A roof window sits in the plane of a pitched roof and is flashed into the covering around it. It is not a dormer and it is not a rooflight on a flat roof, and the three have different products, different prices and different building control positions. Almost everything that goes wrong with one is the flashing rather than the window.
The opening types, from centre pivot to fixed roof windows
Centre pivot windows rotate about the middle of the frame, which makes them easy to clean from inside and means the top half swings into the room. Top hung windows open outwards from the top, keeping the whole opening clear, which suits a low mounted window you want to lean out of. Fixed units do not open at all and are the cheapest way to get light where ventilation comes from somewhere else. Which you need is decided by where the window sits relative to head height and whether it is your ventilation as well as your daylight.
Loft windows come in a manufacturer's grid of sizes
Roof windows come in a fixed range of widths and heights so they fit between rafters at standard centres, and the practical constraint is usually the rafter spacing rather than the look. Going wider than one rafter bay means cutting a rafter and trimming the opening, which is structural work. That is the single question worth asking before choosing a size: does this fit between the rafters I have.
Flashing is the whole job
Every roof window is sold with a flashing kit matched to the covering, and the kits differ for plain tiles, interlocking tiles, slate and low pitch. Using the wrong kit, or improvising with mortar and sealant, is the reason most roof windows leak. The second reason is pitch: every model has a minimum pitch it is certified for, and below that it needs a different product or a kerb. Ask which flashing kit is in the quote and what the roof pitch is.
Escape, safety glass and where the regulations bite
Where a loft becomes habitable, an escape window may be required and it has to meet minimum opening dimensions and a maximum sill height, which is why an escape rooflight is a specific product rather than any window that opens. Overhead glazing is also treated differently from vertical glazing because of what happens if it breaks, so laminated inner panes are common. Approved Documents B and K cover those two points and a supplier should be able to say which of their range complies.
What is a skylight, a rooflight or a roof window?
The three words are used interchangeably in Britain and the products are not always the same. Roof window normally means a unit in the plane of a pitched roof with an opening sash. Rooflight often means a fixed unit, and on a flat roof it means a kerb mounted product entirely. Skylight is the everyday word for any of them. That matters when pricing, because a flat roof rooflight over a kitchen extension includes a kerb and a different flashing arrangement, so quotes using the same word can describe two different jobs. On an extension or an orangery with a flat perimeter roof, assume the kerb mounted product and ask who builds the kerb.
Conservation roof windows, listed roofs and windows for flat roofs
Conservation roof windows sit lower in the roof plane with a central glazing bar and a black finish so they read as a traditional cast iron rooflight, and they are what a conservation area or a listed building usually requires. On a flat roof a pitched roof window is the wrong product: you need a flat roof rooflight on a kerb, which is a different specification with its own drainage and walk on considerations. Anything that is not designed to be walked on is a fragile surface, which is why the HSE treats rooflights as a named fall risk.
Questions people ask about roof window types
Do I need permission to fit a roof window?
Roof windows within the permitted development limits on a house often need no planning permission, though listed buildings, flats and some conservation areas are different. Building regulations apply either way, because you are altering the roof structure and adding a thermal element.
Can a roof window be fitted without cutting a rafter?
Yes if you choose a width that fits the existing rafter bay, which is why sizes come in a grid. Anything wider means trimming the opening with structural timbers.
Why is my roof window leaking?
Most often the flashing kit is wrong for the covering, was improvised, or the roof pitch is below the window's certified minimum. Sealant around the outside is a symptom of one of those rather than a fix.
Can I put a pitched roof window in a flat roof?
No. A flat roof needs a rooflight designed for a kerb with its own drainage detail. Fitting a pitched roof product below its minimum pitch is a guaranteed leak.