The short answer
A well-made roof lantern with an aluminium frame will usually outlast the sealed glass units inside it and the flat roof it sits on. Frames commonly stay sound for several decades, the glass units for somewhere between fifteen and twenty-five years, and the roof covering around the base for a similar span to the roof itself. What shortens a lantern's life is rarely the frame. It is a poorly detailed upstand, a flat roof that ponds, or a failed unit left in place.
So the useful question is which part of the lantern you mean, because the parts age at different rates and are replaced in different ways.
The frame and its finish
Aluminium does not rot, rust through or swell, and a thermally broken frame with a good powder coat can look much the same after twenty years as it did on day one, give or take some dulling on south and west faces. The coating is the part that ages: chalking, fading and, near the coast along the Exe estuary or the Exmouth seafront, some surface pitting if salt is never washed off.
Timber lanterns are a different matter. Hardwood frames can last a very long time, but they need repainting or restaining on a cycle that depends on how exposed the roof is. A lantern over a kitchen island, with warm steamy air rising into it, will work its finish harder than one over a hallway.
uPVC frames sit lower on the scale. They resist moisture, but sunlight and heat can yellow white plastic and make the profiles move slightly, so a lantern in full sun may look tired before it fails. For the comparison in full, see aluminium or uPVC roof lanterns.
The sealed glass units
This is the part most likely to go first. A sealed unit is two or three panes held apart by a spacer and bonded with a perimeter seal, with a gas fill or just air between them. Over the years that seal ages, and the first sign is condensation or a hazy film between the panes that cannot be wiped off. Overhead glass works harder than a vertical window: it faces the sky, so it takes the full swing of heat and cold, and water sits on its low-angle faces for longer.
A warm-edge spacer and a well-supported unit help. So does a lantern that is allowed to drain, because standing water at the glazing bar edges leans on the seal. When a unit does fail, the frame is normally fine. Replacing the glass alone is a smaller job than replacing the lantern, and it is not notifiable to Building Control when only the sealed unit changes in an existing frame. Our page on misted and failed rooflight replacement sets out how that works.
Gaskets, glazing bars and sealant
The softer parts wear before the hard ones. EPDM gaskets harden and shrink after long exposure to sun, and external sealant lines crack. Neither ruins the lantern. They are the parts a tradesperson would expect to renew once or twice over the life of the frame, and a lantern designed with mechanical glazing bars and drained, ventilated rebates copes with them better than one that relies on sealant alone.
Check on a dry day from the ground, or from inside, for these signs:
- a cloudy band or droplets inside the glass edge;
- staining on the ceiling lining near the base of the frame;
- a gasket that has pulled back from the glass;
- powder coating that has bubbled or flaked at the corners.
The roof and upstand underneath
A lantern is only as long-lived as the roof it stands on. The upstand, or kerb, is a box built up from the deck, insulated and covered in the same membrane as the roof. If that covering reaches the end of its life, the lantern has to be lifted to reroof properly, whatever condition the lantern is in. Roof coverings differ widely: GRP and EPDM can run for decades when laid well, while an old felt roof will often need renewing sooner. Ask the age and type of your roof before you buy a lantern, and if the roof is near the end of its life, do both together.
Timing matters because the cost of access, scaffolding and making good inside is shared when both jobs happen at once. Details of how the base is built are in our guide to roof lantern upstands and kerbs.
What makes a lantern last longer
Most of what extends a lantern's life is decided before it is ordered.
- A proper fall
- The roof around the lantern should shed water towards the gutters or outlets, not hold it against the upstand. Ponding shortens the life of the membrane and the glazing seals alike.
- Good ventilation inside
- A lantern over a kitchen sees humid air. A well-ventilated room and a laminated or low-emissivity inner pane keep condensation off the frame surfaces where it would otherwise sit.
- Clean glass and clear drains
- Leaves and moss collect in the gutters and gaps around a lantern in the tree-lined streets of Heavitree and Pennsylvania. A gentle wash with a soft brush and water, a couple of times a year, keeps coating and gaskets in better shape.
- Specification fitted to the roof
- A lantern on an exposed roof needs a frame, glazing bars and fixings chosen for that exposure.
The finish inside ages too
People rarely think about the inside of a lantern's life, but the lining and the plaster beneath it are on view every day. A lining finished with the correct materials, with crisp beads and a sealed paint line, will keep its edge for years. One that was boarded in a hurry and left bare tends to crack at the corners within a season or two, particularly where the ceiling flexes. If you are replacing a lantern, that is an opportunity to look at the lining as well. See roof lantern ceiling finishes for the options.
When to replace rather than keep going
Replace the glass when the units have misted but the frame is straight, the coating sound and the roof beneath it in good order. Replace the whole lantern when the frame is corroded or distorted, the glazing bars no longer hold the glass firmly, or the roof needs relaying and the lantern has to come off anyway. Taking out and renewing a whole lantern, frame and glass, is notifiable under the Building Regulations, as a controlled fitting that has to satisfy Part L. The notification is ours to make, and the work carries a 10-year workmanship guarantee.
If you are weighing it up, a survey will tell you which parts are worth keeping. Send us photos through the quote form, and the written quote will come with a finish schedule that records the new lining and ceiling finish. For the broader picture of what is involved, start with our roof lantern installation page.