Is It Cheaper to Replace the Glass or the Whole Roof Window?
On paper, replacing just the sealed glass is cheaper than the whole roof window. In practice it rarely wins, because a unit old enough to have misted usually cannot be re-glazed at all: the matching sealed unit is discontinued, and the frame and flashing around it are aged too. A glass-only swap only makes sense on a young window with a sound frame, which is rarely the one that has failed.
To be straight with you: Apex does not offer glass swaps or resealing. Roof window replacement is what we do, and this page explains honestly when re-glazing would have been the sound choice and why, on the windows that actually fail, it usually is not available.
The cost comparison, on the surface
Take the figures at face value and re-glazing looks like the obvious saving. A replacement sealed unit is a fraction of a whole new window, and if a glazier could simply lift out two misted panes and drop in fresh argon-filled glass, the sum would favour it every time. That is the comparison people run in their head, and for a wall-mounted window it often holds.
A roof window is a harder case, for two reasons that do not show up in the headline price. First, the glass is not the only thing that has aged. Second, on an older unit the matching glass frequently no longer exists. Both need weighing before the cheaper-looking option turns out to be the false economy.
The frame-condition test that actually decides it
The question is not really “glass or whole window”. It is “is the frame worth keeping”. If the frame, the hardware and the flashing have years of service left, new glass is a sensible repair. If they do not, new glass is money spent propping up a window that is going to fail around it anyway. Here is how to judge the frame, all of it from inside the room.
Press the bottom rail of the sash
Rot starts at the bottom rail of the sash, where water sits. Press it firmly with your thumb. If the timber gives, it has gone, and once rot is in the sash it is generally in the frame too, because the same conditions produced both. A soft rail settles the question on its own: new glass in a rotten frame is wasted.
Work the handle and the hinges
Open and close the window a few times. If the sash is stiff, will not hold its angle, or no longer pulls down tight onto the gasket, the mechanism is worn. Hardware is sometimes obtainable up to about 20 years old and often not beyond, so a tired mechanism on an old window rarely has a fix waiting.
Check the reveal and the age
A cold, mould-prone reveal points to a failed or missing insulation collar, a whole-window issue new glass will not touch. And note the manufacture date on the sash plate. A unit past 15 to 20 years has aged flashing beneath it whatever the timber looks like from inside.
Why re-glazing usually is not available on the window that failed
This is the part that turns a cost comparison into a straight answer. Roof window generations change roughly every 10 to 15 years, and each change moves the glazing bead and the glass thickness. A sealed unit is sized to its generation, so a replacement pane for a 2005 window is not the same component as one for a current model even at the identical size code.
For a unit old enough to have blown, and misting typically appears at 15 to 20 years, that matching sealed unit is usually discontinued. The bead detail it clips into and the thickness it was made to no longer exist in the catalogue. So the cheaper-looking option often is not on the table: you cannot re-glaze what nobody makes glass for. This is the same obsolete-parts problem set out on our replace not repair page and, for the glass specifically, our misted glazing answer.
The honest conclusion
Put the two facts together and the picture is clear. On a unit old enough to have misted, the sealed unit is usually no longer available, and the frame timber and the flashing are 15 to 20 years old and near the end of their service. Spending on glass there, even if you could source it, buys a season on a window whose frame and weathering details are done. Full replacement, with a new frame, new insulation and underfelt collars, and a flashing kit matched to your covering, is the route that actually ends the problem, and it carries a 10-year workmanship guarantee.
A glass-only swap only makes genuine sense on a young window with a sound frame: no rot in the rail, a mechanism that still seals, a warm reveal, and a generation recent enough that the matching unit is still made. That window exists. It is just rarely the one that has failed, because a young sound unit does not usually mist in the first place. When the numbers do favour re-glazing, a specialist glazier is the right call rather than us, and we will tell you so at survey rather than sell you a replacement you do not need.
What it means locally
The Ipswich pattern makes the answer even plainer. A great many roof windows went into loft conversions across the town between 2000 and 2010, so the units misting now are almost all past the point where a matching sealed unit can be had. On the coast at Felixstowe and Harwich the frames age faster still under salt, so the frame test tends to fail before the glass question is even reached. Inland, at Kesgrave and Martlesham, the timber lasts longer, but the generation gap on parts is the same wherever you are.
If you want the full cost picture for the replacement route, our replacement cost page sets out the installed figures by size, and the roof window installation hub covers the whole approach.
Request a fixed quote for a roof window replacement in Ipswich, Suffolk or north Essex, and we will give you an honest read on your window at survey.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.