Frampton Cotterell skylights and rooflights

Four- and five-bedroom houses can put one room in the loft, and that room can change jobs over the years. It starts as a bedroom, becomes a study, goes back to being a bedroom when someone returns home. A roof window has to suit all of those, which is a harder brief than it sounds.

By day, a desk wants steady light that does not throw glare across the screen. A window above and behind the monitor can bounce sky off it, so a blind that can be drawn partway earns its keep. By night the same room has to sleep a person, and the blind needs to be a blackout one, since a pale June morning is bright well before most alarms. The two jobs ask for two different blinds, or for one that does both.

Reach matters in a room with a sloped ceiling. A window high on the slope is out of arm's length from a desk chair, which is where electric and solar operation help. An opening roof window can be run from a switch or a remote on the wall, and the same controls can lower the blind.

Overnight, a closed loft room can become the damp end of the house, with breath gathering on the glass, where the cold of the night shows first. A morning airing clears it, and a roof window that opens a little makes that easy.

Storage needs thought too. Under a sloping ceiling, the wall space that fits a wardrobe or a bookcase is the part with the most height, and a window set there takes it away. A window beside the full-height wall, with the bed at the low end, uses the room better. Furniture is easier to move than a roof window, so sketch the layout before the window is fixed.

Where the window sits comes from the floor, not from the roof. The part of a loft room with full standing height is small, so it is worth marking against the slope, with the window over the place you will stand, not over the low edge that looks at the skirting. How wide the opening can go is then settled by the rafter spacing.

When does a kitchen extension get used most? For some households it is the evening, after work, when the daylight is going and the lamps are coming on. That is worth remembering when choosing roof glass, because the pane spends those hours reflecting the room back.

With the pendants lit and the sky dark, a flat rooflight or a lantern shows the table, the worktops and the people at them. Nothing glows. The room below is what you see, and the night sky only appears when the room lights are off. A blind across the opening stops the glass turning into a black panel on a winter evening, and it can be drawn when you want the ceiling to feel solid.

A kitchen-diner built onto a family house can carry a large area of glass, so summer is the other test. Warm air collects under a flat ceiling, and an opening unit lets some of it out where a fixed one cannot. Electric or manual compares how the two open, which matters when the glass is above a worktop and you cannot reach it.

Where the glass is placed matters as much as its size. Over the dining table it lights the people sitting at it; over the worktop it lights the cook; over the walkway between them it lights nobody. A tape laid across the floor of the extension, marking the table and the island, shows quickly where the daylight is wanted, and the lantern or rooflight is then centred on that.

The structure comes first, though. Cutting through a flat roof usually involves trimmed joists, with their load carried to either side, and the deck gets a look before anything is ordered. A lantern stands on a raised kerb called an upstand, made to the maker's details, with the roof covering finished against its sides.

Where the extension is still to be built, the glass belongs to the extension's own assessment, and the position is confirmed with the council ahead of the work. On an existing roof, the limits on how far a rooflight stands out are in the planning guide, and we run through them for your house.