The Velux roof windows installed before the year 2000 use a dimensional coding system that no longer corresponds to current references. Finding the right size to replace a glazing, install a blind, or change the entire opening requires understanding how these old Velux dimensions were designated, and especially how they translate into the contemporary catalog.
Old frame and connection kit: what installing in original format really implies
Most guides focus on correspondence tables between codes. However, the starting point for a successful installation is upstream, at the level of the frame, this opening cut into the structure to receive the window.
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On Velux installed before 1990, the frame was often sized with tolerances different from those of recent models. Since 2023-2024, Velux France offers specific connection and sub-frame kits to allow for a replacement in the same dimension, without having to redo the roofing or modify the structure. This changes the game: it is no longer systematically necessary to enlarge or reduce the existing opening to install a modern model.
Roofing craftsmen also report an increase in condensation and rot issues on the frames surrounding Velux windows from before 1990. When the wood is degraded, simply replacing the window is not enough. A diagnosis of the frame is necessary before any order, to avoid installing a new opening on a weakened structure. This guide on old Velux dimensions details the markers to check on models from before 2000.
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Old generation Velux identification plate: where to find it and how to read it
Identifying the dimension involves the identification plate fixed to the window frame. On older models, this plate is most often located on the upper part of the frame, visible when the opening is in the rotation position.
Old references follow a format like GGL 304, GGL 606 or GGL 1, where the numbers indicate the size and sometimes the type of glazing. The three-digit codes (304, 306, 308, 606, 608) correspond to precise width and height dimensions that do not always have a strict equivalent in the current range.
When the plate is worn or missing, there are two reliable options left:
- Measure the overall width and height of the frame (not just the glazing), then compare with the correspondence tables published by Velux.
- Check the code inscribed on the edge of the opening or on the rotation hardware, which sometimes bears a readable reference.
- Contact Velux customer service with the measurements taken, as some intermediate formats do not appear in the simplified tables available online.
Field feedback varies on the reliability of measurements taken solely on the glazing. Only the overall dimension of the frame allows for an exact correspondence.
Correspondence between old codes and current Velux codes
The transition from an old coding system to the current nomenclature is based on a simple principle: each old numeric code has been replaced by an alphanumeric code (letters + numbers). Thus, GGL 304 corresponds to the current format MK04, and GGL 606 to the format SK06.
However, some very old formats (pre-1991) do not have a direct correspondence. Velux has modified some dimensions by a few millimeters between generations, which can pose problems for blinds and accessories but remains manageable for the replacement of the window itself thanks to the connection kits mentioned earlier.
| Old code | Current code | Common usage |
|---|---|---|
| GGL 1 / 102 | CK02 | Small openings, bathroom |
| GGL 304 | MK04 | Standard format, bedrooms |
| GGL 306 | MK06 | Common format in renovations |
| GGL 606 | SK06 | Large opening, living area under the roof |
| GGL 608 | SK08 | Large format, living room |
This table covers the most frequent references. Non-standard formats require direct verification with Velux, as tolerances vary according to the year of manufacture.
Blinds and accessories: compatibility with old dimensions
The question of compatibility mainly arises for blackout and filtering blinds. Third-party manufacturers generally use the current coding. For an old Velux, it is therefore necessary to identify the corresponding code before ordering.
A blind designed for an MK04 will fit a GGL 304, but a blind intended for an SK06 may not necessarily fit a GGL 6 from before 1991 due to dimensional discrepancies. The available data does not guarantee universal compatibility on the oldest models: case-by-case verification remains the only reliable method.
Financial assistance for replacing an old Velux
Replacing an old, highly energy-inefficient roof window may benefit from MaPrimeRénov’ and energy savings certificates, even in non-strictly habitable attics. The main condition concerns the installed glazing: it must be certified with Uw and Sw values compliant with the updated requirements in 2024.
This assistance can significantly reduce the cost of installation on an old frame. Guides focused on dimensions almost never address this aspect, even though it often conditions the decision to replace or not an old window that is still functional.
Before starting the work, checking eligibility on the France Rénov’ site allows for sizing the actual budget. Replacing in the same dimension, without redoing the structure, simplifies the file since the intervention remains limited to about half a day for a standard format.
The choice between keeping an old Velux and replacing it ultimately hinges on three criteria: the condition of the frame, the availability of the exact corresponding code, and the level of financial assistance that can be mobilized. When these three parameters converge, installing in the original format remains the most economical and least intrusive solution to improve insulation without modifying the roof.