Roof lanterns & flat roof rooflights
In short
On a flat roof the daylight comes from a lantern or a flat glass rooflight sitting on a timber upstand, and almost everything that goes wrong is at that upstand — its height, how the covering is dressed up it, and whether water can get away around it.
Also called: Roof lantern · Flat rooflight · Skylight on flat roof · Lantern light
Signs you need this
- A flat-roofed extension or kitchen that needs daylight
- Water tracking down the internal reveal of an existing lantern
- Standing water against the upstand after rain
- An upstand that looks very low relative to the roof surface
- Condensation running off the glass of an older lantern
- A leaking flat rooflight on a felt or GRP roof
Flat roof glazing sits on a kerb — a timber upstand built up off the deck — and the glazing unit fixes to the top of it. That upstand is the whole job. The lantern or rooflight itself is a manufactured product that arrives sealed and does what it does; the roof around it is what decides whether the room below stays dry.
Height is the first thing. The roof covering has to turn up the upstand and be properly terminated, and there needs to be enough height above the finished roof surface that water and debris cannot get over the top — generally 150mm as a minimum. Upstands built too low, which is common when somebody was trying to keep the profile down, let water in as soon as there is any depth on the roof.
Drainage around it matters just as much. A lantern in the middle of a flat roof creates a dam: water arriving from the higher side has to get round it. If the falls do not take it away, it stands against the upstand, and standing water against a junction finds its way through eventually. On a roof of any size, the fall has to be designed around the opening rather than assumed.
The covering detail at the upstand is where workmanship shows. On EPDM it is a properly bonded upstand with the corners formed correctly and a termination bar or trim at the top. On GRP it is laminated up the kerb as part of the same continuous skin, which is one of the reasons GRP suits lanterns well. On felt it is a proper upstand detail rather than a strip stuck up the side.
Condensation is the other half of the story and it accounts for a large share of the complaints. A big area of glass over a kitchen or a dining space is a cold surface above a humid room, and water forms on it and runs down onto the internal reveal — which reads as a leak. Modern lanterns have thermally broken frames and good glazing which reduces it substantially, but ventilation in the room below still matters.
One structural point worth flagging: a lantern is heavy, and the opening in the deck has to be trimmed to carry it. On a new build or extension that is designed in. On a retrofit into an existing flat roof, the deck has to be opened, trimmed with proper timbers, and the upstand built off that — it is not a job of cutting a hole and standing a frame on it.
What the job covers
- Deck opened and the opening properly trimmed
- Upstand built to adequate height above the finished surface
- Falls designed so water gets away around the opening
- Covering dressed and terminated correctly at the upstand
- Lantern or rooflight set, sealed and fixed to the manufacturer's detail
- Thermally broken frames specified to limit condensation
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your roofer has to honour it. On roofing it would also be close to meaningless: access is often the biggest single factor, and it varies enormously house to house. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
How high should the upstand be?
Generally at least 150mm above the finished roof surface. Lower than that and water, debris or a build-up after heavy rain gets over the top. Low upstands are common where somebody wanted to keep the profile down, and they are one of the more frequent reasons a lantern leaks.
Water is running down the inside. Is it leaking?
Often it is condensation rather than ingress. A large area of glass over a kitchen or dining room is a cold surface above humid air, and water forms on it and runs onto the reveal. Modern thermally broken frames and good glazing reduce it a great deal, but the room still needs ventilating.
Can a lantern be retrofitted into an existing flat roof?
Yes, but it is a real job rather than cutting a hole. The deck has to be opened and properly trimmed to carry the load, the upstand built off that, and the covering dressed up it correctly — which in practice usually means re-covering at least that area of the roof. It is much cheaper as part of a re-cover than on its own.
Does water stand around my lantern?
If it does, the falls were not designed around it. A lantern in the middle of a flat roof dams the water coming from the higher side, and unless the fall takes it round, it stands against the upstand. Standing water against a junction is what eventually finds a way through, so it is worth correcting rather than living with.
Need this doing in Macclesfield?
Tell us the job and it goes straight to our local roofer — one call back, usually the same day. Free quote, no obligation.