Roof lantern leaks: causes and what to do

If your roof lantern leaks, the cause is usually not the glass but the connection around it: the roof covering against the upstand, the wall flashing or an ageing sealant joint. A pitch that is too shallow or a blocked drain also causes leaks. First rule out condensation. Then trace the spot systematically, from low to high.

First rule it out: is it condensation?

Water under a roof lantern is by no means always a leak. Condensation appears spread across the glass, on cold mornings or after cooking and showering, even in dry weather. A leak comes back after rain in the same spot, often along an edge or in a corner. If in doubt, first read condensation on a roof lantern.

Why a roof lantern leaks: the most common causes

A leaking roof lantern almost always has one of these causes. They have one thing in common: the water finds its way along a joint or connection, not through the glass.

• Connection of the roof covering to the upstand
• Wall flashing on a roof lantern against a taller wall
• Upstand too low, so water or snow comes over the edge
• Pitch too shallow, so water stands on the glass
• Aged or loosened sealant joints and gaskets
• Blocked drainage in the gutter, bottom rail or downpipe

Flashing and upstand

The upstand is the raised kerb on which the roof lantern rests. The roof covering runs up against it and the lantern frame drops over it. If something goes wrong there, such as an EPDM strip that does not run high enough or a loose seam in bitumen, water runs behind the finish and into the house. Tradespeople usually keep the upstand about 15 centimetres above the roof covering, so that puddles and melting snow do not come over the edge. See the upstand of a roof lantern, connecting to EPDM and connecting to bitumen.

With a wall-abutting lean-to roof lantern, the wall connection is the sensitive point. It should be made with a wall flashing profile plus an EPDM lead substitute or zinc flashing, matched to the wall type. Water seeping down through porous brickwork above can also come out behind the flashing. In that case the cause lies in the wall, not the roof lantern.

Pitch, sealant and drainage

The pitch has to carry water away quickly. If the pitch is too shallow, water stands, leading to dirt and algae. That is why a DagLichtDesign roof lantern has a standard pitch of 8°, just above the 7° needed for quick drainage. If the upstand is crooked or has settled, water can run the wrong way. More in the right pitch for a roof lantern.

Sealant needs maintenance. Sun and temperature changes make it harden and come loose. So check sealant joints every year. A blocked gutter or drain lets water back up over the edge. A gable lantern has a self-cleaning drainage system in the bottom rails, but leaves in the gutter the water runs into remain a risk.

How to trace a leak

Start low and work upwards. If you spray at the top straight away, the water runs past every joint and you will not know which one is leaking. Work safely from a ladder or the adjacent roof and never walk on the glass.

• Note when it leaks: with every shower or only in wind
• Clean gutters, bottom rails and drains first
• Run water with a garden hose, one side at a time, low to high
• Wait a few minutes per side and have someone watch inside

What to do next

Have the repair done by the builder or roofer who installed the roof lantern, because it is usually about the connection to the roof. The flashing details are in the installation manual, and he can call us with questions on working days from 7:00 to 17:00 via advice and service. With an acute leak, cover the spot temporarily and put a bucket under the drip.

Frequently asked questions