The answer
A lantern stays watertight because several layers each do part of the job, and none relies on a bead of sealant alone. The lantern stands on a raised upstand so surface water has to climb to reach the joint. The roof membrane is dressed up the upstand and sits beneath the lantern's base. The glazing bars carry small channels that drain any water that gets past the seals. And the flat roof around it has a fall, so water leaves instead of standing. Take away any layer and the others still work, which is the point.
Start with the upstand
Everything begins with the height of the base. A roof lantern is fixed to a raised upstand, a short box built up from the roof structure, so the joint between lantern and roof is held well above the level the water reaches.
On a flat roof, water runs and sometimes pools. If the glazing sat flush with the surface, a puddle after heavy rain would be pressing directly at the seal. The upstand lifts the joint clear, so rain has to rise up and over a barrier before it could get in, and in practice it never does.
The height also has to suit the roof build-up. Too low an upstand cramps the membrane detail, which is where leaks start.
That is why we work out the upstand at the survey and not on the day. It is decided by the thickness of what is underneath, the fall of the roof and the lantern chosen. For a pitched lantern the base has to be level and square, so the kerb is checked with a spirit level before the frame arrives.
The membrane that climbs the kerb
The waterproof layer of a flat roof is its membrane, which may be a single-ply sheet, a liquid-applied coating, a bitumen felt system or fibreglass, depending on the roof. Around a lantern it does something extra. It is turned up the outside of the upstand and tucked beneath the lantern base, so a continuous skin runs from the roof surface to a point above any water level.
This is the main defence. The lantern's seals are the second line, and the membrane is the first. If every sealant joint failed, water would still meet a sheet running up the kerb and overlapping the base.
The corners are the fragile spots. A sheet has to fold or be welded at the corners of the upstand, and the quality of that work decides how long it lasts. Good detailing uses preformed corners or carefully welded folds in place of cut and patched pieces, with the upturn finished under a trim so rain cannot run behind it.
Glazing bars that drain themselves
Even a well-made lantern lets small amounts of moisture reach its inner parts. Wind-driven rain pushes water into joints, and warm air inside carries vapour that condenses on cold metal. A good lantern is designed to expect that.
The glazing bars, the rafters between panes, have a channel behind the visible face and a gasket on either side of each pane. Any water that gets past the outer seal runs down the channel and out at the base through small drainage holes, called weeps, onto the roof outside. Condensation inside the bars drains the same way.
This is the reason a lantern should never be sealed solid with silicone along every joint. It looks tidy for a season, and then traps water in the bars where it cannot escape, which corrodes metal and causes the very leak it was meant to stop. A drained and ventilated system is more reliable than a sealed one.
A well-built lantern keeps the weeps clear of paint, sealant and debris. They are small and easy to block during decoration or roof work, so we keep them in mind when the lantern is fitted and when the roof is finished around it.
Gaskets, ridge and the joints between parts
Each pane of glass sits in a gasket and is held by a cap that clamps it. The gaskets are compressed rubber or similar, and their job is to give a weathertight seal that can still move with the temperature changes of the seasons.
At the ridge and hips, where the sloping panes meet, the joints are shaped to shed water outward, and a ridge cap covers the top. Hip and corner details are where two sloping planes meet, and they are normally the most complex part of the frame. Good lanterns come with these details designed and tested by the manufacturer, which is one reason we use made-to-measure units from established makers over improvised ones.
Gaskets are also the part that ages first. Sunlight, frost and years of movement slowly harden them. That is gradual, and it is picked up if someone looks at the lantern once a year.
An autumn look is well timed: from outside, check each pane's edges for gaps or a cap that has moved, and from inside, check the frame bottom for water marks.
The roof around the lantern must drain
A flat roof is not flat. It is laid to a gentle fall that sends water to a gutter or an outlet. If the fall is wrong, water stands, and standing water is hard on every joint it touches.
Around a lantern the issue is sharper, because the upstand is an obstruction. Water running down the roof meets it and has to divert round both sides. If the roof falls towards the lantern, water collects at its upstream edge and sits against the kerb. A well-planned roof avoids that by keeping the lantern from sitting in a hollow, and by forming a small sloped area behind it, called a cricket, that sends water round the sides.
This is another reason the survey comes first. We check the fall of the roof, the position of the outlets and whether the lantern site is suitable. Fixing a drainage problem is a roofing job, and it is better done at the same time as the lantern than separately.
A wet spell tests all of this together: day after day of rain, never drying out, with standing water on a roof that was meant to shed it. In a Nottingham winter the roof is wet far more often than it is dry, so we plan for that case, not for the dry afternoon on which the lantern is first admired.
What we do about it
Watertightness is a matter of workmanship in the details, which is why it is covered by our written 10-year workmanship guarantee. The way we go about it is simple to state.
We survey the roof before quoting, identifying the covering, the fall and any weak spots. We specify the upstand to suit the build-up of the roof. We dress the membrane up the kerb and finish it under the lantern base. We leave the weeps clear and the drainage paths open. And we check the fall of the roof around the lantern so that water leaves instead of standing.
If the lantern is going onto a roof that is already there, the condition of the old covering comes into it, and our answer on fitting a lantern to an existing flat roof goes into it in more depth. To see how the frame material affects the base detail, read aluminium or uPVC roof lanterns.
You get a fixed written quote after a free survey, and it does not change on the day. If you already have a lantern that gives you concern, tell us what you see, and photographs are welcome on WhatsApp at 07880 284133.