Roof lanterns corrode near the coast because airborne chloride from salt-laden air settles on every exposed surface and attacks the passive oxide layer that protects stainless steel and aluminium. Standard A2 grade 304 stainless fixings pit and rust-stain within two or three winters inside about a mile of open sea. Standard powder coating chalks and blisters. The answer is 316L stainless fixings throughout and a marine-specification coating system, specified at order rather than added later.
This is not a marginal effect. It is the single most visible early failure on coastal lantern installations in Suffolk and north Essex, and it is entirely avoidable at the specification stage for a modest cost.
What chloride actually does
Stainless steel is not stainless because the metal does not corrode. It is stainless because chromium in the alloy forms a thin, self-repairing oxide film on the surface that stops corrosion progressing. Chloride ions break that film down locally and prevent it re-forming, which produces pitting: small deep points of attack rather than general surface rust.
Grade 304, sold as A2, contains chromium and nickel but no molybdenum. Grade 316, sold as A4, adds around 2 per cent molybdenum, and molybdenum is specifically what resists chloride pitting. The L in 316L refers to low carbon content, which improves corrosion resistance at welds and in the heat-affected zones around them.
Three coastal conditions make this worse than the chemistry alone suggests:
- Deposition without washing. Salt aerosol settles on surfaces continuously. On sheltered faces, under laps, and around fixing heads, rain never rinses it off, so chloride concentrates rather than dissipating.
- Wetting and drying cycles. Each cycle leaves the salt behind and concentrates the remaining solution. A surface that is permanently wet or permanently dry corrodes more slowly than one that alternates.
- Crevices. A fixing head against a frame, a lap joint, a gasket seat: these are oxygen-starved gaps where the passive film cannot rebuild. Crevice corrosion is why fixings fail before the surrounding metal shows anything.
Galvanic corrosion, the second mechanism
Put two dissimilar metals in contact with an electrolyte between them and you have made a battery. Salt water is an excellent electrolyte. Aluminium in direct contact with steel in a coastal environment corrodes preferentially and quickly, because aluminium is the less noble metal in the pair.
On a lantern this shows up at every point where a steel fixing passes through an aluminium profile, and around any steel structural element near the frame. The correct response is physical isolation: proprietary nylon washers, isolation tape or a coating between the two metals, so the circuit is broken. We isolate rather than hoping, because hoping has a visible failure mode.
The coastal specification
| Component | Standard inland | Within a mile of open sea |
|---|---|---|
| Fixings | A2 grade 304 stainless | 316L stainless throughout, including concealed fixings |
| Frame coating | Standard powder coat | Marine-specification pre-treatment and coating system |
| Upstand height | 150mm minimum | 200mm, because driven rain climbs |
| Flashing laps | Standard laps | Extended laps, mechanically terminated |
| Dissimilar metals | Good practice to isolate | Isolated without exception |
Marine-specification coating means a different pre-treatment and a coating system rated for coastal exposure, which most manufacturers offer as an upgrade rather than as standard. It adds a modest amount to the unit cost and lengthens the finish guarantee considerably. It also adds a week or two to the lead time, which is worth knowing when you are planning.
Concealed fixings matter most. The fixings you can see get rinsed by rain. The ones inside the frame, under the capping and through the upstand do not, and those are the ones holding the lantern to the building. Specifying 316L only for the visible fixings misses the point entirely.
Where this applies in Suffolk and north Essex
The full marine specification applies within roughly a mile of open sea. That takes in Felixstowe, Harwich, Frinton, Clacton, Aldeburgh, Southwold and the rest of the open-coast belt, where there is nothing between the property and the North Sea.
The estuarine towns are a different case and we treat them differently. Woodbridge, Wivenhoe, Manningtree, Shotley and Pin Mill sit on tidal saline water with genuine wind exposure across open water, but they do not carry open-sea salt loading. Upgraded fixings make sense there. Full marine specification usually does not, and we are not going to sell Woodbridge a Southwold specification.
Inland, at Stowmarket, Hadleigh, Bury St Edmunds, Lavenham or Colchester, chloride is not a design factor at all. The constraints there are structural and conservation-related, and paying for marine coating would be money spent on a problem that does not exist.
What to do about an existing corroded lantern
Once fixings have pitted, they do not recover. The corrosion product occupies more volume than the metal it came from, which forces joints apart and opens the weather line, so a corroded lantern typically starts leaking at the corners not long after the staining appears. Replacing individual fixings on a twenty-year-old unit does not address the coating failure, the perished gaskets or the state of the upstand timber underneath.
We replace rather than repair. On a coastal replacement, the new unit goes on in full marine specification, the upstand is rebuilt to 200mm where it needs it, and the installation carries a 10-year workmanship guarantee.
One thing that genuinely helps a correctly specified coastal lantern: rinse it with fresh water once or twice a year. Salt film builds on the glass and the frame and self-cleaning coatings do very little against it. A hose from ground level is enough, and nobody should be going up on a roof to do it.
Related questions
- Why do roof lanterns leak at the corners? covers what corroded fixings do to a joint.
- When should a roof lantern be replaced? lists corroded fixings as a trigger sign.
- What time of year is best for fitting a roof lantern? matters on exposed coastal sites.
See also aluminium roof lantern installation and upstand construction, under roof lantern installation.
Get the right specification for your postcode
We set the specification by actual exposure at survey, not by a blanket rule. Marine specification is included as standard on any job within a mile of open sea. 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.