Signs a rooflight needs replacing

1. Misting between the panes

Haze you cannot wipe off from either side. The seal has failed. Not urgent, not a leak, and permanent. The unit has lost most of its insulating value.

2. Condensation on the room side that never used to happen

Often not the glass at all. A blocked ventilation filter is the most common cause, and it takes two minutes to clean. If cleaning it does not help, the glazing may be at the end of its life or the perimeter detailing may be the issue.

3. Draughts

Hardened or split gaskets. Usually a replaceable part rather than a new window, and worth doing.

4. Staining around the reveal

Tide marks, discoloured plaster or peeling paint at the top of the reveal. This means water has been getting in for a while, from outside or as condensation inside the build up.

Worth investigating promptly, because it usually means the timber around the opening is getting wet.

5. Stiff or noisy operation

Usually hinges wanting lubrication. If it persists after that, the frame may have moved, which suggests the opening or the timber has shifted.

Two that mean act now

Soft timber. Press the frame around the bottom of the sash and the reveal. If it gives, water has been getting in and delay makes it worse and more expensive.

Active water ingress. Any water actually entering during rain. That is not a planning matter.

The age test

If it went in more than twenty years ago and shows any of the above, replacement is usually better value than repair, because you get current glazing, new seals and new hinges rather than one part renewed on a worn window.

Seven things worth acting on

Rooflights rarely fail suddenly. They give warning, and the signs below are the ones that mean the unit is at or near the end of its life rather than needing a clean or an adjustment.

Misting inside the cavity

Moisture in the cavity that cannot be wiped off either face. The perimeter seal has failed, the argon has gone and the coating is degrading. Not urgent, not repairable, and it does not improve.

2. A sash that will not close properly

Check the gasket first, because grit and moss on the compression seal hold a sash a fraction open and that is easily cleaned. If the seal is clean and the sash still catches at one corner, the frame is out of square, which on an old unit usually means it has moved or the timber has gone.

3. Soft timber

Press the frame at the sill and the lower corners. Timber that gives under a thumb is decayed, and decay in a window frame does not stop. This is the sign that turns a comfortable decision into an urgent one.

4. Water on the reveal every winter

If it is on the plaster rather than the glass, it points at a missing insulation collar. That can be corrected without replacing the window, but on a unit already twenty years old it is usually the moment to do both.

5. Cracked or blown seals and gaskets

Perished rubber that has gone hard or split. Replaceable on some models and, at fifteen to twenty years, generally a sign the rest is the same age.

6. Draughts

A window you can feel across the room. Usually gaskets, sometimes a frame that has moved, and on an old unit it is rarely worth chasing separately.

7. Age plus company

The one nobody thinks of as a sign. If a unit is over twenty years old and there are others of the same age on the same slope, they are all in the same window. Access is a fixed cost shared across everything done in the same visit, so doing them together is meaningfully cheaper than doing them one at a time as each fails.

What is not a sign

Condensation on the inner face of the glass on cold mornings, which is a ventilation matter. Rain noise, which is normal. Thermal ticking as the frame moves, which is also normal.

Where to start

Photograph the data plate on the sash. From that we can tell you what the unit is, how old it is likely to be, whether the size is still made and what the options are, without anyone visiting.

The unambiguous ones

Misting between the panes. The sealed unit has failed. Not urgent, and not repairable. The staining that builds up as moisture condenses and evaporates repeatedly is permanent, so leaving it indefinitely costs you light as well as heat.

Soft timber in the frame or sash. Push a fingernail into the corners. Rot in the frame is the end of the window.

A sash that no longer closes evenly against its gasket. Frequently a distorted frame, and it means draughts and eventually water.

Water coming in during rain that is not explained by a blocked drainage channel or debris on the sill.

The ones worth watching

A hardened, cracked or flattened gasket that no longer recovers after compression. Sometimes replaceable, and on an old unit it is a sign the rest is the same age.

Hinges that have become stiff, or a sash that no longer holds position. Adjustable at first, then not.

Lifted or corroded external cladding, which stops protecting the timber underneath.

Condensation on the inner face of the glass in conditions where it did not use to happen, which suggests the unit has lost its gas fill even if it has not yet misted.

The age arithmetic

Sealed units last fifteen to twenty five years. Under fifteen with a sound frame, reglazing is usually right. Over twenty, hinges, gaskets and mechanism are near the end too, and new glass in an old frame buys a few years before everything else needs doing.

Since access is frequently the largest cost, and it is identical either way, that arithmetic usually favours replacing the whole unit on an older window.