Roof lanterns and flat-roof glazing: the leak risks
When a lantern leaks, the glass is almost never the culprit. The failure is at the kerb it sits on and how the waterproofing meets it.
The kerb is the roof, not the window
The upstand the lantern sits on has to be built to a proper height and the membrane dressed fully up it and over the top. Stopping the membrane short and relying on sealant is how they leak.
Height above the finished roof
A kerb that's too low lets standing water and driven rain get above the waterproofing line. Adequate upstand height is the single most important dimension.
Drainage around it
A lantern in the middle of a flat roof splits the drainage. Falls need to take water round it and to the outlet, not into a pond against the upstand.
Condensation versus leaks
Water running down the inside of the glass in cold weather is usually condensation from the room below, not a roof failure. Ventilation and trickle vents address it.
Sequencing with the builder
Getting the roofer and the glazing supplier to agree the kerb detail before it's built saves a lot of retrofitting later.
FAQs
That's typical of an upstand that's too low or a membrane not dressed high enough. Light rain drains; heavy rain overwhelms the line.
Sometimes. Where the kerb detail itself is wrong, the lantern usually has to come off to correct it properly.
The kerb is built and waterproofed first, then the lantern is set onto it.
Related reading
More on materials & systems and the jobs that usually come with it.
- Loft condensation: ventilation, not leaksMaterials & systems
- Storm damage: what insurance covers and what it does notRepairs & problems
- Roof cost by house type: terrace, semi, detached and bungalowCosts & budgeting
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