Do roof windows come in standard sizes?
Yes, and this is the single biggest practical difference between a roof window and almost every other glazed opening in a house. A pitched-roof window is a manufactured product built to a fixed grid of widths and heights, held in stock, and shipped with a flashing kit, a blind, an insulation collar and a set of accessories all built to the same dimensions. You choose from the grid rather than specifying a size. That constraint sounds limiting and in practice it is the reason a roof window can be surveyed on a Tuesday and fitted the following week. The codes that describe those sizes are a subject in themselves and are set out on the roof window sizes and how they are coded page. What follows is what the grid means when you are standing in the room deciding.
How many sizes there actually are, which is fewer than people expect
A manufacturer’s pitched roof window range runs to roughly thirty to forty combinations of width and height across the whole catalogue, and a merchant holds perhaps a dozen of those on a shelf. The widths sit in six or seven steps between about 550mm and 1,340mm. The heights sit in seven or eight steps between about 700mm and 1,600mm.
Not every width pairs with every height. The narrow widths stop short of the tallest heights because a tall narrow sash is unwieldy to rotate, and the widest frames do not appear in the shortest heights because the proportion looks wrong and the glazing gets heavy. So the real grid is a sparse rectangle rather than a full one, and a request for something like 1,340mm wide by 780mm high generally has no product behind it. This is worth knowing before a room layout is drawn around a dimension somebody assumed.
Where the grid came from, and why it is timber rather than glass
The sizes were not chosen for the glass. They were chosen for the roof. Rafters in this country are set at 400mm or 600mm centres, and a roof window either fits between two of them or it needs a rafter cut and trimmers designed across the opening. That is the difference between a one-day job and a structural calculation submitted to Building Control.
So the narrow widths exist precisely to slip between rafters. A 550mm frame goes into the clear space between two trusses at 600mm centres. A 780mm frame spans one rafter at 400mm centres, needing one trimmed member. A 1,140mm frame spans two rafters at 600mm centres. Every width in the grid maps onto a rafter arrangement that exists on real roofs, which is why the numbers look arbitrary until you stand in a loft with a tape measure. Trimming detail is on the trimming rafters page.
The steps, and what each one is really for
Reading the grid as a set of intentions rather than a list of numbers makes the choice much faster.
| Approximate frame width | What it is designed to land on | Where it turns up in Gloucestershire |
|---|---|---|
| 550mm | Between two trusses at 600mm centres, no cut member | Quedgeley, Abbeydale, Bishops Cleeve, Ashchurch and Yate estates |
| 660mm | One rafter trimmed at 400mm centres, narrow rooms | Victorian rear additions in Gloucester and Tewkesbury |
| 780mm | The general-purpose size, one trimmed rafter at 400 or 600 | Interwar semis at Longlevens, Hucclecote and Warden Hill |
| 940mm | Wider daylight, one or two trimmed rafters depending on spacing | Loft conversions on the vale terraces |
| 1,140mm | Two rafters at 600mm centres, or a pair of bays at 400 | Larger rear slopes and Forest of Dean cottages |
| 1,340mm | The widest single unit, always a designed trimmer | Big rear elevations, usually where two units would be better |
The heights move in coarser jumps, roughly 700, 980, 1,180, 1,400 and 1,600mm, and there the constraint is the run of slope rather than the rafters. Add the flashing lap needed above and below and a 1,400mm window wants a good deal more than 1,400mm of roof.
Standard does not mean interchangeable
Each manufacturer has its own grid, and the grids do not line up. A Fakro or Keylite frame at the same nominal width as a VELUX is often within twenty or thirty millimetres of it but not identical, and the codes describing them are different systems entirely. Roto uses a different notation again.
That matters in one specific and expensive way. Every accessory orders against the brand’s own code: the flashing kit, the blind, the insulation collar, the underlay collar, the awning, the insect screen. A blind bought for one brand’s nominal size will not fit another’s frame of the same nominal size, and neither will the flashing. So the brand is effectively chosen at the moment the window is chosen, and everything afterwards follows from it. Severn fits VELUX, Fakro, Keylite and Roto, and the comparison of what separates them is on the which roof window brand is best page.
