The short answer
Yes. A roof lantern can be cut into a finished flat roof, provided the joists around the opening can be trimmed and strengthened, a raised kerb can be built above the deck, and the roof covering can be rejoined to it. The work is a structural and waterproofing job first. The lantern itself is the last thing to arrive.
What cutting into a finished roof really means
The opening is small on the plan but it touches every layer of the roof, so the job is planned from the ceiling upwards.
A flat roof is a sandwich. Plasterboard sits at the bottom, then the joists, then the deck, then insulation, then the waterproof covering on top. A lantern needs a hole through all of it, and a frame that stands proud of the surface. So you are not just lifting something onto the roof. You are altering the structure that carries it, and then rebuilding the weatherproofing around the new edge.
That is why a lantern over an old extension is a different conversation from a lantern specified before the extension is built. On a new build the openings are drawn in and the joists are laid to suit. On an existing roof we work with what is already there: the joist direction, the joist size, the deck and the covering. The survey is where we find out what that is, and it decides most of what follows.
Because this page is about the roof you already have, we assume the extension is finished and in use. The room below is furnished, the ceiling is painted, and you would like the mess kept to one corner. That shapes the order of work as much as the structure does.
Joists, trimmers and the opening
The weight of a lantern and the load it collects have to be carried round the hole, not through it.
When we cut a joist, the ends that remain need something to rest on. Timber trimmers go across the opening on each side, and the cut joists hang from them. The trimmers carry the load into the uncut joists either side, and those may need doubling up. Where the span is long or the opening is wide, a steel beam may be the better answer. The page on whether a lantern needs a steel beam round the opening explains when that happens.
Two things are checked before anything is cut:
- which way the joists run, because an opening that lines up with them needs fewer cuts than one that crosses them;
- what is in the roof space already, such as a service run, a flue or an old opening that was closed off.
The lantern also adds weight and collects snow. The structure is assessed for both, and the detail is in our answer on snow load on a roof lantern. New structural work in a dwelling falls under the Building Regulations, and where the work is notifiable we handle the Building Control notification.
The kerb is what makes it watertight
A lantern never sits on the flat surface. It stands on a raised frame, and the roof covering is carried up the sides of that frame.
The kerb is a box of timber, built on the deck round the opening and usually around 150mm or more above the finished roof surface. That height is deliberate. The waterproofing is dressed up the outside face of the kerb and is tucked beneath the lantern's own cill, so water running across the roof meets a vertical barrier rather than a joint lying flat.
Insulation goes inside the kerb, and in a dwelling it has to be continuous with the insulation in the roof so that the opening does not become a cold spot. That is also where a good kerb helps with sound: a dense, lined kerb stops the lantern frame from sounding hollow in rain. Our page on lantern kerbs and upstands goes through the build in detail.
The kerb height also affects how the lantern looks from the garden. A taller kerb lifts the whole unit, so it is worth settling before the order goes in. Our answer on how high a lantern sits above the roof sets out the usual range.
Joining new waterproofing to old
The covering on the rest of the roof is not renewed, so the new work has to meet it cleanly.
How that joint is made depends on what the roof is covered with. The three common types behave differently, and the choice is made at the survey.
| Existing covering | How the new kerb joins it | What we check first |
|---|---|---|
| Single-ply membrane | A new membrane is welded to the old at the kerb base | Whether the old sheet is sound enough to weld to |
| Felt or built-up | New layers are lapped into the old and taken up the kerb | The age of the layers and how well they bond |
| Liquid-applied | The coating is carried up the kerb and overlaps the old | That the existing coating is compatible |
If the covering is at the end of its life, it is cheaper and neater to renew the whole roof while the access is there, than to join a new kerb to a tired surface. We say so at the survey, and the quote shows it as a separate line so you can see the choice.
The fall of the roof matters here. A flat roof is laid to a slight slope, often a fall of 1:80, and the kerb must not block water from reaching the outlet. On a low-fall roof we may add a small cricket, a little sloped upstand behind the kerb, so that water flows round it instead of pooling. That is one of the details that separates a tidy job from one that stands water after every shower.
Living in the house while the roof is open
The opening is made in a single day where the roof allows, and closed to the weather the same day.
The order is planned so the roof is never left exposed overnight. The room below is cleared and covered, the ceiling is cut from underneath or from above depending on access, and the trimmers go in before the lantern is lifted. A temporary weatherproof cover protects the opening until the lantern is fitted. The full sequence is on our page about how a roof lantern is installed.
Inside, the ceiling is made good round the new opening with a lined reveal, finished flush and left ready for decoration. The lantern's glass is also chosen with the room in mind. A laminated inner pane is the sensible option over a kitchen, because it is safer overhead and quieter in rain. Overhead glazing is not a critical location under Part K, but good practice still calls for safety glazing. Our page on why the inner pane should be laminated explains that choice, and the written quiet spec with your quote records the glass, the seals and the upstand for your room.
A planning check comes before any of this. Many lanterns on an existing flat roof fall within permitted development, but not every house has those rights, so it is checked for the address. The page on planning permission for a roof lantern covers the common cases.
Questions about lanterns on existing roofs
Can any flat roof take a lantern?
Nearly any sound flat roof can, but the structure has to be checked first. A very shallow roof with thin joists may need strengthening before it carries the extra load, and the survey will say so.
Would a flat rooflight be simpler for my roof?
It can be, because a flat unit needs a lower kerb and a lighter opening. It gives a different kind of light, though. Our guide to choosing between a lantern and a flat glass rooflight compares them, and the flat roof rooflight installation page covers the flat option.
How large an opening is sensible in an old extension?
It depends on the joists and the room. A smaller opening means fewer cuts and a lighter trimmer. The roof lantern size guide shows how size relates to the floor area below.
Will the ceiling need redecorating?
The area round the opening will, because new plasterboard and a lined reveal are added. We leave it ready to paint, and the rest of the ceiling is untouched.
If you have an extension with a flat roof and would like to know what is possible, get a quote and we will survey the roof and write up the options. Our roof lantern installation page covers the wider service.