Sun tunnel installation in Birmingham

A sun tunnel is a small, round rooflight that carries daylight down a reflective tube. A clear dome on the roof catches the light, the tube carries it across the loft, and the ceiling diffuser spreads it into the room below.

Because the opening is small and the tube is narrow, a sun tunnel can reach rooms that a rooflight or roof window cannot, such as a hallway under a loft that is too shallow or too full to take a window.

Some homeowners call it a skylight and some trade suppliers call it a rooflight with a tube. Whichever name you use, the parts and the questions are the same.

What the room looks like afterwards

In the room you see a flush white disc in the ceiling, rather like a large downlight. The tube itself is hidden in the ceiling void, so the finish is a plastered or plasterboard ring around the diffuser.

Outside, the dome sits low on the roof, which makes it far less noticeable from the street than a roof window of the same room.

Why rigid carries more light

Light loses a little energy every time it bounces off the inside of the tube. A rigid tube has a smooth, mirror-like finish and few joins, so the light bounces less often on its way down and arrives brighter at the diffuser.

A flexible tube is ribbed, which means the light meets a rougher surface and scatters more. Stretched out straight, it carries a decent amount of light. Bent hard in two places, it carries noticeably less, which is why we try to keep the run as straight as the loft allows.

The dome and its flashing

The dome is a clear plastic bubble, usually acrylic or polycarbonate, set in a base that is flashed into the roof covering. Water is kept out by the flashing, which is why it has to suit your roof: a tile or slate roof takes a flashing dressed into the covering, while a flat roof takes a base that sits on a kerb.

Domes are clear for the most light, and some makers offer a tinted or coated dome that cuts heat and glare on a roof facing the afternoon sun. Our quote lists the dome type beside the tube so you can see what each choice does to the light.

Roof pitch matters too. On a steep slope, the dome catches light from one side of the sky more than the other, while on a flat or gently sloping roof it gathers light more evenly through the day.

What decides the light

Four things decide how bright the room feels: the size of the dome, the length of the tube, the number of bends in it, and the amount of sky the dome can see. A bigger dome gathers more light, a shorter tube loses less of it, and a roof slope that faces open sky does better than one shaded by a tall neighbour or a mature tree.

The room matters as well. A small bathroom with pale tiles will feel bright with a modest tunnel, because the walls bounce the light around. A long hallway with dark floors and dark doors needs a bigger tunnel or a second one to feel properly lit.

Brightness changes with the weather and the time of year, so we tell you which of the options on the quote will give more light than the others, and by how much in plain terms.

Halls and landings

A hallway is the classic case, because it often has a front door with a small window at one end and nothing else. A tunnel in the middle of the ceiling lights the whole length evenly, and on a bright day you can switch the hall light off altogether.

A landing at the top of the stairs benefits in the same way, and the light often spills down the stairwell to the hall below. If the stairs turn, the light reaches the corners better than a single downlight does.

Bathrooms and wardrobes

A bathroom with no window usually relies on a fan and a light, and a tunnel adds natural light without opening the room to the outside. Frosted diffusers are popular here, and the light kit gives you a ceiling light for the evening from the same opening.

A walk-in wardrobe benefits from the colour accuracy of daylight, which makes it easier to tell navy from black. The tunnel is small enough to fit between the joists of many wardrobe ceilings.

Rafters and joists

Standard tunnels are made to slot between rafters at common spacings, so on many roofs the dome, the tube and the diffuser can all be fitted without cutting a structural timber. Where a rafter does fall in the way, the opening is trimmed with added timbers so the load is carried round it.

The Building Regulations apply to a fresh opening in a roof, and a trimmed rafter is covered by them as well. Notification to Building Control is something we take care of when the job is notifiable.

On a pitched roof the tube is usually longer, because it runs from the slope of the roof down through the loft to a ceiling directly below. The base is flashed into the tiles or slates, and we match it to your roof covering, whether that is plain clay, concrete interlocking tiles or slate.

On a flat roof the tube is often shorter, because the ceiling sits only a short way below the roof deck. The dome sits on a small raised kerb, and the roof covering turns up it, which is the same principle as a larger rooflight. Our answer on fitting a sun tunnel in a flat roof explains the upstand and the flashing.

A flat roof also gives you a choice between a tunnel and a small flat rooflight over the same room. Our page on flat roof rooflights sets out that alternative so you can compare them.

Tunnel or small flat rooflight

A small flat rooflight gives you a view of the sky and more daylight than a tunnel of the same footprint. A tunnel gives a smaller pool of light, but it can reach a room that has other rooms or a loft above it.

If the room is directly below a flat roof with a shallow void, a rooflight often suits. If there is a loft, a bedroom or a long run between roof and ceiling, the tunnel is usually the answer.

Tiles, slate and older roofs

On an older tiled roof tiles next to the opening are lifted, cut or replaced to take the flashing, and any that break are replaced with the closest match we can source. If the roof looks as if it needs more attention before it can take the work, you hear that from us at the survey.

When the roof covering is close to worn out, we tell you straight away and before anything is ordered, since a new opening sits best in a covering that will last longer than the tunnel's installation.

Building Regulations

A new roof opening is regulated as building work, and our fitting follows the current rules. If the job is notifiable, we make the notification ourselves, and a registered building control approver can be used in place of the council.

Heat loss through the opening is one of the things the rules look at. Because a sun tunnel is measured differently from a flat pane of glass, Building Control decides how Part L 2021 is applied to it, and each tunnel on the quote carries the maker's published thermal figure, which puts the options on one line for comparison.

On the roof

We work from the outside first, setting up access, lifting the tiles or cutting the roof covering and forming the opening. The base and flashing go in and are tied into the covering, and the dome is fitted on top.

A temporary cover keeps the weather out while the tube goes in below, so the roof stays closed to the rain.

In the loft

In the loft we fit the tube, join the sections, and seal each joint so light and air stay inside. The insulation is laid back around the tube once it is in place.

Then we check the run from below, looking up the tube for gaps or kinks.

In the room

In the room we cut the ceiling opening, fit the diffuser and make good the edge. We clear up as we go, and dust sheets protect the floor below the opening.

Finally we look at the diffuser in daylight to confirm it sits level and the light is even.

A tunnel is fitted from three places: the roof, the loft and the room.