Ceiling positions and where the tunnel can land

What constrains it

Structure. Rafters, joists, purlins, binders and trussed rafter webs. The tunnel passes between them, not through them, and nothing structural is cut for a sun tunnel.

Services. Water tanks, pipework, cables, ductwork and downlights all live in the space between roof and ceiling.

The roof position. The collector needs a clear area of covering, away from valleys, hips, chimneys and other units.

Prioritise the straight run

This is the useful principle. A straight drop delivers materially more light than a bent one, so it is usually worth moving the diffuser a metre to get one.

A tunnel landing slightly off-centre in a hallway but running straight beats a centred one with two elbows, every time.

Where diffusers usually land well

Hallways and landings: centrally along the length, or above the point people move through.

Bathrooms: over the middle of the room rather than directly over the shower, so it is not fighting steam and is easier to reach for cleaning.

Walk-in wardrobes and utility rooms: anywhere clear, because these are small.

Where they do not

Directly above a door, where the light does least. Tight into a corner, where half the output lands on a wall. Or immediately beside an existing ceiling fitting, which looks unresolved.

Deciding on site

We look in the loft and in the room before committing. Five minutes with a torch settles what a plan cannot, and moving the position at that stage costs nothing.

The position is a negotiation between two ends

A sun tunnel has a roof end and a ceiling end, and both have constraints. The best installations come from being flexible about at least one of them, because the single biggest determinant of how much light arrives is whether the tube runs straight.

Every bend costs more light than a metre of straight tube. A route with two bends is worth reconsidering, and moving the ceiling position half a metre is frequently a better trade than accepting a dogleg to keep the diffuser centred.

In most rooms the diffuser does not need to be dead centre, and nobody notices that it is not.

What is in the way at the roof end

You need a clear patch of roof, structurally free between rafters, with open sky above it.

Rafter spacing. A tunnel is sized to fit between rafters without cutting one. On modern trussed rafters at 600mm centres that limits the diameter, which is one reason 350mm is the common size. On an older cut roof, spacings are irregular and the clear span at the intended position has to be measured rather than assumed.

Purlins. In a traditional cut roof a purlin runs horizontally partway up the slope and it cannot be cut. A tunnel goes above or below it.

Obstruction above. A chimney, a tree canopy or a taller neighbouring building shading the dome position costs more than orientation does. Stand where the dome would go and look up: if you can see a clear patch of open sky, it will work.

What is in the way at the ceiling end

Ceiling joists. The diffuser opening has to sit between them, which sets a grid of possible positions rather than a free choice.

Services. Lights, extract fans, sprinklers, ductwork and pipework all compete for the same ceiling, and in a small bathroom the ceiling is already busy. A tunnel diffuser is a fixed diameter disc and it needs its position agreeing with everything else.

Where a light fitting would have to move to accommodate it, consider the optional light inside the diffuser instead, so one opening does both jobs.

Ceiling type. Plasterboard is straightforward. Lath and plaster needs more care because the plaster keys through the laths and can crack away beyond the cut. Artex cuts fine and is harder to make good invisibly, and Artex of a certain age may contain asbestos, which is tested rather than guessed at.

What is in the way in between

The loft, and it is busier than people remember.

Cold water tanks and expansion vessels, frequently sited directly above a bathroom because that is where the services are. Soil and vent pipes, for the same reason. Structural members: purlins, binders, struts and, in a converted or partially converted loft, new steels. Cables and, occasionally, ductwork.

The awkward reality is that the rooms with no daylight are the rooms in the middle of the plan, and the middle of the plan is where the loft is most congested. That is the fundamental tension in siting a tunnel.

Storeys, and what each means

Bungalow, roof straight into the ceiling. Frequently under a metre, and this is where tunnels perform at their absolute best. Light loss compounds with length, so a short straight run delivers close to the theoretical maximum.

It is also why every room in a bungalow is a candidate: there is no floor above to negotiate.

First floor room in a house, through the loft. One to two metres. Straightforward, effective, and the standard case.

Ground floor room, through the loft and a first floor void. Three to five metres, and this is where diameter has to compensate. Step up a size, keep it as straight as possible, and be realistic about the result.

