wider tube ever replace a second tunnel?

Only within limits. A wider tube lights more floor area and a shorter, straighter run keeps more of that light, but neither changes the fact that a room split into two distinct areas is usually lit better by two tubes than by stretching one.

Do two tunnels in the same room need two separate diffusers?

Yes. Each tube ends in its own ceiling diffuser, so a room with two tunnels has two fittings visible in the ceiling, generally positioned over the two areas they are lighting.

Should two tunnels in one room match in size?

Not always. Where one end of the room needs more light than the other, for example a kitchen counter against a landing beyond a doorway, the two tubes can be different diameters to suit each area rather than matched for the sake of it.