Manufacturers offer a wide range of standard sizes, from small units around half a metre wide to substantial ones over a metre wide and nearly two metres tall. What limits the choice on a given roof is rarely the catalogue: it is the spacing between rafters or trusses, the pitch, and whether the window has to satisfy an escape requirement.
How sizes are expressed
By a code rather than by dimensions, on most ranges.
Each manufacturer uses a letter and number system where the letter indicates the width and the number the height, and the actual millimetres are in a table.
Ordering by the code rather than by approximate dimensions is what avoids a unit that does not fit.
What the frame size means
The outside of the frame, which is not the opening or the glass.
The structural opening is slightly larger than the frame, and the clear glass is meaningfully smaller. Manufacturers publish all three.
Which one a quoted size refers to is confirmed rather than assumed.
The width constraint
Set by the rafter or truss spacing on most roofs.
Standard centres allow a certain clear width, and the narrower sizes in every range are made specifically to fit within it.
What that allows is on can a roof window be fitted between trusses.
Going wider than one bay
Possible on a cut roof, and a structural job on a trussed one.
Trimming a rafter with a designed detail lets a wider unit go in. Cutting a truss requires a designed replacement for what it was doing.
The height constraint
Set by the slope rather than the structure.
A tall unit needs enough slope above the eaves and below the ridge to sit in, and on a shallow roof or a low knee wall that runs out quickly.
It is measured up the slope rather than calculated from the room.
The escape constraint
Which frequently decides the size before anything else.
Where a loft room is a bedroom, one window has to provide a stated clear opening at a stated sill height, and only particular sizes and actions qualify.
The requirements are on can a roof window be an escape window.
Two windows rather than one large
Frequently better on several counts. Two units in adjacent bays give more glass than one wide one would on a trussed roof, light a room more evenly, and need no structural work.
How far apart is on how far apart should two roof windows be.
Stacking two vertically
An option on a tall slope and worth knowing about.
Two units one above the other, weathered by a combination kit, give a tall glazed area where a single unit of that height is not made or will not fit.
It reads well from inside and it is a more demanding weathering detail.
Made to size
Rare on roof windows, unlike flat roof units.
Because the sizes are standardised around rafter spacings, most installations use a catalogue size and the opening is formed to suit it.
Which is the opposite of a flat roof replacement, where the opening dictates the unit.
Replacing an old unit
Where the existing opening decides the size.
An old window’s size code is on its data plate, and the equivalent current size is generally available, which makes a like for like replacement straightforward.
Finding that code is on how do I find my size code.
Going bigger than the existing opening
Possible and it is new work rather than a replacement.
Enlarging means altering the structure and the covering, and the detail is on can a roof window be replaced with a bigger one.
How much glass a room needs
A useful rule rather than a calculation.
Somewhere between a twelfth and an eighth of the floor area as a starting point for a room otherwise poorly lit, less where a gable window already contributes.
How much light one actually delivers is on how much light does a roof window give.
What a small unit still achieves
More than people expect, because roof glazing sees the whole sky.
A modest window in a landing or a stairwell transforms a space that a much larger wall window would only improve.
Conservation sizes
Smaller across the board, and deliberately.
Conservation units are modest because the originals were, and because a large flush unit in an old roof stops reading as an original feature.
Sizes on a chalet or knee wall roof
Where the available slope is shorter than the roof suggests.
On a chalet bungalow the floor sits high relative to the slope, so the usable height between the knee wall and the ceiling line is limited and a tall unit will not fit.
It is measured from inside against the finished floor, and it is on the Findon Valley page.
Matching an existing pair
Worth attention where a second window is added years later.
Ranges change, so a current size that matches an old one dimensionally may differ in frame depth or external profile, and on one slope that reads.
Identifying the original range before ordering is what avoids a visible mismatch.
How the measuring is done on a pitched roof
We measure the actual bay width and the available slope before choosing a size, and we settle any escape requirement first because it narrows the range before anything else.
The whole picture is on the roof window installation page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.