Why Do Roof Lanterns Leak at the Corners?

Roof lanterns leak at the corners because corners are where the most movement happens and where the least material bridges it. Aluminium expands and contracts along the length of every bar, and all of that movement arrives at the corner joint. Gaskets shrink back from mitred ends over time, leaving a gap. In cheaper systems that gap was only ever filled with mastic, and mastic has a service life of around five years. A corner leak is almost never a spot defect.

The distinction matters because it decides what you do about it. A one-off puncture can be patched. A corner leak is the weathering design reaching the end of its working life, and patching it treats the place the water appeared rather than the reason it appeared there.

The four things happening at a corner

Thermal movement concentrates there. A 3m aluminium eaves beam moves roughly 2mm between a cold January night and a hot July afternoon on a south-facing roof, and the surface temperature range on dark powder-coated aluminium under glass is wider than the air temperature range. That movement runs along the bar and has to be absorbed somewhere. It is absorbed at the joint.

Gaskets shrink back from mitred ends. EPDM and silicone gaskets are extruded and cut to length, and over years of thermal cycling they retract. The retraction shows first at cut ends, which is exactly where the mitres are. A gasket that has pulled back 4mm at each of four corners has opened four holes in the weather line.

The corner is a three-plane junction. The eaves beam runs horizontally, the hip bar runs up at 20 to 25 degrees, and the upstand turns 90 degrees underneath. Three surfaces meeting in three planes is a difficult detail to weather in any building component, and it is the reason good systems use a moulded corner cap or a machined casting there rather than a straight cut.

Water arrives there in quantity. A pitched lantern sheds water down its faces, and the volume converges at the eaves. At the corners it changes direction. Anywhere water changes direction it slows down and dwells, and dwelling water finds gaps that running water passes over.

Why the mastic answer fails

In a well-designed system, the corner is weathered by overlapping components and a moulded end cap, so the joint is protected by geometry. Water has to travel uphill to get past it. In a budget system, the corner is a butt joint filled with mastic, and the sealant is the only thing between outside and inside.

Sealant is a gasket between two components, not a waterproofing layer. It has a service life, it hardens, it loses adhesion at one edge, and it does all of that faster where there is movement and ultraviolet exposure. Both are at their maximum at a lantern corner. So the design works for four or five years and then does not, and no amount of re-application changes the underlying arrangement. That is why we do not offer resealing: it re-starts a five-year clock on a detail that was never going to last.

The upstand underneath

There is a second, quieter cause of corner leaks, and it is not in the lantern at all. If the upstand is twisted, meaning the top surface is out of level across the diagonals, the frame bolted to it is racked. A racked frame does not compress its gaskets evenly, and the compression is worst at the corners because that is where the frame is stiffest and least able to conform.

This is why a brand new lantern can leak at a corner within a year of installation, and it is always an installation fault rather than a product fault. The top of an upstand should be checked across both diagonals before the frame goes anywhere near it. Our page on when the upstand is the problem, not the lantern works through the wider picture.

How to tell a corner leak from condensation

These get confused constantly and the confusion sends money to the wrong place. The test is timing.

Directional dependence is the most useful single clue. If it only leaks in a south-westerly, the corner facing that way is the one that has gone.

What this looks like on the Suffolk coast

Corner failures run earlier here than inland. Within about a mile of open sea at Felixstowe, Harwich, Frinton, Aldeburgh or Southwold, three things compound. Driven rain arrives horizontally rather than vertically, so it attacks the vertical faces and the underside of laps that were designed for water running downwards. Chloride from salt-laden air degrades gasket surfaces and attacks the fixings holding the corner components together. And the constant wetting and drying cycle keeps salt concentrated at fixing heads rather than washing it away.

A grade 304 stainless fixing at a lantern corner in Felixstowe will commonly show rust staining within two or three winters, and the corrosion product expands, which opens the joint further. We specify 316L stainless throughout on coastal work for exactly this reason. The estuarine towns, Woodbridge, Wivenhoe, Manningtree, Shotley and Pin Mill, carry moderate salt loading with real wind exposure across open water, so upgraded fixings are sensible without the full marine package.

Related questions

• Should you reseal or replace a failing roof lantern? covers the economics of the decision.
• Why do roof lanterns corrode near the coast? explains the fixing side.
• Roof lantern leaking in heavy rain: what to do now if it is happening today.

For the full diagnostic method see diagnosing where the water really gets in, under roof lantern installation.

Get it looked at properly

We survey the whole weathering design rather than the corner that is showing, and if replacement is the answer we say so with the reasoning. We do not offer resealing or patching on any rooflight. Request a fixed quote.