Short answer: Yes. Fitting a roof lantern on an existing flat roof is routine work, provided the joists can carry the extra weight and the opening is trimmed properly. The real work is the kerb: it has to be built, insulated and then joined into the old roof covering so water cannot get behind it. A survey settles all three before any price is given.
The joists come first
Putting a lantern on an old flat roof begins below the roof, not on top of it. A flat roof on a kitchen extension is usually a deck of boards or ply laid over timber joists, with insulation and a waterproof covering on top. It was designed to carry itself, the covering, a little snow and the odd person walking about. A lantern is a heavier thing: a frame in aluminium or uPVC, sealed glazed units, and the kerb it stands on, all concentrated around one opening.
So the first visit is a joist check. We look at the ceiling side where there is access, open a small inspection hole where there is not, and note the joist depth, the spacing and the span. Back extensions on Victorian and Edwardian terraces and on 1930s semis often have modest joists at generous spacing. Newer extensions on the Wirral and around St Helens are usually better built, but not always.
Where the existing joists are not up to the job, the answer is rarely to drop the lantern. It is to strengthen: doubling joists, adding a beam to share the load, or bearing the new kerb on a stronger line. Anything beyond a simple case gets an engineer's calculation, and that calculation goes in with the Building Control notification, which we handle. The size of the lantern is a separate question, handled in matching a lantern to the room.
Trimming the opening
A lantern needs a hole, and a hole in a flat roof cuts through the joists that give it its strength. Trimming is the carpentry that puts that strength back. The joists beside the opening are doubled where needed, and the cut joists end on new trimmers across the opening, held with joist hangers, so their load is carried round the hole instead of stopping in mid-air.
It is also the point where the fall of the roof matters. Water on the old roof runs towards a gutter or outlet, and the new opening interrupts that run. On the uphill side of the kerb the water meets a wall and has to be turned round either end of it. A small sloping saddle behind the kerb, which roofers sometimes call a cricket, steers it away instead of letting it sit there. Leaving it out is a classic way for a well-made lantern to end up with a permanent puddle against its upstand.
Inside, the ceiling is cut back to the line of the opening and the sides are lined, usually with plasterboard on a timber frame, to form a clean light well from ceiling to glass.
Building the kerb
The kerb is the upstand the lantern sits on. Its job is to raise the glazing above the roof surface, with the covering dressed up the outer face. On a retrofit it comes in two common forms. One is a timber kerb built on site from treated softwood or ply, then insulated on its inside face and clad. The other is a factory-made insulated kerb supplied to size. Which suits depends on the opening, the existing build-up and how much the roof already carries.
Either way, the kerb is fixed to the trimmers and the structure around them, not simply screwed to the old deck. It must be square and level, because a lantern is a rigid frame and will not follow a kerb that has twisted. Height above the finished roof surface matters just as much. An existing roof can have a thick build-up, and the kerb must rise above it far enough to keep water out. Heights, and the difference between site-built and factory kerbs, are covered in flat roof rooflight upstands.
Tying the new kerb into the old covering
This step decides whether adding a roof lantern to a flat roof leaves it dry. The old covering has been down for years and the kerb is brand new. Where they meet, one continuous waterproof layer has to run from the old roof, up the face of the kerb, and finish at the top where the lantern frame covers its edge.
How that is done depends on what is already there. Felt, GRP and single-ply membranes such as EPDM are all worked differently. The old surface is cleaned back to sound material, and the new layer is lapped onto it using products that are compatible with the old covering. Mixing materials that do not bond is the usual reason a join fails, which is why the survey records the covering type and its condition before a quote is written. If the covering is tired right across the roof, it often makes sense to renew all of it while the roof is open. Felt, rubber and GRP each get a fuller answer in rooflights on felt or rubber flat roofs.
If the existing roof is under a guarantee from the firm that laid it, find out what a new opening does to that cover before work starts. Our own work is guaranteed for 10 years, and you get that in writing. To start, ask for a free quote; the survey of the roof comes first.
- Joist depth, spacing and span recorded at survey
- Trimmers and doubled joists specified round the opening
- Kerb fixed to structure, square and level
- Saddle built behind the kerb to turn water round it
- New membrane compatible with the old covering
- Building Control notification made before work starts
Before you book the survey
Does a lantern on an old roof need Building Regulations approval?
Since lanterns count as rooflights, the Building Regulations 2010 make a new one controlled work, and building control has to be told. We handle that notification for you. The fuller picture is in does a roof lantern need approval.
Can the old covering stay as it is?
Often it can, if it is sound and of a type the new membrane will bond to. If the survey finds it tired, renewing it while the roof is open is sensible, and the fixed written quote says which route applies.
Will the roof be open to the weather?
No. The opening is cut and made weathertight in the same visit, then the kerb, membrane and lantern follow in a planned order. The survey gives you the programme for your roof, and you can send photos on WhatsApp first.