Rooflight and skylight fitting in Blaina

In January the day starts late, and the hall of a semi or terrace is the last place in the house to notice. The front door is shut, the doors off the landing are closed, and the stairs sit in the middle of the plan with no wall of their own to the outside. The hall light goes on without anyone thinking about it.

A sun tunnel is made for that spot. It has three parts: a roof dome that collects daylight, a tube running down through the loft, and a ceiling diffuser at the bottom that spreads the light into the room. Because only daylight travels that way, the diffuser gives nothing after dark and the ordinary fitting does the work. It is a way to take the edge off a gloomy stairwell by day, and it is not a lamp.

Two things decide whether one is possible. The first is the loft: the tube needs a clear path from the roof to the ceiling beneath, past purlins, a water tank or boxes of stored things. The second is the roof covering, because the flashing around the dome is made to suit it, so the survey records whether the slope is tile, slate or something else. Where the loft is not a straight drop, deciding whether a rigid or a flexible tube suits it is the next question.

Keep a tube away from the room where someone sleeps. A tube can let early daylight into the room on a summer morning, and a sleeping room does better with a roof window and its blackout blind. Hallways, landings, dressing spaces and small bathrooms are the places where early daylight is welcome. Whether a sun tunnel needs planning permission is a separate question, answered in our page on sun tunnels and planning.

Stored boxes can be moved aside, but a water tank is harder to shift, so a route that clears it is the first thing we look for. Choosing where the diffuser goes is worth a few minutes on the landing. It does most for the stairs when it sits over the treads or the turn, and less when it is tucked in a corner.

Terraces with a back addition tend to put the bathroom there, under the lower roof, where the ceiling follows the slope. A roof window is a natural fit, since it gives a bathroom daylight without a wall facing the neighbours. A bathroom is also the room where the window works hardest.

By day a window in the slope is hard to see into. After dark the picture changes: the lights are on inside, the glass behind them is black, and anyone standing at the window position can be seen from a house higher up the hill. Whether that is a real question for your home is best settled by standing in the room at the survey and looking out. A blind settles it either way.

Steam is the other half of the story. Moisture settles on the coldest surface, and glass is often it. Pick a window that opens, run the extractor longer than seems necessary, and wipe the sill. That keeps the frame in better order than hoping the room dries itself.

On the numbers, Approved Document L (Wales) sets out guidance that a roof window should meet 1.4 W/m²K or Window Energy Rating Band B. It is a point of comparison between models, and we go through it with you when we suggest one.

Putting in a new roof window is building work in the Building Regulations' terms. When the job is notifiable, we handle the notification, and it goes to local authority building control or a registered building control approver. The size of the opening depends on the slope and the joists around it, and it is best marked out on the ceiling inside before anything is cut outside. Replacing an old window in its existing opening is a smaller job, and our page on replacing an old skylight shows how the two differ.