Roof window sizes and how they are coded

What the code describes, and the two things it does not

A size code gives the nominal outer dimensions of the frame in centimetres. It does not give the structural opening, which is larger, and it does not give the glass area, which is smaller. Both of those are derived from it and both matter more in practice than the frame size does.

The frame is the reference because it is the fixed manufactured object. The opening has to be built around it with a fitting tolerance, and the glass is whatever is left inside it once the frame section and the sash section are deducted. A window quoted as 780 by 1400mm has a visible glass area appreciably smaller than that, and a rough opening appreciably larger.

Height digits, and the jumps between them

The two digits after the letter fix the height, again in centimetres. The common set runs 01 at 70cm, 02 at 78cm, 04 at 98cm, 06 at 118cm, 08 at 140cm and 10 at 160cm. Intermediate codes turn up on older and discontinued ranges, so a 1980s window may carry a number that no longer appears in a current list.

The jumps are the useful thing to notice. Between 118 and 140cm there is 220mm of extra length, which is a significant amount of run of slope on a shallow rear addition roof and a significant amount of extra daylight on a steep one. There is no size in between, so a roof that will take 1300mm of slope gets a 1180mm window and not a 1400mm one.

From frame size to structural opening

The rough opening in the roof is the frame size plus a fitting tolerance on each dimension. As a working rule, allow around 40 to 45mm on the width and 45 to 60mm on the height over the nominal frame, and check the exact figure the manufacturer publishes for the range being fitted, because it has changed between generations.

That tolerance is not slack. It is the space the frame gets packed and shimmed in, and it is what allows a square frame to be set truly square in a roof that is not. Old roofs are rarely square: rafters twist, purlins sag, and a 1900 terrace can be out by 15mm across a metre. Without the tolerance the frame gets forced, the sash binds, and the window will not shut evenly.

Rafter centres, and the widths that exist because of them

The width grid is not arbitrary. It exists because of the spacings roofs are built at, and the arithmetic is unforgiving.

A 600mm truss centre leaves around 550 to 570mm of clear width between two trusses. A 55cm wide window plus its fitting tolerance needs about 595mm, which is why fitting a C code unit on a trussed roof usually means trimming right up to the trusses either side and no further. Any wider and part of a truss has to come out and be carried on a designed member.

A traditional cut roof at 400mm centres gives roughly 350mm clear in one bay, 750mm across two, and 1150mm across three. So a 66cm window takes two bays and one cut rafter, a 78cm takes two bays comfortably, and a 114cm takes three bays and two cut rafters. Each cut rafter transfers load into the trimmers and then into the rafters either side, and beyond one bay that becomes a calculation rather than a routine detail.

Victorian rafter spacing in a vale terrace, measured rather than assumed

Gloucester’s Victorian and Edwardian terraces, and the older stock through Tewkesbury and the vale villages, were built in imperial and they were built by eye. Rafters at nominally 16 or 18 inches on centre come out anywhere between 380 and 480mm in reality, and the spacing frequently varies along the same slope.

That has a direct consequence for the size code. A window chosen off a drawing that assumes 400mm centres can arrive to find 460mm centres and a purlin 60mm out of where it was expected, and the intended structural opening no longer lands between the same rafters. Which is why every survey on older stock records actual measured centres at three points on the slope, plus the position of any purlin, binder, strut or collar, before a code is chosen. The trimming detail is on the trimming rafters page.

Run of slope, and the height code the roof will accept

Width is set by the rafters. Height is set by how much clear slope there is between the eaves, any purlin, and the ridge, and this is where Gloucestershire roofs bite.

A window needs its own length plus room above and below for the flashing to lap into whole courses of covering. As a working figure, allow around 250 to 350mm of covering below the apron and at least a full course plus a lap above the head. On a vale rear addition with 1.8 to 2.4 metres from gutter to abutment, a 1400mm window plus those laps has already used the lot, and a 1180mm unit set high is usually the right call.

On an interwar hipped rear slope at Longlevens or Warden Hill, a purlin at mid height often leaves around 1.2 metres of clear vertical run below it. That is a 980mm high window at most, which pushes the specification towards a wide short code such as P04 or U04, or towards two units in adjacent bays.

What the code will not tell you: the glass area

Two windows with the same code from different generations of the same range can deliver noticeably different amounts of daylight, because frame and sash sections have got slimmer over the years.

Manufacturers publish a glass area and a daylight area for each unit, and on a modern unit the glass is typically somewhere around 65 to 75 per cent of the nominal frame area. That ratio matters most on small codes, where the fixed frame section eats a larger share. Two C04 windows have more total frame and less total glass than one M08 of similar overall area, which is worth knowing when the choice is between one large unit and two small ones.