Frame size, structural opening and daylight are three different numbers
The size on the code is the outside of the frame. The hole in the roof is bigger. The glass is smaller. People conflate all three and then find the room does not do what the drawing suggested.
The structural opening is typically the frame size plus 40 to 60mm on the width and rather more on the height, depending on the brand and on whether the installation is standard or recessed. The visible glass is the frame size minus the frame and sash sections on all four edges, which on a small unit removes a surprising proportion: a 550mm wide frame gives around 380mm of clear glass, so roughly a third of the width is structure. On a 1,140mm frame the same sections take a much smaller share. That is the real argument for one larger unit over two small ones where the roof allows it, and the real reason two small ones still often win, because they need no structural work at all.
Made-to-measure pitched roof windows, and what they cost you in time
Bespoke sizes do exist in the pitched-roof world. Conservation rooflights in cast or pressed steel are made to a set of traditional sizes and can be produced outside them. Some manufacturers offer special sizes on extended lead times. And a flat rooflight on a kerb, which is a different product, is routinely made to whatever dimension the opening is.
The trade-off is consistent. A stock roof window is available within days, arrives with a matched flashing kit off the same shelf, and has blinds and accessories held in stock for years afterwards. A special size means a lead time measured in weeks, a flashing set that may need adapting, and a blind that has to be ordered at the same time because it will not be available casually later. Where the room genuinely needs a dimension the grid cannot give, that trade is worth making. Where it does not, rounding to the nearest grid size costs 40mm of daylight and saves a month. Made-to-size units for flat roofs are covered on the flat rooflight standard sizes and made to measure page, where the logic runs the other way round.
Old openings that match nothing currently made
Replacement work runs into this constantly. A roof window fitted in 1985 was built to a grid that has since been revised, and frame profiles have changed even where the nominal code survived. A 1990s unit and its modern equivalent of the same code can differ by 20mm at the fixing line, which is enough that the new frame does not land on the old brackets.
There are three usual outcomes and all of them are ordinary. The current unit of the same code fits with the opening adjusted slightly, which is the common case and is done while the covering is off. The opening is enlarged to the next size up, which on a like-for-like visit with the scaffold already there is far cheaper than doing it later as its own job. Or the timber trimmers are packed to suit. The size code stamped on the old sash, usually on a plate at the top of the frame behind the ventilation flap, is read at survey so this is settled before anything is ordered.
Choosing from the grid rather than arguing with it
The practical method is to work backwards. Measure the clear run of slope from the top of the tiles at the gutter to the abutment or ridge, subtract the lap the flashing needs above and below, and that fixes the tallest height code the roof will accept. Measure the rafter centres and their positions, and that fixes the widths that avoid structural work. What is left is usually two or three real options, and the choice between them is about the room.
Gloucestershire tightens that funnel more than most counties. A Victorian rear addition roof in a Gloucester or Tewkesbury terrace often gives only 1.8 to 2.4 metres from gutter to abutment, which caps the height code well below what people expect. A hipped interwar rear slope at Longlevens narrows towards the corner and has a purlin across the middle, so the clear zone is often 1.2 metres of vertical run across two bays. A trussed estate roof at Quedgeley gives 550mm of clear width and nothing more without a designed member. In all three, two units in adjacent bays deliver more glass than one wide one and need no structural design. Sizing for the room specifically is on the what size roof window should I get page, and the size code system itself sits with the VELUX window installation hub.
Next on this subject.
- Question What is a roof window? A roof window is a glazed unit that sits in the plane of a pitched roof, in line with the…
- Question How much does a roof window cost to fit? Fitting a roof window is priced from the roof, not from the window. Two houses a mile apart in Gloucestershire…
- Question What size roof window should I get? Start with the room and let the roof edit the answer. The useful method is to work out how much…
- Question Can a roof window be an escape window? Yes, and on most loft conversions in Gloucestershire a roof window is the escape opening. It only works if three…