The short answer
Makers sell roof windows in a fixed range of widths and heights, each marked with a letter and number code, so you pick from a chart rather than ordering a custom size. The best fit usually sits between two rafters so no rafter has to be cut. Check the pitch range for the model, and measure inside the loft before you order, because the chart is only a starting point.
How the size codes work
The big roof window makers publish a size chart in which every window has a code. A typical code is two letters that stand for a width and a height group, followed by a two-digit number that sets the exact opening. Two windows with the same two letters are the same width and height class, and the number tells you which one.
The codes are manufacturer specific. One maker's code cannot be read across to another's, and a chart for a centre-pivot model is not the same as one for a top-hung or a flat roof unit. Always read the maker's own chart for the exact model, and give your roofer the full code, not just a rough size.
The chart usually lists three figures that people confuse: the overall frame size, the glazed area you see through, and the rough opening needed in the roof. The frame is bigger than the glass, and the opening is bigger than the frame. Daylight depends on the glass figure. The carpenter cuts to the opening figure.
Matching the window to the rafters
The most economical roof window sits between two rafters with no timber cut at all. Rafters on older Swindon houses often sit at wider centres than on newer ones, and hand-cut roofs can vary along the length. Open the loft hatch with a tape and measure the clear gap between the inside faces of the rafters at the point where the window will go.
A window narrower than the gap is secured to the rafters with packing pieces. If it is wider, a rafter must be cut and the load carried round the opening by a pair of new timbers called trimmers. That is a bigger job, covered on the page about trimming rafters.
It is common to choose a slightly narrower window to avoid trimming, but that gives up some daylight. The trade-off is yours to make, and the survey shows which routes are open on your roof.
Pitch, height and where the window sits
Every roof window has a range of roof pitches that it suits. A window used at a pitch outside its stated range can leak or open awkwardly, so check the model's limits on the maker's data sheet. Low pitches need special flashing or a different type of unit, and very steep pitches change how far the sash swings.
Height matters inside the room. A centre-pivot window is usually positioned so that a person standing can look out and so the handle is within reach. If the sill is too low, you may have trouble with furniture; too high, and you cannot see out or open it comfortably. A window above a stair has different needs from one over a bed.
If you are fitting a window to a loft room, think about where the floor and ceiling meet the roof slope. Our page on where to put a roof window goes through those choices, and the page on loft conversion windows covers the rest.
Big or small: what changes with size
A bigger window lets in more light, but a big sash is heavy, harder to clean, more costly to replace and may need a trimmed opening. A smaller one is easier to fit and often cheaper, but a room with a single small window can still feel dim.
Two small windows side by side are sometimes better than one very large one. They can be spaced across a wide roof to light both ends of a room, they fit between rafters more easily, and they give two points for ventilation. A narrow window running down a steep slope can bring light into a stair without taking up much roof.
Glass area is not the only variable. Frame colour, glazing type, an electric opener or a blind add to the weight and price. The page on electric roof windows explains what a motor needs in terms of power and room.
Thermal rules and flat roof units
Part L of the Building Regulations sets energy limits for new and replacement windows in an existing home. For a roof window the target is Band B on the Window Energy Rating scale, or 1.4 W/m²K. A rooflight, the unit for a flat or low roof, must reach 2.2 W/m²K. The sizes on a chart do not change that, but the glazing option you pick has to meet the figure.
Flat rooflights are measured in a different way. They are often ordered to a specific opening and fixed on an upstand, not selected from a chart of letters and numbers. Our page on flat rooflights shows how those are specified, and the page on flat skylight leaking shows why the detail at the edge matters.
Rain noise also varies a little with the glass you choose, as covered on the page about roof window rain noise.
Measuring before you order
Make a short list before you buy. First, find the make and code of any existing window, which is printed on a label inside the frame. Second, measure the clear gap between rafters, the roof pitch and the distance from the ceiling or floor to where you want the window. Third, take photographs of the loft and the roof from outside.
If you send these to us, we can tell you which sizes are likely to work and which will mean a trimmed opening. The survey then checks the real roof, and you receive a written quote with photos of it, so nothing is a surprise.
Replacing like for like is usually simpler, because the opening already exists, and the page on roof window replacement sets out the steps. If you are only starting out, the skylights and roof windows hub is the place to begin.