Pane, sash or window: the three levels of renewal
A VELUX skylight can be renewed at three levels, and the right one depends on the age of the window and what else has worn out besides the glass.
The smallest step is a new pane in the existing sash. The sash is the part that pivots when you open the window, and the insulated glass unit sits inside it behind a glazing profile. On some models a replacement pane can be supplied and fitted into the old sash, but the pane has to match the sash exactly, including its thickness, coatings and the laminated inner layer that overhead glazing needs.
The middle step, and the usual one for a misted VELUX, is a replacement sash. A new sash arrives from the factory with the glass unit already fitted, sealed and tested. The old sash lifts off its hinges, the new one goes on, and the frame in the rafters, the flashing on the roof and the lining inside the room are left alone. From the room it looks like a new window. From the street it looks the same as before.
The largest step is a whole new roof window. The old frame, flashing and often the lining come out, and a current window goes into the same opening or a resized one. That is a bigger job, and it is also the only route that brings the frame, seals and insulation up to date along with the glass. If you want to understand why the glass failed in the first place, our guide on why rooflights mist between the panes covers edge seals and desiccant.
Read the data plate before anything is ordered
Every VELUX window carries a data plate that tells you its type, size and glazing, and that plate decides which replacement parts exist.
Open the window and look along the top of the frame. The plate shows a type code such as GGL or GGU, a size code such as MK04 or CK02, a glazing code and a production mark. From those, the right sash or pane can be matched to the window. The size code matters in particular, because it tells you whether the window is a current size or an older one.
Windows made before April 2001 use an older sizing system. Replacement sashes for very old windows are harder to match, and parts for some early models are no longer made. For those, a new window with a replacement flashing designed for the older installation depth is the practical route: it fits the existing opening without disturbing the plaster lining. Our guide to replacing a misted roof window walks through the same decision for pitched windows of any make.
Why a factory sash beats loose glass
Glass made to fit by a local glazier can look like the cheaper option, but it has to meet the same specification as the unit it replaces, and on a roof window that is a high bar.
The inner pane of a roof window over a living space is laminated, so that if it breaks it holds together rather than falling into the room. The outer pane is toughened. Between them sit a low emissivity coating and a gas fill that set the window's heat loss, and on many units a solar control layer as well. A sealed unit made to fit the sash has to reproduce all of that, bond correctly into the sash profile and keep the drainage path clear. A replacement sash does all of this at the factory, and it comes with the manufacturer's own product warranty on the glass.
A new sash also renews the parts that wear alongside the glass: the sash seals against the frame, the ventilation flap and the handle. On a window that has been opened and closed for fifteen or twenty years, those parts are often as tired as the pane.
When a whole new window is the better upgrade
Replacing only the sash keeps the old frame, flashing and insulation, so it makes sense when those parts are sound and the window is otherwise the one you want.
A full new window is worth weighing when the frame shows wear, when there are draughts or cold spots around the frame rather than the glass, when the flashing is tired or the tiles around it are due for work, or when the window is from before 2001 and parts no longer match. It is also the moment to change what the window does. A new window can be larger to bring more light into a loft bedroom, electric or solar operated so a high window is easy to open, or fitted with a different glazing type for the slope it faces. A change of size may affect the opening in the rafters and, in some cases, planning, which our answer on whether a replacement rooflight has to be the same size covers.
For a wider comparison across flat rooflights and lanterns as well as roof windows, see glass only or full unit replacement.
Choosing the new glass for the slope it faces
Whichever route you take, the new glass does not have to match the old one, and renewal is a good time to choose glazing for the way the slope catches the sun.
A Cambridge loft sits under a wide, open sky, so the slope's orientation decides much of what a roof window gives the room. A north slope gives soft, even light that rarely needs solar control. A south slope takes several hours of direct sun in summer, and a pane with solar control coating, or an external awning blind, keeps a loft bedroom comfortable in June. East and west slopes bring low sun that glares across a desk or a screen in the morning or evening.
| Slope | Light in the room | Glass to consider |
|---|---|---|
| North | Soft and steady | Standard low-E |
| East | Bright mornings, low glare | Low-E plus a blind |
| South | Strong sun and heat | Solar control |
| West | Warm evening glare | Solar control or awning |
A clearer new pane can also make the room noticeably brighter than a misted one, as our answer on whether a new rooflight is brighter than the old one explains. How we weigh orientation, glass and blinds together is set out in how we plan the light.
Questions people also ask
Does a replacement VELUX sash need Building Control?
Renewing the sash on an existing window in the same frame is generally treated as a like-for-like replacement of a part. Replacing the whole window is controlled fitting work under the Building Regulations, and the new unit has to meet Part L. We handle the notification where the work is notifiable, and our guide on Building Control for replacement rooflights has the detail.
Can a new sash be fitted from inside the room?
Usually, yes. A centre pivot sash is turned and lifted off its hinges from inside, so a sash swap rarely needs scaffolding. A full window replacement involves the flashing and roof covering, so access outside depends on the height and position of the roof.
Will a replacement sash match my other VELUX windows?
A sash made for your window's type and size code matches its frame, and the outside view is unchanged. The glass tint may look slightly different from older neighbouring panes if you choose a different coating, which is worth deciding at the survey.
How long does a sash replacement take?
Changing a sash is a short job, typically well under a day per window once the correct part has arrived. A whole new window in the same opening takes longer, and fitting a new lining adds more time again. To see which route suits your window, book a light survey through misted and failed rooflight replacement or request a quote.
