Skylight and rooflight installation in Llanhilleth

February mornings in a bathroom with a roof window begin with fogged glass. A shower fills the air with moisture within minutes, and sloping glass can be cooler than the air around it, so moisture can settle there.

A window made for this room opens, so the air can leave. It also helps to have an extractor that carries on after the shower is off. Without either, drops can form on the glass and run down onto the sill, so it helps to know how the room is aired now.

The spot where the window goes counts for as much as the pane. A sash above the bath or behind the basin is a stretch to reach, and an electric or solar roof window can answer it, worked from a wall switch or a handset. VELUX states that the solar and electric models in its roof window range have a rain sensor built in, which closes the window by itself in wet weather. That is a feature of those VELUX models only.

Tell us whether the room has a bath, a shower or both. A bath under the slope means a person lying back looks up at the glass, which changes where the blind should sit. A shower puts the glass at standing height, and the extractor and the steam path are what to plan around.

Privacy changes after dark. By day a window facing the sky is rarely overlooked, but a lit bathroom at night is easy to see into from higher windows in neighbouring houses. Stand where the window would go and look at what can see it. A blind often settles the matter.

Windows, roof windows and rooflights are all controlled fittings, which makes fitting one building work. Where the work is notifiable, we handle the Building Regulations notification with local authority building control or a registered building control approver.

On a March evening the desk in a loft study has two lights on it: the lamp, and whatever is left of the sky. A roof window above a desk can send daylight over the shoulder rather than across the face, and the same glass has to be reckoned with at both ends of the day.

Afternoon sun on a screen is the first problem. A blind that you can lower part way can take the glare off without taking the room into darkness, and it is chosen with the window so that the two suit each other.

After dark the second problem arrives. The pane turns black and works as a mirror, throwing the lamp and the monitor back at you. Pulling the blind down over the glass tidies this up, and so does placing the desk a little off the line of the window.

Fresh air is the third. A study with the door shut and the heating on gets stale, and a sash that opens a little lets it clear. For a window that is out of reach, an electric opening rooflight or an electric roof window can be worked from a wall switch.

Ventilation in a study is easy to forget until June. A closed room under a roof with the computer running warms up, and the sash that opens is the simple answer to stale air, with the blind kept down on the sunny side.

If the room doubles as a guest bed, the blind works twice: against the afternoon glare and against a bright early morning, which is a long way from the study use of the same window.

Tell us whether anyone works there during the day, since afternoon glare is a question of who is looking at the screen. Say what the room is used for and for how many hours. A study, a guest bed and a playroom want different things from the same slope.