Should You Reseal or Replace a Failing Roof Lantern?

Resealing a failing roof lantern typically buys twelve to eighteen months. It does not address failed sealed units, perished gaskets, corroded fixings or wet timber in the upstand, which is usually the real problem by the time water shows inside. If the lantern is over fifteen years old, has more than one misted pane, or has leaked in the same place twice, resealing is a payment plan for a replacement you will buy anyway. We do not offer resealing, and this page explains the arithmetic behind that.

The honest version of this trade is that patching is easy money. A reseal takes half a day, costs a few hundred pounds, and the customer is happy for a year. We would rather tell you what a reseal actually is.

What a reseal does and does not touch

Reading down that column is the whole argument. A reseal addresses one failure mode out of seven, and only for a while. The reason it is offered so often is that it is the only intervention that can be done from outside in an afternoon without lifting anything.

The arithmetic over five years

A reseal costs a few hundred pounds and buys roughly twelve to eighteen months on a unit that has genuinely started to fail. Do it three times over five years and you have spent a meaningful fraction of a replacement, and at the end of it you own the same twenty-year-old lantern, now five years further into decay, with five more years of water in the upstand timber.

The running cost sits on top of that. A lantern from the mid-2000s has a whole-unit U-value somewhere around 2.8 to 3.2 W/m²K. A modern thermally broken unit is 1.2 to 1.4. Over a 4m² lantern that difference is heat leaving a kitchen every hour of every winter, and it does not appear on a repair invoice.

Then there is the part nobody prices. Water that has been getting into an upstand for three years has been in contact with timber the whole time. Sealing the surface does not dry it out; it seals the moisture in. When the lantern finally comes off, the upstand rebuild is bigger and sometimes the deck around it has gone too. Delay makes the eventual job more expensive, not less.

The tests that settle it

Five checks, and any two of them together mean the unit is past economical repair.

1. The age test. Over fifteen years, the gaskets are at end of life across the whole unit regardless of what the frame looks like. Over twenty, the sealed units are on borrowed time too.
2. The second pane test. One misted pane can be bad luck. Two means the units were made in the same batch, have had the same thermal cycling, and the rest are close behind.
3. The repeat leak test. If water has come in at the same place twice, the first intervention did not find the cause. A third attempt usually does not either.
4. The fixing test. Rust staining running down the frame from fixing heads means the fixings are pitting. Corrosion product expands and forces joints open, so the leak path is being actively widened.
5. The upstand test. Press the internal reveal at the base of the lantern. Soft, discoloured or damp plasterboard means water has been present long enough to reach the timber, and no surface treatment reaches that.

You can do the first four from the ground and from inside. The fifth is what we check at survey, and it is usually the one that decides the answer. Our page on when the upstand is the problem, not the lantern goes into what we look for.

What a replacement actually involves

We lift the old unit off, expose the upstand and inspect the timber properly, photographing what we find. Where the upstand is sound we rebuild the covering detail and set the new lantern on it. Where it is not, and on failed lanterns it frequently is not, we rebuild it, which is the only intervention that ends the problem rather than deferring it. The new unit goes on with a 10-year workmanship guarantee behind it.

On a like-for-like replacement with a sound upstand that is a two-day job, and the roof is open for hours rather than days. Modern profiles rarely match older kerb dimensions exactly, so some adjustment of the upstand footprint is normal even when the timber is fine.

Suffolk and north Essex context

A large number of lanterns went onto extensions across Ipswich, Kesgrave, Rushmere, Colchester and the surrounding villages between 2003 and 2010. Those units are now fifteen to twenty-three years old and they are reaching end of life together, which is why the resealing question comes up so often here at the moment.

On the coast the timeline is shorter. At Felixstowe, Harwich, Frinton, Aldeburgh and Southwold, chloride attacks fixings and degrades gasket surfaces years earlier than inland, so a coastal lantern of the same age is usually in worse condition than its inland equivalent. Resealing a corroded coastal unit is the least durable version of an already short-lived intervention. Estuarine locations, Woodbridge, Wivenhoe, Manningtree and Shotley, sit between the two.

Related questions

• When should a roof lantern be replaced? gives the full checklist of trigger signs.
• How much does it cost to replace an existing roof lantern? puts figures on the alternative.
• Why do roof lanterns leak at the corners? explains why the sealed joint fails first.

See also diagnosing where the water really gets in and misted and blown glazing, under roof lantern installation.

Get an honest assessment

We survey the whole installation, including the upstand, and tell you what we find with photographs. We do not offer resealing or patching on any rooflight, so there is no version of this where we benefit from talking you into one. Request a fixed quote.