Replace a roof lantern when any two of these are present: water has come in at the same place more than once, more than one pane has misted, fixings are rust-staining the frame, the frame is visibly distorted or racked, the profile is obsolete so parts are unavailable, or the unit is over twenty years old and has no thermal break. One of those alone may be manageable. Two together mean the unit has reached the end of its working life.
Most roof lanterns fitted to a reasonable standard last twenty to twenty-five years inland, and fifteen to twenty on the open coast. The failure is rarely sudden. It is a sequence of small signs over three or four years, and the useful skill is recognising the sequence early enough that the upstand underneath is still dry.
The six trigger signs
- Repeat leaks in the same place. The single most reliable indicator. A one-off leak from storm damage is an event. Water arriving at the same corner in successive winters means the weathering design has reached the end of its life, not that a spot needs attention.
- More than one misted pane. Sealed units in a lantern were made in one batch and have had identical thermal cycling. When the second one goes, the others are not far behind. One misted pane is bad luck; two is a pattern.
- Corroded fixings. Rust staining running down the frame from fixing heads. The corrosion product occupies more space than the metal it came from, so it forces joints apart and widens the leak path while you watch.
- A distorted or racked frame. Sight along the ridge and along the eaves beams. A visible bow or twist means either the upstand beneath has moved or the frame has been fighting an unlevel base since the day it was fitted. Gaskets cannot seal against a frame that is out of shape.
- An obsolete profile. Lantern systems from the 2000s are frequently out of production, and gaskets, capping bars and end caps are not available. A component you cannot buy is a component you cannot replace.
- No thermal break, and cold to touch. If the internal frame is genuinely cold in winter and runs with condensation, there is no polyamide break in the profile. That is not adjustable. It is what the lantern is.
The checklist
| Check | How to do it | Points towards replacement if |
|---|---|---|
| Age | Extension build date, or ask the previous owner | Over 15 years coastal, over 20 inland |
| Misting | Look up on a bright overcast day | Two or more panes affected |
| Leak history | Think back over three winters | Same location twice or more |
| Fixings | Look for staining down the frame from screw heads | Any rust staining visible |
| Frame line | Sight along the ridge and eaves from the garden | Visible bow, twist or opening joints |
| Internal reveal | Press the plasterboard at the base of the lantern | Soft, damp or discoloured |
| Coating | Rub the frame with a light cloth | Chalky residue on the cloth |
All of that can be done from inside and from the garden. Nobody should be climbing onto a flat roof to run a checklist, particularly not a wet one.
The upstand decides how urgent this is. A tired lantern over a dry upstand is a planned job you can do next spring. A tired lantern over an upstand that has been wet for two winters is a job worth doing sooner, because the timber decay is what turns a two-day replacement into a bigger one.
When you can reasonably wait
Not everything needs acting on immediately, and we would rather say so.
A single misted pane in an otherwise sound unit, with no leaks and no staining, can be left. It is cosmetic and it does not accelerate anything else. Chalked powder coating on its own is also cosmetic; a coating that has lost its gloss is not letting water in. A lantern that is simply thermally poor, cold and expensive to sit under but watertight, is a comfort and running-cost decision rather than a building-fabric one, and it can wait for a moment that suits your budget.
What does not wait is water reaching timber. Once the internal reveal is damp, the clock is running on the upstand.
Suffolk and north Essex timing
The extension boom across Ipswich, Kesgrave, Rushmere, Martlesham, Colchester and the surrounding villages between roughly 2003 and 2010 put a very large number of lanterns onto roofs at the same time. Those units are now fifteen to twenty-three years old and reaching end of life together. If your extension dates from that period, the checklist above is worth running this year rather than next.
Coastal properties run five years ahead of that curve. At Felixstowe, Harwich, Frinton, Clacton, Aldeburgh and Southwold, chloride attack on fixings and coatings brings the fixing and coating triggers forward considerably, so a fifteen-year-old lantern there is often in the condition an inland one reaches at twenty. Estuarine locations, Woodbridge, Wivenhoe, Manningtree, Shotley and Pin Mill, sit between the two with moderate salt loading.
Inland, the deciding factor is more often structural or thermal than corrosive. On the older timber-framed stock across mid Suffolk, a replacement is also the moment to correct an upstand that was never properly insulated, which is worth planning for rather than discovering.
Related questions
- Should you reseal or replace a failing roof lantern? covers the economics.
- How much does it cost to replace an existing roof lantern? puts figures on it.
- What time of year is best for fitting a roof lantern? helps with the planning.
See also when the upstand is the problem and misted and blown glazing, under roof lantern installation.
Get it assessed
We survey the lantern and the upstand, photograph what we find and tell you plainly whether it needs doing now or can wait. We do not offer repairs, resealing or patching. Request a fixed quote.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.