How Long Does a Rooflight Sealed Unit Last?

Glass does not wear out. What fails in a double glazed unit is the edge seal, the thin band of sealant and spacer that holds two panes apart and keeps moist air out of the gap between them. A desiccant in the spacer soaks up the small amount of moisture that gets in over time, which is how a good unit stays clear for so long. When the desiccant is full and the sealant has aged, the gap fills with room air and the pane clouds.

That means the lifespan of a unit is the lifespan of its edge, and a few parts are involved.

• Edge seal the sealant ring that keeps the gap airtight
• Spacer the strip that holds the two panes apart
• Desiccant the drying material inside the spacer
• Gas fill argon or similar between the panes, for insulation

Better-made units use sound sealant and a good spacer, so they hold on for longer, while cheaper ones tend to give up earlier. Two units of the same size can look identical from the room and still age very differently. That quality gap is invisible once the unit is in the roof, and it is one reason we show the make and specification on every written quote.

A window in a wall is shaded by an overhang and drains water straight away. A rooflight sits flat or at a shallow angle, so it takes the full weight of the sun, collects water along its lower edge and swings through the day's temperature changes with nothing to soften them. Each swing makes the glass and frame expand and contract a little, and over a long run of seasons that works at the seal.

South-facing units in strong sun age their sealant fastest. Flat rooflights that hold water on the glass or kerb give the edge more to cope with. Low pitches drain slowly, and our guide on the minimum pitch for a roof window explains why the angle matters. A steeper slope gives the sealant an easier life, because water runs off before it can sit against the edge. The same reasoning applies to the kerb on a flat roof, where a well-formed fall keeps the lower edge of the glass from standing in damp.

Dark-framed or tinted units can run hotter again, and heat is the quiet enemy of sealant. A shaded rear slope on a Victorian terrace is usually a gentler place for a unit to live than a bright south-facing flat roof on a modern extension. None of this means a unit is doomed, only that two roofs on the same street can age at different speeds.

A unit rarely fails all at once. It gives notice, and the notice is easy to read once you know where to look. Look for these on a bright, dry day, from inside the room, and again on a cold morning when the glass is at its coolest.

1. A faint haze or fine droplets that appear between the panes and do not wipe off.
2. A white or rainbow film on the inner face of the outer pane.
3. Condensation that returns to the same corner each cold morning, even after cleaning.
4. A noticeable chill coming from the glass compared with a newer unit next to it.

Any one of these points to a seal that is letting go, and two together make it close to certain. The detail on each sign, and how to tell them from ordinary surface condensation, is on our page about the signs of a failed rooflight.

When a seal goes, the unit is replaced. If the frame and sash are sound, only the sealed glass unit comes out, and a new one goes into the same frame. If the frame has also aged, a whole new rooflight or roof window is the better spend, and it lets you add an opening sash where air matters, such as in a bathroom, landing or loft room.

Our page asking is a misted rooflight worth replacing helps you weigh the two routes. How each job is carried out sits under the misted and failed rooflights hub, and our 10-year workmanship guarantee covers what we fit.

On terraces in Bristol and Bath stone houses alike, the survey is free and the quote comes in writing, itemised, with opening options shown and nothing to sign. You see the choices before you decide anything.