Do roof lanterns leak?

The frame is not usually the failure

People assume the weak point is the aluminium and the gaskets, because that is the visible part. It rarely is.

A lantern frame is a drained and ventilated system. It does not rely on a single line of sealant to keep water out. Water that gets past the outer cap gaskets, and some always does in driving rain, runs down internal channels in the rafter bars, collects at the eaves beam and discharges over the outside of the kerb. That is how the system is designed to behave. What it depends on is those channels being clear and continuous, the end caps being fitted the right way round, and the drainage slots at the eaves not being blocked by sealant applied by somebody trying to be helpful.

The one frame-related failure worth naming is a unit that has been forced out of square during fitting. A lantern that has been racked to fit an opening that was not square puts the gaskets under uneven compression and one bar will weep before the others.

Where the kerb meets the frame

The eaves beam sits on the top of the kerb, and that joint is the most important 30mm on the whole installation.

It is made with a continuous bed of the correct sealant on a clean, dry, flat kerb top, with the frame set into it and fixed down at the specified centres so the bead compresses evenly. Three things spoil it: a kerb top that is not level, so the bead is thick in places and squeezed to nothing in others; fixing into wet timber; and sealant applied to a dusty surface. All three are invisible the day the job finishes and all three show up in the first sustained south-westerly with rain behind it.

Water that gets past here does not appear at the lantern. It runs inside the kerb construction and shows on the ceiling a metre away, which is why this failure is so often misdiagnosed.

Where the covering meets the kerb

The roof covering has to run up the outside of the kerb and terminate under the lantern’s flashing or drip. How far up, and how it is held there, is set by the covering.

Felt is dressed up and mechanically fixed, with the fixing line above the water line. Single-ply is welded and finished with a termination bar and a sealant. EPDM is bonded, with the corner dressed rather than cut. GRP is laid in one continuous piece over the angle. The failures are the same in every case: a termination that stops too low, a corner that has been cut and butted instead of dressed, and laps that run the wrong way relative to the fall so water is presented at the open edge of a joint rather than the closed one.

Standing water is the real enemy on a flat roof

The kerb has to stand the frame clear of water that sits and water that is driven. The working figure is 150mm from the finished surface of the covering to the underside of the frame, and there are good reasons not to go below it.

A flat roof in heavy rain carries a film of moving water. Add wind and that water is driven up any vertical surface it meets. Add a blocked outlet, a leaf-filled gutter or a fall that has flattened with age, and a shallow pond forms. A kerb at 75mm survives none of that. A kerb at 150mm survives all of it, and is still doing its job in fifteen years when the outlets have not been cleared for a while.

Building a kerb across an existing fall makes this worse, because the kerb dams the water that used to run past. The falls either side have to be corrected with tapered insulation or firrings so the water goes round rather than sits. Upstands, kerbs and warm roof build-ups sets out the construction.

Fixings through the wrong plane

Every screw through a waterproof layer is a hole, and there are only two acceptable answers: put it somewhere the water never reaches, or make it good with a detail designed for the purpose.

The failures we see are fixings driven through the covering on the flat of the roof to hold something down, fixings through the face of a kerb below the termination line, and fixings through the top of a flashing rather than under its lap. Each of them is a hole in a surface that water crosses. On a windy site a fixing that was watertight when driven works loose over years of thermal movement, and then it is an open hole.

Marks that are condensation, not rainwater

A significant proportion of what gets reported as a leaking lantern is water that was already in the house. Condensation forms on the coldest surface in the room, and on a new lantern that is often the frame at the eaves or the plaster of the reveal.

The tells are reliable. Condensation appears on cold, still nights and on the inside face, often when it has not rained. Rainwater appears during or shortly after rain and is usually accompanied by a wet mark that spreads. Condensation is worst in kitchens and bathrooms, where moisture is generated, and it is worst where the insulation stops short at the reveal so a cold bridge runs round the opening.

The fixes are different too: better ventilation, a continuous insulation line up the kerb, and a vapour control layer that meets the frame. We have written about the mechanism in what causes condensation on a rooflight and about the kitchen-specific case in roof lantern condensation in a kitchen.

Age, and the units that reach the end of their life

Sealed units do fail eventually. The perimeter seal loses its integrity, the desiccant saturates, and moisture forms between the panes. That is not a leak into the room. It is a misted unit, and the answer is a new unit in the existing frame, which on a lantern is a day’s work and does not disturb the kerb.

Older lanterns, particularly timber ones that have not been maintained, have a second failure route: the frame itself moving as the timber cycles, opening the gasket lines. Aluminium does not do this. It is one of the practical reasons most new lanterns are aluminium or aluminium over timber, which we compare on aluminium and timber roof lanterns compared.

Questions that reveal whether an installer has done this before

Ask any installer these, ourselves included, and listen to whether the answers are specific.

• What height will the kerb be above the finished covering, and why that number?
• How is the insulation carried up the kerb, and where does the vapour control layer terminate?
• How does the existing covering terminate against the new kerb, and is it being renewed?
• What happens to the falls either side of the kerb?
• Where does water that drains inside the frame come out?

An installer who answers those in millimetres and named materials has built one properly. An installer who answers by naming a brand of lantern has not. Fifteen years of this work, Building Control notified by us, and a ten-year guarantee that exists because those four junctions are where the risk actually sits.