Short answer
A roof lantern has real drawbacks: it normally needs planning permission on an existing flat roof, it lets in summer heat, it can run with condensation in cold weather, it needs an upstand and a flat roof that drains, and its glass is hard to reach for cleaning. Each can be kept small by design, and a survey shows which apply to your roof.
Free survey / fixed written quote / Building Control notification handled / 10-year workmanship guarantee
Planning permission comes before anything else
On the roof, a lantern usually rises by more than 15 cm. Added to an existing flat roof, that height normally puts it outside permitted development, so a planning application is needed. Inside a conservation area, the Gower AONB, Bannau Brycheiniog National Park or a World Heritage Site, a new rooflight in a roof slope needs planning permission in Wales whatever its height.
Where the lantern belongs to a new extension, the council gets the question from us before anyone guesses at the answer. The roof lantern planning permission guide covers the ground, and it is the first thing to read before you settle on a design.
Heat in summer, damp in winter
Glass over a room lets in sun as well as light, so a lantern above a south-facing extension can warm the room in summer. An opening vent lets warm air out, and coatings on the glass are an option, so the room can be planned for it. Winter has the opposite effect. Steam from a kitchen condenses on cold surfaces, and a lantern has a lot of glass. The answer is in the room as much as the lantern: better glazing, and a vent or fan that gets used.
The survey notes which way the room faces and what happens in it before we suggest a specification, because the same lantern behaves differently over a cooking kitchen and over a quiet reading room.
| Drawback | What helps |
|---|---|
| Summer warmth | Opening vent, coating, orientation |
| Winter condensation | Better glass, ventilation |
| Rain noise | Glazing choice, build-up |
| Dirty glass | Self-cleaning option, reach |
A lantern asks a lot of the flat roof
The lantern sits on a builder's upstand, and the maker sets what that upstand should be. Brett Martin's installation bulletin calls for a height of 150 mm or more and suggests building it dead level. A lantern can accept a pitch of up to 5 degrees, but the bulletin warns that it will probably look lopsided.
Water also has to leave the roof around the upstand, so the surface needs a fall. If the existing roof is out of true or the upstand has failed, that needs looking at before the glass. Forming a fresh opening usually alters the structure of the roof too, and the lantern's load has to be carried, or the roof strengthened to carry it.
Keeping the glass clear
A lantern is hard to reach with a cloth, and the glass sits where rain and grime collect. Self-cleaning glass is available as a rooflight option. Pilkington Activ is a glass coating that works with daylight to break dirt down and with rain to rinse it off, and Pilkington names roof glazing among its applications. Where nobody can reach the glass without a ladder, a coating like that is worth asking about, and the survey notes how the glass can be reached, which matters for the cleaning as much as for the fitting.
Weighing the list up
None of these makes a lantern a poor choice, and none is a reason to rule one out before the survey. They are the questions a lantern asks of your extension. A south-facing kitchen with a vent and a fan, on a sound flat roof with room for an upstand, answers several of them before the survey starts. A shaded room under a roof that needs rebuilding answers fewer, and a different rooflight may suit it better. The survey puts the room, the roof and the council's view side by side before any figure is written down.
Cost, and the other ways to get the light
Cost belongs on the list of drawbacks, since the structure underneath, the glazing and the upstand all weigh in. The answer on what a roof lantern costs sets out what moves the figure. If the list is putting you off, a flat roof rooflight, a walk-on rooflight or a sun tunnel brings light in with a different shape of roof work.
A survey of your roof and room comes free, and a fixed written quote comes after it. We back the workmanship with a 10-year guarantee in writing. Phone 01792 929062 or request your survey.