The complication here is the first floor void. Where the tube can run within a floor void, between joists, it is invisible. Where it cannot, it has to pass through a first floor room, which means boxing in.

Boxing in, and when it is worth it

A cased duct in the corner of a room, treated as a piece of joinery and decorated to match, is honest and neat if it is designed rather than improvised.

Where a route exists behind a knee wall, in a cupboard, through a bulkhead over a stair, or within a stud partition running in the right place, use it. Those routes are invisible and they are worth hunting for.

Where boxing in through a habitable room is the only option, price it honestly against the alternative. At that point the total cost of tunnel plus joinery plus decoration may exceed a rooflight in a different room that would serve the house as well.

Loft conversions

A converted loft has no open roof space, so the tube passes through the converted room and needs enclosing.

Two additional considerations. The conversion has a designed structure, including new floor joists and frequently steels, and nothing in that is cut for a tunnel without checking.

And the insulation sits at rafter level rather than ceiling level, so the tube passes through insulated construction. Where it crosses that layer the detail has to maintain both the insulation and the vapour control layer, sealed to the tube. Get that wrong and you have a cold bridge and an air leak in a warm roof, which is a worse outcome than the same error in a cold loft.

Where the answer is genuinely no

A vaulted or sloping ceiling following the rafter line, with no space above it. There is nowhere for a tube to run, and the alternative is a rooflight in the slope itself, which in that situation is straightforward.

That is the only common situation where a tunnel cannot be made to work at all.

Two positions rather than one

Worth considering in any room bigger than a WC.

Two smaller tunnels spaced apart read as ambient daylight across a space. One larger tunnel in the middle reads as a spotlight, with the corners of the room still dark, even where the total aperture is similar.

The light is the same; the distribution is completely different, and distribution is what makes a room feel daylit rather than lit.

Two positions also give more scope for finding straight runs, because you are no longer trying to land a single tube on one particular spot. And where two are fitted on the same visit the second costs meaningfully less, since the access, protection and setting up are shared.

How to settle it before anybody visits

Send three things: a photograph of the room ceiling, a photograph of the loft space above it showing what is up there, and a note of which room and how it is used.

From those we can usually confirm whether a straight run is available, recommend a diameter, and frequently identify a position a metre from the obvious one that halves the run.

That single change is worth more than a size increase, and it costs nothing to make at the planning stage.

Start at the ceiling and work upward

The instinct is to find a workable spot on the roof and see where the tunnel comes out. That produces disappointing installations.

Decide first where the light needs to land in the room, then find the straightest available route from there to a workable roof position. Where the two cannot be reconciled, compromise on the roof end, because a dome moved along a slope costs nothing while a bend in the tube costs light.

What constrains the ceiling opening

Joist spacing. The tube passes between joists without cutting them, which sets a practical maximum diameter and fixes the position within the bay.

Ceiling construction. Plasterboard is straightforward. Lath and plaster is brittle, cracks beyond the cut, makes more mess, and on a listed building may be part of what is protected. A concrete soffit needs coring and may not be permissible at all.

A vaulted ceiling following the rafters has no void above it, so there is nothing for a tunnel to run through. A roof window is the answer there instead.

Services. Cables, pipes and light fittings already in the void.

Where to put it within the room

Not necessarily in the centre.

In a hallway, light near the middle of the run reads better than light at one end. In a landing, near the top of the stairs is where it does most good, because that is where people need to see.

In a bathroom, over the basin or the shower rather than over the door, so the light falls where you are looking.

And in a long hallway, two smaller tunnels spaced along it beat one large one in the middle, because they spread light rather than concentrating it in a pool with dark ends.

The offset question

A short offset between the roof position and the ceiling position, achieved with a purpose made elbow, is worth it to land the light where it is wanted.

A long dogleg to reach a preferred position is not, because the light lost to two bends is substantial and the result disappoints.

The judgement is roughly this: one gentle turn to gain a genuinely better position, yes. Two or more turns to satisfy a preference, no.

What the survey should confirm

The loft contents and the route, the joist and rafter spacing at both ends, the ceiling construction, and whether the intended roof slope raises any consent question. All four before the diameter is chosen.