Do roof lanterns leak?
A correctly detailed roof lantern does not let water in, and a badly detailed one nearly always does. The distinction is almost never the lantern itself. Modern aluminium lanterns are gasketed, drained systems made to a consistent standard, and the glass is the most reliable component on the roof.
What varies is the two or three hundred millimetres of roof immediately around the lantern: the height of the kerb, the fall of the roof approaching it, the way the covering terminates, and how the corners were formed. Those four details decide the outcome, and none of them is visible once the job is finished.
Where water actually gets in
In order of frequency, the entry points are the junction between the covering and the kerb, the corners of that junction, the fixings through the base frame, and a long way behind them, the glazing itself.
Water does not need a hole. It needs a route and a driving force. On a flat roof the driving forces are gravity into a low spot, capillary action along a lap, and wind pressure pushing water up a vertical face. A detail that would be watertight in still rain can be defeated by all three acting on a January night with an easterly behind them.
The upstand height, and the water it holds back
The industry figure of one hundred and fifty millimetres from the finished roof surface to the top of the kerb is not decoration. It is the depth of water that can accumulate on a flat roof in a heavy storm before it drains, plus the height wind driven rain can be pushed up a vertical face, plus a margin.
Where a roof drains cleanly and sits sheltered, some manufacturers will warrant less. Where the roof is genuinely flat, has a history of standing water, or is exposed, one hundred and fifty is the minimum rather than the target. Lanterns bedded straight onto a deck with a fifty millimetre kerb, because somebody wanted a lower profile from the garden, are the most common design failure in this whole subject.
How the covering is terminated at the kerb
The covering has to go up the kerb and over its top edge, so that the lantern base frame lands on top of a continuous waterproof surface rather than beside one.
Each covering has its own correct method. A single ply membrane is welded up the face and dressed over the top with hot air welded corner pieces. GRP is laid up in situ, with matting taken continuously round the corners and up over the edge. Felt is bonded in layers with a separate upstand piece and a capping sheet. Where the covering merely stops short of the kerb and a bead of sealant bridges the gap, the joint is being asked to do the job the covering should be doing, and sealant has a shorter life than a roof.
Gaskets, and how a lantern sheds water internally
Aluminium lanterns are not sealed systems. They are drained and ventilated ones, which is a better idea. Each rafter bar carries a gutter formed into the extrusion below the glass line, and the assumption built into the design is that a small amount of water will get past the outer gasket.
That water runs down the internal gutter of the bar, discharges into a channel in the eaves beam, and exits through drainage slots at the bottom of the base frame onto the roof covering. This is why blocking those slots with sealant, which people do when they see water emerging, converts a working system into a bath. The slots are meant to be open.
Water that is condensation, not rain
Before assuming ingress, work out which of two things is happening. Surface condensation forms on the room side of the glass when warm moist air meets a cold surface, and it appears on cold nights, most commonly over a kitchen or a bathroom. It is physics rather than a fault, and it is managed with ventilation and with warmer glass.
Misting between the panes is different again, and means the sealed unit has lost its edge seal and the desiccant is saturated. That unit has reached the end of its working life and wants replacing. The tell for genuine ingress is that it correlates with rain and with wind direction rather than with cold and cooking.
Driven rain, and the Essex east coast
Exposure is a real design input, not a footnote. Wind driven rain in this county comes predominantly from the south west, but the punishing events on the coast come off the North Sea from the north east, and they arrive with very little to slow them down.
Properties on Canvey, along the Blackwater and Crouch estuaries, and on open ground around Bradwell and Burnham see materially higher driving rain than a terrace in the middle of Chelmsford. In those positions the kerb gets built taller, the corners get extra attention, and the lantern is specified with the higher weather rating rather than the standard one.
Standing water beside a lantern kerb
Water that has not cleared forty eight hours after rain is ponding, and ponding against a kerb is a sustained test rather than a passing one. A membrane joint that copes with rain running past it will eventually be found out by water sitting on it for weeks.
Ponding around a new lantern usually has one of three causes: a roof laid with insufficient fall in the first place, deflection in the joists after the opening was cut, or the lantern itself placed on the low side of the roof so it dams the flow to the outlet. The third is a design decision and it is avoidable. Position the lantern so water runs past it and away, not into it.
Finding the source when a mark appears on a ceiling
Water travels before it appears, sometimes several metres along a joist or a service run, so the wet mark on the plasterboard is rarely below the entry point. Tracing it is a methodical exercise rather than a guess.
The usual sequence is to correlate the appearance with weather and wind direction, then check the drainage slots in the base frame are clear, then hose test the roof in sections from the low point upwards, watching from inside. A thermal camera is useful for finding wet insulation within the build-up. Only once the entry point is known is there any point in touching it.
What the ten-year guarantee covers
Sunspire’s guarantee is on workmanship, which is precisely the half of the job that decides whether a lantern stays dry. The glass and the frame carry their own manufacturer warranties, typically ten years on the sealed units and longer on the powder coating.
The reason the workmanship is guaranteed rather than the product is that the product was never the risk. Kerb height, falls, laps and terminations are the variables, they are all set on site, and they are all cheap to get right on the day. That is the whole argument for treating a lantern installation as a roofing job with glass in it, rather than a glazing job on a roof.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements