An old roof window is usually better replaced than repaired for three reasons that stack up together: the parts you would need are often obsolete, rot in one part means rot in the parts you cannot see, and the thermal performance has fallen so far that no repair recovers it. Once a window is old enough to be failing, repair tends to buy a season while the underlying problem carries on.
Apex does not offer repair, resealing or component swaps on roof windows. That is not a sales position. It is what the three reasons below lead to every time on a unit that is genuinely worn out. Here is the honest case, reasoned rather than dressed up in invented figures.
Reason one: the parts have gone
Roof window generations change roughly every ten to fifteen years, and each change moves the frame profile, the hardware fixing points and the glazing bead. Parts for a current-generation window are readily available. Parts for the previous generation are around for a while after it is discontinued. Parts for anything older than about twenty years, particularly pre-2000 units, are largely gone.
That includes the sash, the hinge set, the gaskets, the handle assembly and the sealed unit itself, because the glass thickness and bead detail changed with the profile. We have opened up 1990s windows on Ipswich terraces where every single component was technically repairable and not one of them was obtainable. A repair you cannot source parts for is not a repair. The parts side of this is set out in more detail on our obsolete VELUX window page.
Reason two: rot does not stay in one place
Older roof windows are laminated pine with a lacquered or painted internal finish and an aluminium capping outside. Rot starts at the bottom rail of the sash, where water sits longest and the finish fails first. Press the bottom rail with your thumb, and if it gives, the timber has gone.
The trouble is that the conditions which rotted the sash, poor ventilation, an unmaintained finish, years of trapped moisture, acted on the frame at the same time. So once rot is in the sash it is generally in the frame too, and the frame is the part built into the roof that you cannot swap without taking the window out. Replacing a rotten sash on a sound frame is occasionally reasonable. Replacing a rotten sash on a frame that is quietly rotting behind the capping is money spent on a window that fails again from the part you did not touch. The progression is covered on our rotten VELUX frame page.
Reason three: the thermal performance has collapsed
A sealed unit from the mid-2000s started life at roughly 2.8 to 3.2 W/m2K. Once it has blown, the argon has escaped and the low-emissivity coating is degrading, so the real figure now is worse than that. A modern replacement is around 1.1 to 1.3 W/m2K. That is not a marginal improvement, it is a different order of heat loss, and no repair to an old unit recovers it because you cannot re-gas or re-coat a failed unit.
Part L makes the point in regulation. A replacement rooflight in an existing dwelling has to meet a whole-unit U-value of 2.2 W/m2K or better. A blown old unit does not, and cannot be brought back to it. Replacement is the route that both fixes the problem and meets the standard.
What repeated repair attempts actually cost
We will not invent a statistic here, so reason it through instead. Each repair on an old window carries the cost of access, a scaffold or tower, a tradesman’s time, and whatever part could be found or fabricated. That is rarely trivial once a roof is involved. If the part lasts a season or two and then the next component in the same worn window fails, you pay the access cost again for the next repair. Two or three of those, and you have spent a meaningful share of a new window without ever fixing the frame, the flashing or the U-value.
Meanwhile the clock on the parts availability is running down, so a repair that is just about possible this year may be impossible next year, at which point you replace anyway, having spent the repair money first. The logic points one way on a genuinely old unit: the repair spend is a deposit on a replacement you will make regardless.
Where repair does make sense, and why we still do not offer it. On a recent window, a single worn friction pad or a handle is a fair repair, and a good glazing firm may do it. We do not, because roof window replacement is the whole of our trade and repair is not something we offer. What we will tell you honestly at survey is whether your window is young enough that a repair is worth chasing elsewhere, or old enough that replacement is the sensible call. The timing judgement sits on our when to replace page.
What it means for your window
If your window is misting, the misting itself is only a symptom, and whether it is worth acting on depends on which side the moisture is, which we cover on misted VELUX glazing. If it is rotting, the rot has usually spread further than the visible part. If it is old and performing poorly, the U-value case decides it on its own. In practice all three tend to arrive together on a fifteen to twenty year old unit, which is why replacement so often wins.
When you do replace, you get the whole thing sorted at once: new unit, new insulation and underfelt collars, flashing matched to your covering, and a 10-year workmanship guarantee. If cost is the sticking point, our glass versus whole-window cost page explains why the whole window is the better value even when only the glass has visibly failed. Roof window installation and replacement is all we do.
Request a fixed quote and we will give you an honest read on whether your window is worth keeping or ready to replace.
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.