Signs a rooflight needs replacing

This is the sign most people notice first. A cloudy patch, a fogged corner or fine droplets that appear and disappear with the weather, always between the two layers of glass and never on a surface you can wipe, means the seal at the edge of the sealed unit has let moist air in. The unit has lost the dry gap that insulates, and it will not clear on its own.

On its own, a misted pane is a glass problem, and a new unit in a sound frame is often the answer. It becomes a sign the whole rooflight is due when the misting appears in several panes at once, or in a frame that shows other wear from the list below. We go through the causes in why rooflights mist between the panes.

Condensation on the room-facing glass is a different thing. It forms when warm, damp air meets a cold surface, and it wipes off. A little on a winter morning in a bathroom or kitchen is ordinary. It is worth noticing when it is heavy, when it runs onto the sill, or when it is worst at the edges of the glass.

Frequent heavy condensation points to cold glass or a cold frame, which is a thermal weakness in an old unit or a frame without a thermal break. Water running onto a plastered sill stains it, softens paint and, over years, can work into the lining. If the glass is single, early double glazing, or a unit with a plain aluminium spacer, a modern replacement behaves very differently.

Look at the lining and the ceiling around the opening, because they show what is happening where you cannot see it. Tide marks, brown or yellow patches, bubbled paint, black spotting at the sill or hairline cracks running away from the corners tell a story.

A ring of stain below the frame

Often condensation that has run down the glass for a long time, or cold air meeting damp air at an uninsulated sill.

A stain that grows after rain

Water getting past the flashing, the kerb or the seal at the frame edge. That is a weathering problem at the roof, and a sign the frame and its surround should be looked at together.

Soft or crumbling plaster

Moisture has been in the lining for some time. In a lime ceiling, softening can be a sign of a sealed modern paint trapping damp, which is a finish issue as well as a roof issue.

Photograph any stain before and after wet weather so the pattern is clear. It will help us tell cold-surface condensation from water coming in from outside, and it goes onto the finish schedule so the ceiling is treated properly when the work is done.

The frame is the part that decides whether the rooflight has years left. Pine frames need their paint or lacquer to stay sound, and once the finish fails on the outside edge, water gets in at the bottom rail and the corners. Aluminium and steel frames last longer, but corrosion shows as white powder or rust bloom at joints, and a lantern's glazing bars can shift when the fixings or the kerb below move.

• Press the bottom of a timber frame with a screwdriver handle. If it gives, the timber is soft.
• Look for open joints at the corners, or a gap between the sash and the frame that is wider on one side.
• Check that daylight does not show round the closed edge of an opening rooflight.
• On a lantern, sight along the ridge and the glazing bars for dips, bows or uneven gaps.

Any one of these on its own may be fixed by replacing a component. Several together mean the frame has reached the end of its useful life, and a new sealed unit would be wasted on it.

Rubber and foam seals harden with age and sunlight. A seal that has gone brittle, cracked or flat no longer presses against the sash, and the first sign is draughts, rattling in wind or a whistle round an opening rooflight. Hinges and stays wear too. If a window is stiff to open, will not stay where you put it or does not close square, the mechanism is struggling, and an electric opener that strains or stops is working against a frame that has moved.

These are the details people put up with for years, because each is small. Together they add up to a rooflight that lets heat out and noise in. Where the seals and the gear are both worn, replacing the whole unit is generally more sensible than replacing parts.

Glass is stable, but the materials around it are not. Plastic glazing such as polycarbonate or acrylic, common on older flat rooflights and domes, yellows, crazes and goes brittle under sunlight, and fine surface cracks spread over time. A dome that has gone cloudy and brown has stopped doing what it was installed to do.

Cracked glass is a different category. A crack from a stone, a branch or thermal stress should be dealt with promptly, and a cracked outer pane means the sealed unit will soon fail anyway. If you have an unlaminated inner pane overhead, it is worth bearing in mind that laminated glass is the better choice when it is replaced, because it holds together if it breaks.

Age on its own is a reason to look hard, though not a verdict. Many rooflights fitted before the 2010s have single glazing or early double glazing without a coating, with aluminium spacer bars and no thermal break in the frame. They work, but they lose heat, run cold at the edges and gain a lot of heat on a sunny roof.

Building Regulations set a standard for replacement rooflights through Part L, which means that a new rooflight will perform markedly better than one fitted a generation ago. A rooflight that is cold to the hand in winter, hot to stand under in summer and cloudy at the edges is telling you something about its build. That is a better reason for replacement than any single fault.

One sign deserves its own mention, because it only shows up after someone has tried a smaller fix. If you have had a misted unit changed, or the outside cleaned down by a previous owner, and the stain at the sill or the corner of the ceiling comes back, the glass was not the cause. Something else is carrying water or cold into the opening, whether that is the flashing, the kerb, the frame joint or an uninsulated lining.

At that point replacing the whole rooflight and forming the lining again is usually more sensible than another round of partial work. The roof side is dealt with by flashing and weathering that suit the roof covering, whether slate, tile or a flat roof membrane. The inside is dealt with by insulating the lining and making the ceiling good in the right material, which in an older Devon house may be lime. Painting over the mark never lasts, because the cause is still there.

It helps to keep a short note of dates: when you first saw the mark, what the weather was doing, and whether the room was in use. That sounds fussy, but it is the quickest way for a surveyor to tell condensation from water coming in.

Some clues only appear in the room, and people read them as decorating problems. A paint line round the lining that keeps cracking is the plaster moving against the frame, often because the lining was not insulated or fixed well in the first place. A cold patch on the ceiling close to the opening, which you can feel with the back of your hand, is a thermal bridge: the frame or the lining is letting heat leave the house.

In a loft room with a rooflight over the bed or the desk, the draught from a worn seal can be felt on your face on a windy night. In a kitchen extension with a lantern, a room that overheats by early afternoon and cools quickly at dusk is showing you glass that offers little control of solar gain.

None of these prove the rooflight has to go, but they add weight to the other signs. When three or four are present together, the case for replacing the whole unit is strong, and a survey can price the whole job with the finish inside written down.

A few plain questions sort most cases before anyone visits. How old is the rooflight, and do you know who fitted it? Is the problem in one pane or all of them? Does it change with the weather, the time of day or whether the heating is on? Has anything been done to it before, such as new glass or fresh paint on the frame?

The answers shape the survey. A rooflight under ten years old with one misted pane is very likely a glass job. One that is twenty years old, with a worn frame and a stain that moves, is very likely a whole replacement. Between those two lies a lot of ground, which is why a person looking at the frame, the roof and the ceiling together is worth more than any checklist, including this one.

Take photographs from inside and outside, in daylight, and note when the misting or marks appear. A photograph of the label on the frame edge helps with sizing. Then ask for a survey rather than a guess, because the right answer depends on the frame, the roof, the ceiling and what you want from the room.

A survey tells you whether a new unit is enough or whether the whole rooflight should come out. If it is a glass change, replacing just the glass in a rooflight sets out what to expect. If the whole thing is going, remember that a full replacement is notifiable under the Building Regulations, and that we handle the Building Control notification. The ceiling matters too, and replacing a rooflight without damaging the ceiling shows how it is protected.

Every quote comes with a finish schedule that records the ceiling, the lining and how it will be finished, so the room looks complete when the new rooflight is in. Read the whole picture on our misted and failed rooflight replacement page, or ask for a quote and send us your photographs.