01Yes, with checks
A lantern can sit on an old flat roof if the roof can take it
Fitting one is routine. The real work is finding out what is under the felt before you choose the glass.
Plenty of south Essex homes have a flat roofed extension from the 1960s to the 1990s: a back kitchen, a dining room, a lean-to added when the family grew. Those rooms are often dull in the middle, and a lantern is the obvious cure. So can a roof lantern go on an existing flat roof? Yes. We fit a rooflight or skylight lantern onto a flat roof that is already there, and it does not mean rebuilding the extension.
What it does mean is a proper look at the roof first. A lantern is heavier than a flat rooflight and it stands up on a raised frame, so three things have to be right: the structure beneath, the kerb it sits on, and the waterproof covering that is dressed up to meet it. If any of the three is in poor shape, it is better to know before the old roof is cut open.
- Joists that are sound, sized for the span and not sagging
- A roof covering with plenty of life left, or one due for renewal anyway
- Enough height under any fascia, gutter or upper window to allow a lantern
- A roof that drains properly, with no ponding near the opening
- Insulation you can reach and improve around the new opening
02Structure
What the roof has to carry
Glass and a frame weigh far more than the felt and board they replace, and the opening needs framing round it.
A flat roof is a deck of boards on timber joists, with insulation somewhere in the build-up and a waterproof layer on top. When we cut an opening, the joists that run across it are interrupted, so the cut ends have to be carried by new timbers, called trimmers, fixed around the edge of the hole. For a larger lantern that frame may need to be steel or an engineered beam, and the loads should be checked by a structural engineer. We would rather tell you that on the survey than have it surprise you in the middle of the job.
If the roof already has a rooflight in it, life is simpler. The opening is there, the framing round it exists, and the question is only whether it is the right size and strong enough. Swapping an old dome for a lantern in the same hole is a common route, and a lantern with a slightly larger footprint can often be fitted by adjusting the existing kerb.
Remember too that the finished height of a lantern affects the planning position, which is covered in do roof lanterns need planning permission. Check that before you settle on a size.
What we look at on the survey
- Joists
- Size, spacing, span and condition of the timber
- Deck
- Boards or sheet material, and any rot or movement
- Covering
- Felt, EPDM or GRP, and how old it is
- Insulation
- Thickness and whether it is a warm or cold roof
- Falls
- Whether water runs off or collects near the opening
03Kerb and cover
Where the lantern meets the roof
The kerb is the part that makes a lantern warm, and the roof covering has to climb it in one clean piece.
A lantern does not sit straight on the deck. It stands on an upstand, a raised collar of timber or a manufactured kerb, usually 150mm or more tall. That height lifts the glass clear of the roof surface and gives the covering something to run up against. The kerb should be insulated on its outside face, and that insulation needs to join the roof insulation with no gap, otherwise a chilly band forms around the opening however good the glass is. Our page on what a roof lantern upstand is goes through the detail, and the kerbs and upstands guide compares the options.
Next comes the covering. If the existing roof is in good order, the new membrane can be joined to it. If it is near the end of its life, this is the moment to renew the whole roof, because you do not want to open it twice. Whether the lantern suits your covering depends on the material, and we cover that in rooflights on an EPDM roof and rooflights on a GRP roof.
| Existing roof | What usually happens | Watch for |
|---|---|---|
| Felt, in good order | Covering joined or replaced locally | Old layers lifting at the edges |
| EPDM rubber | New membrane bonded to the upstand | Correct adhesive and clean surfaces |
| GRP fibreglass | Laminated up the kerb in one go | Cracks or crazing in the surface |
| Tired of any type | Renew the whole roof first | Opening a roof twice |
04Warmth
Keeping the extension warm after the change
A lantern brings in more daylight and more glass, so the roof around it should be as warm as the day it was built.
Older flat roofs often have little insulation. Some have none at all above the ceiling. If we are opening the roof anyway, that is the cheapest point to add insulation round the lantern, and sometimes across the whole deck. A better roof helps the lantern look good on your heating bill, not just in the room.
The lantern itself should be specified for heat. Every unit we supply has warm-edge spacers and low-e glass, in a frame designed to limit cold bridging. A new lantern has to meet current Building Regulations, which means the work is notifiable, and we handle the Building Control notification for you. For more on how a lantern feels in January, read are roof lanterns cold in winter.
Ready to see whether your roof is a candidate? Book a free survey and written quote through our quote page. For the wider range of lantern types and finishes, the roof lanterns page is the place to start.
Will my old flat roof take the weight of a lantern?
Does the whole roof have to be replaced?
Not necessarily. If the covering is sound, the new membrane can often be joined to it around the kerb. If the roof is already worn out, renewing it at the same time avoids opening it twice.
Will the extension ceiling need rebuilding?
The opening is framed and the inside of it lined with plasterboard, which is then decorated. The rest of the ceiling stays. A lantern with a plaster lining can also be tidied to match, and we protect floors and furniture while we work.
Can a lantern replace an old dome rooflight?
Yes, and it is a common swap. We check that the existing opening and its framing can carry the heavier lantern, and the kerb is usually rebuilt or adapted to suit.
How long does it take?
It depends on the size of the lantern and the state of the roof, but a straightforward job is measured in days, not weeks. Our survey will give you a clear programme in writing.