Fakro and Keylite numbering, and why codes never cross brands

Fakro and Keylite both publish their own numbered size tables, and the physical dimensions largely mirror the same industry set of widths and heights, because they are all designed around the same rafter spacings. The codes themselves are not interchangeable.

An order placed with a VELUX letter and number against a Fakro window is an order for a different size, and a flashing kit or blind ordered on the wrong maker’s code will not fit the frame it arrives at, because the fixing points and channel profiles differ. When a window is being replaced, the brand and the code both get identified from the data plate on the sash rather than inferred from a tape measure. The differences between the brands themselves are on the brand comparison page.

Roto’s slash notation, and the widths that sit slightly off the grid

Roto expresses size as two numbers separated by a slash, roughly the width and the height in decimetres, so a unit described as 7/11 is approximately 74cm wide by 118cm high.

The useful thing to notice is that Roto’s widths are not identical to the others. A Roto width of around 74cm sits close to but not on the 78cm that dominates the rest of the market, and the same applies at 54 and 65cm. That is fine on a new installation, since the opening is built to the frame that has been chosen. It becomes relevant on a replacement, because a Roto opening does not necessarily suit a different maker’s frame without adjusting the trimmers.

Reading the code off a window that is already in the roof

On a replacement the existing code is the starting point, and it is usually printed on a data plate on the sash. On a centre pivot unit, rotate the sash through and the plate is generally visible on the top edge or the side of the sash, giving the range, the size code, the glazing code and a serial number.

Photograph it rather than transcribing it, because the strings are long and a single wrong character orders the wrong product. Where the plate has gone or is unreadable, the frame gets measured directly, the covering profile is measured for the kit, and the code is worked back from that.

Discontinued codes on 1980s and 1990s units

A window put in thirty years ago may carry a size that is still made, a size that has been dropped, or a size that is still listed but with a frame profile that has changed. All three happen.

Where the code survives, a like for like replacement drops into the existing structural opening and the internal plaster usually survives untouched. Where the profile has changed, the fixing line moves, and the frame may need packing out or the trimmers adjusting. Where the size has gone entirely, the choice is the nearest current size with the opening modified to suit, which is often an opportunity worth taking. A 1980s unit is frequently smaller than what the same room would be given today, and while the roof is open and the scaffold is up, going up one code costs far less than it would as a separate job later. The replacement page covers the rest of that decision.

Two codes side by side, and the gap that is fixed

Where two windows go next to each other, the space between the frames is not a design choice. The manufacturer sets it, commonly around 100mm horizontally and around 250mm vertically for a window above another, and a combination flashing kit is made for that exact arrangement.

So the setting out arithmetic for a pair is the two frame widths plus the fixed gap plus the fitting tolerance on the outer edges, and the structural opening is designed around that total. On a hipped interwar rear slope, two C04 units in adjacent rafter bays with the fixed gap between them is frequently the answer that gives the most glass with no structural design at all. Layout of multiple units is covered on the pairs and triples page.

Everything else orders against the same code

Once the code is settled, five other items follow from it without further decisions: the flashing kit for the covering, the underlay collar, the insulation collar, any blind or insect screen, and any external awning. Each exists in a version matched to that code.

Which is why the code is the item on a survey note that everything else hangs off, and why changing it late is expensive. A window ordered, then reconsidered, takes five other line items with it. The kit side of that is on the flashing kits page, and the blind side on the blinds page.

Sizing for the daylight the room wants

The roof sets the ceiling on what is possible. Within that, the useful benchmark is glazed area against floor area. A rule of thumb that holds up well in a room in the roof is a glazed area of around a fifth of the floor area for a room that reads as genuinely bright, and around a tenth for one that reads as adequately lit.

On a 12 square metre loft bedroom that means somewhere around 2.4 square metres of glass for the bright version, which is two M08 units rather than one. Two windows also light a room far more evenly than one, since a single unit in the middle of a slope creates a bright zone under it and shadow in the corners. That is worth knowing before the roof decides the answer for you.

What to measure before asking for a figure

A useful first email needs less than people expect: a photograph of the roof from the garden or the street, a photograph of the room inside, what the covering is if you know it, and roughly where the daylight should land. From that comes an indicative range, then a survey, then a fixed price.

What the survey then records, and what turns into a code, is the pitch off a digital level, the rafter or truss centres measured at three points, the depth of the rafters, the position of every purlin, binder and strut, the clear run of slope between eaves and ridge, the covering type with its profile height and batten gauge, and the finished floor level from the conversion drawings. The code falls out of that list rather than being chosen at the start of it. The costs page explains how the two figures work.