Roof window installation in Crawley, glazed for the room under it

A roof window is a glazed, opening frame set into a pitched roof, in the same plane as the tiles or slates around it. It brings daylight into a loft room, a landing or a stairwell, and it is the everyday answer to the question of how to light a room that has no wall to put a window in.

The idea is simple. A sloping roof has a lot of surface and a room under it has very little wall. Put glass in the slope and the light comes down at an angle, reaches the far side of the room and lights the ceiling, which throws it back again. A single roof window can do more for a loft bedroom than a dormer of the same cost, because it takes sky from above rather than a strip of horizon from the side.

What separates it from a fixed skylight is the frame. A roof window has a sash that opens, usually on a pivot or a hinge, and a sash that can be cleaned from inside, ventilated on a warm night and, where the room is a loft conversion, used as a way out in a fire. It sits in a flashing kit that is built for the roof covering around it, so the joint between glass and roof is made of metal and profiled seals, not sealant.

The survey is done from inside the loft as much as outside, because the rafters, the insulation and the ceiling line decide the window, and the roof covering decides the flashing. We measure the roof from both sides before anything is ordered.

Inside, the first things to establish are the rafter spacing and size, the pitch, and what is between and under the rafters: insulation, a vapour layer, boards, plaster. Rafters in older roofs are often narrower than you would expect and spaced at whatever the carpenter had in mind. In a post-war roof they tend to sit at something near 400 to 600mm centres, and a window is chosen to fit the gap or to need only a modest change. The guide to standard roof window sizes and rafter spacing lists the usual frame widths and heights and how they sit against common spacings.

Then we stand where you will stand. We hold a tape from the floor to where the sill would come and check it against a person sitting on the bed, at the desk, in the bath. The height at which the window sits in a sloping ceiling changes whether you can reach the handle, look out from a chair and see the sky from standing. The guide on setting the height of a roof window in a sloping ceiling goes into it, and it is one of the choices that is easiest to get right on paper and hardest to correct afterwards.

The two common sash types open in different ways and suit different rooms. A centre-pivot window turns on a bar at its middle, while a top-hung window hinges at its head and swings out and up. Each one changes where you stand and how much of the opening you can use.

A centre-pivot sash rotates about a horizontal axis near the middle of the frame. The handle sits at the top, so you pull it down from above, and the lower half of the sash swings outward as the upper half swings in. A full turn presents the outside of the glass to the room for cleaning. It is the usual choice for standard-height roofs because the handle is easy to reach when the sill is low, and the open sash does not block the room.

A top-hung window hinges along its head, and the bottom edge swings away from you and upward. The handle is at the bottom, which suits a window set high in the slope, where a top handle would be out of reach. Some top-hung windows, known as projection or balcony types, lift out and up so far that a person can stand in front of a rail. The explanation of what a roof window balcony is shows how that works and what it needs from the roof.

A roof window that opens helps the room breathe, and a window that does not open well can leave a room damp. Condensation on the inside of a roof window is a question about air, not only glass, and trickle vents are part of the answer.

Warm, moist air rises to the ceiling, and in a loft room the ceiling is also the roof. Breath, cooking steam and bathing fill the top of the room with water vapour, and the coldest surface it meets is often the glass and the frame. A film of water on the glass at a cold dawn is not always a fault in the window. The explanation of why a roof window gets condensation on the inside goes through the causes, which are usually the room, the lining and the ventilation rather than the unit.

Part F of the Building Regulations sets the ventilation a habitable room needs, and a loft conversion has to meet it. Many roof windows include a slim ventilation flap at the head of the frame that gives background air without opening the sash. Others rely on a trickle vent or on the ability to park the sash ajar. The answer on whether roof windows need trickle vents sets out when one is wanted and when the window already has it built in.

The lining is what you see of a roof window from inside, and the blind is what you touch every day. Both are easy to overlook in the quote and easy to regret afterwards, so both are planned at the survey rather than left to the last day.

The reveal is the sloping return between the window frame and the ceiling. It is boarded, insulated and finished so there is no gap between frame and plaster. The line is usually splayed a little, so light spreads into the room rather than dropping as a shaft. A bare, unsealed reveal leaks warmth and sound as well as air, and it is where condensation starts. The guide on roof window linings and making good the reveal covers the boards, the insulation collar and the finish, and the work we do to leave the room tidy when we go.

Blinds do two jobs. They control glare and they control heat. A slope faces the sky, so a south or west roof window can overheat a loft room on a warm afternoon, and the only full answer is a blind that stops the heat before it crosses the glass. An external awning blind does that. An internal pleated blind softens the light and gives privacy. A blackout blind is what a nursery or a shift worker's bedroom needs. The blinds for roof windows guide sets out the three and how they fit, and the answer on the best blind for a roof window in a nursery is written for that one room.

Crawley is a New Town, and its roofs show it. The 1950s and 1960s neighbourhoods have a great many pitched-roof houses with lofts that have not been touched. Their conversions and their roof windows are the natural way to gain a room without adding to the footprint.

West Green was the first neighbourhood, from 1949, with Northgate, Langley Green and Three Bridges following in the early 1950s and Pound Hill, Ifield, Tilgate, Southgate and Gossops Green through the mid-1950s. Furnace Green came in 1960 and Broadfield in 1969. Later neighbourhoods, among them Bewbush, Maidenbower and Forge Wood, have newer roofs with different pitches and different rafter spacings. What this means for a window is that no two streets can be assumed to share a roof. The pitch, the covering and the truss or rafter layout are read from the roof in front of us, and a survey will show whether the job is straightforward or not.

The noise follows the geography. Langley Green and Northgate lie on the northern edge, nearer the airport. One third-party flight-path site suggests they get climb-out and ground noise, which we treat as indicative rather than official, and we always listen on the day. The town centre and southern neighbourhoods sit outside the loudest band on the same source. The M23 runs close to Maidenbower and the railway runs through Three Bridges, so those streets hear a steadier, lower background. Beyond the borough, homes in Horley, Charlwood and Lowfield Heath sit under the airport's approaches, while Horsham, Haywards Heath, East Grinstead, Reigate and Brighton are each a different mix of road, rail and rain.

These are the questions that come up most on a first visit to a loft under a pitched roof. Each has a longer answer on its own page, and each is the kind of thing that is easier to settle at the survey than from a brochure.