The short answer
There is no single figure, because the life of a sealed glass unit depends on how well it was made and fitted, how much weather it faces and how well water drains away from its edges. A well-made unit in a well-fitted, well-drained frame can serve for many years. The glass itself rarely fails; the seal around it is what gives out.
What actually wears out
People asking about rooflight glass lifespan are usually picturing the glass itself. Glass does not wear out. What ages is the sealed unit it sits in: the spacer bar, the primary and secondary seals, and the drying agent that keeps the gap dry. When those fail, moisture enters, the unit mists, and its insulation drops. The sealed unit life is therefore the life of its seals.
That is also why two rooflights of the same age on the same house can be in very different condition. One may have good drainage and a gentle position, while the other sits in a damp kerb facing the afternoon sun.
What shortens or lengthens the life
Several factors work together, and most of them are fixed on the day of installation.
- Quality of the unit. A unit with a warm-edge spacer and properly applied secondary sealant lasts better than a basic one. Makers' warranties are a guide to their own confidence, but read what they cover.
- Exposure. A rooflight takes more sun and rain than a wall window. Strong sun swings the temperature of the glass through a wide range every day, and the seals flex with it.
- Drainage. A frame that cannot shed water leaves the edge of the unit sitting in damp. Good drainage paths protect the seals.
- Fitting. A unit fitted square, with correct packers and no pinching, is spared uneven stress. A twisted or overtightened unit tends to fail early.
- The frame. A frame that moves, rots or corrodes breaks the seal from the outside.
- Roof type. Flat roofs hold water around a rooflight kerb. Pitched roofs shed it, though the slope can face strong sun.
Because there is no honest single number, be wary of a supplier who offers one without looking at your roof. The sensible approach is to match the unit to its position: a larger south-facing pane over a kitchen needs a better-made unit than a small north-facing window over a landing, and the choice of specification should reflect that. We explain the reasoning in the quote, so you can see why one glass is recommended over another.
Signs the unit is nearing the end
Failure announces itself gradually. Watch for:
- a faint haze or droplets that appear between the panes on cold mornings and fade by midday;
- a persistent cloudy patch near an edge or corner;
- a room that feels colder under the glass than it used to;
- mineral staining on the inside faces of the glass;
- discoloured or cracked edge seals visible round the glass.
The first appearance of mist is the point to act, because from then on the insulation has already fallen. Our mist explainer describes what is happening inside the unit.
How long the frame and flashings last
The glass may not be the part that decides when the job is done. The frame, flashings and kerb age at their own pace, and a frame that has outlived two sets of glass is not unusual. When we replace failed glass we check the frame and flashings at the same time. If they are close to the end of their life, replacing the whole unit usually makes more sense than fitting new glass into a frame that will need replacing soon. Our guide to choosing new glass or a new unit covers that decision.
Coventry's mix of housing matters here. Interwar semis, Victorian terraces and newer extensions each have different roof pitches, coverings and drainage, and a rooflight that suits one may need a different detail on another. We take that into account when we choose the unit and the flashing kit, so the glass has the best chance of a long life where it sits.
Getting the most from a new unit
You can help a new unit last in a few simple ways:
- have the roof around it kept clear of leaves and moss by someone with safe access;
- where it can be reached safely, clean the outer face with water and a soft cloth, never abrasives;
- ventilate the room below, because lower humidity means less moisture trying to find its way into the seal;
- check the room side occasionally for staining or damp, which may point to a flashing problem before it grows.
What we do and what we promise
The free survey looks at the condition of the unit, the frame and how water leaves the roof. A fixed written quote follows before any work starts. A replacement goes in with new flashing and seals on the roof, and where a whole unit is replaced we splay the reveal inside if the roof depth permits and finish it in plaster. Replacing the whole unit is notifiable building work, so Building Control is told by us. Part L governs the new unit, and the work follows current Building Regulations.
Our own commitment is a 10-year workmanship guarantee on the work we carry out. That covers our fitting, flashing and sealing, and it sits alongside whatever the glass maker provides on the unit itself.
