The answer
Replace a skylight when the fault is in the unit itself and not in something around it. Misting between the panes means the glazing seal has failed. A frame that is soft, rotten or warped, glass that is cracked, and a sash that no longer closes tight are all signs the unit is finished. A leak is less clear cut, because the cause may be the flashing or the roof covering and not the window. There is no set age. The decision turns on what the survey finds, and on whether a new unit would also fix the cold and condensation the old one gives you.
Signs the glazing has failed
The commonest reason to replace a skylight is the sealed glazing unit. Two or three panes are bonded together around a spacer, with a gas or air gap between them. Over the years the seal at the edge can break, and moist air gets into the gap.
You see it as a grey haze, droplets or streaks between the panes that no cloth will reach. It tends to appear on cold mornings and clear in warm weather, which is why it is often put down to condensation. Condensation forms on the inside or outside surface and wipes off. Misting inside the sealed gap is permanent and gets worse.
A misted unit still keeps the rain out, but it no longer insulates as designed, and the cloudy glass dims the very daylight you fitted it for. On a dull November afternoon that matters more than in July. Our page on replacing misted rooflights covers how a glazing swap is done.
Cracked or chipped glass and scratched acrylic domes belong in the same group. A crack lets moisture into the unit, and old plastic domes can yellow and turn brittle with years of sunlight.
Frames, sashes and seals
The frame carries the glass and holds the weatherproofing, so a frame that has gone is a reason to replace the whole unit, not just the glass.
On timber roof windows look for soft spots, peeling paint that exposes bare wood, and dark staining at the bottom edge, where water collects. A screwdriver pressed into rot tells you more than a glance. On a metal or plastic frame the signs are corrosion, a warped profile and gaps at the corners.
Then there is the way it works. A sash that sticks, will not stay open, or leaves a draught when closed has lost its seals or its hinges. Those parts can wear out well before the glass does. Look for daylight at the edges of a closed sash, and a cold stream of air on a windy day.
Opening mechanisms have a life of their own. A pole-operated or cord-operated window that has become stiff may only need a new part, but if the model is old, the parts may no longer be made, and a new unit is the practical answer.
Leaks: the unit or the roof?
A drip from a skylight does not prove the skylight is at fault. The water often arrives from the flashing, the upstand or the roof covering around the unit and then runs to the lowest point, which may be the window frame.
That is the reason a survey matters here. On a pitched roof the flashing kit that seals the window into the tiles or slates can fail on its own, or the tiles around it can slip. On a flat roof the membrane dressed up the kerb can split at a corner, and the unit sits perfectly sound in the middle of it.
A few clues help. A leak that appears only in driving rain from one direction points at a flashing. A stain that begins at the top of the frame and spreads down points at the roof above. Water running down the inside of the glass is more likely condensation than a leak.
If the old unit has leaked more than once, or sits in a roof covering at the end of its life, replacing the unit and the flashing together is often more sensible than treating the symptom. The written quote says exactly what is being replaced.
When cold and condensation are the reason
Some skylights work, in the sense that they are weathertight and open and close, and still make a room miserable. A single-glazed or early double-glazed unit loses heat quickly, and the glass surface can get cold enough for condensation to run down it every morning.
Modern glazing is held to a limit. For an existing dwelling, Approved Document L (Part L, 2021) sets 1.4 W/m²K for roof windows and 2.2 W/m²K for rooflights, and a new unit fitted today has to meet the figure for its type. An older unit may fall well short of that, particularly if it is single-glazed.
The comfort gain is easy to notice from the room. A better-glazed unit sits closer to room temperature, so the cold down-draught you feel standing underneath it eases, and the glass stays clear in the mornings more of the time. In a winter room that is the difference between sitting under a skylight and avoiding it.
Replacement for this reason is a judgement call, and no fault has to be present. If the room is uncomfortable for months of the year and the unit is old, the case for swapping it is strong, and we say so in the survey if we think so.
What to expect when you decide to replace
Replacing a skylight starts with the same step as installing one: a free survey. We look at the unit, the frame, the flashing and the roof around it, and we measure the opening. If the fault is in the unit, we say so. If it is in the roofing around it, we say that instead, and the quote covers what the roof needs.
If the opening and its upstand are sound, a new unit can often go into the same position, so the ceiling stays as it is. If the opening is the wrong shape or the roof has changed, the job resembles a new installation. In both cases the price is fixed in writing after the survey and does not change on the day.
A replacement rooflight counts as controlled work under the Building Regulations, so the same rules apply as for a new one, including the energy limits above. Any Building Control notification the job needs is made by us.
The work comes with a 10-year workmanship guarantee in writing. If you are comparing the old unit with a different type, such as a flat rooflight in place of a dome, ask about it at the survey, and our guide to choosing a skylight for a dark room sets out the options. For costs in general, see what affects the price.