The first question a roofer asks about a lantern is rarely about glass. It is a number: what size is the kerb? A flat roof for a new kitchen extension has been framed, the joists are in and the deck is about to be boarded, and somebody has to decide how big the hole is. If the figure comes from a drawing that guessed at the lantern, the kerb is built to the guess, and the glass either does not sit properly or is ordered again to match.
Lantern sizing is not hard, but it has three separate numbers that are easy to muddle: the size inside the kerb, the size outside it, and the height of the upstand above the finished roof. The sections below take them in turn, then look at how a lantern is chosen to suit the room beneath, whether to pick a standard size or have one made, how the structure around the opening is handled, and the short list of facts to send your builder.
Internal kerb size and external kerb size
A kerb is a raised timber box built on the roof deck around the opening. It lifts the lantern clear of the roof surface and gives the roofer something to dress the membrane up against. Every kerb has two sizes, and drawings that give only one are a common source of mistakes.
The internal size is the clear opening inside the kerb, measured between its inner faces. It is the size of the hole in the ceiling, less any lining, and it decides how much structure sits above the room. The external size is the outside of the finished kerb, including the thickness of the timber, any insulation fixed to it and the membrane or covering dressed over it. The lantern's base sits on this outer face, so the external size is the figure the lantern is made to fit.
| Measurement | Where it is taken | What it governs |
|---|---|---|
| Internal kerb size | Between the inner faces of the kerb | The opening in the ceiling and the light well |
| External kerb size | Across the outside of the finished kerb | The size the lantern base is made to sit on |
| Overall lantern size | Across the outside of the lantern base | How far the lantern extends over the kerb |
The difference between the two is the build-up of the kerb wall. Timber thickness, insulation and the weatherproof covering all add to the outside, and different makers specify different overhangs for the lantern base beyond the kerb. This is why a lantern should not be sized from a catalogue figure alone. The maker's drawing for the chosen unit says which dimension it needs to see, and where on the kerb it is measured.
When we send the kerb sizes to the builder or architect, both numbers go in the same document, labelled, for the product in question, before the roof is built. If you are reading a drawing and it simply says 'lantern opening', ask which of the two it means.
Sizing the lantern to the room below
The kerb has to be big enough for the structure and small enough for the roof. In between, the size is a design choice, and it starts with the room underneath, not the glass catalogue.
A lantern is seen from below as a bright panel in the ceiling, and it reads best when it looks deliberate. Centred over a dining table, it frames the space. Pushed to one side, it looks accidental unless it follows a line in the room, such as the edge of the kitchen island or the width of the garden doors. A margin of flat ceiling around the opening makes the lantern look like a feature and not a hole.
Proportion matters in the other direction as well. A lantern that is too small for a wide room looks like a skylight lost in a large roof, and one that is too big can overwhelm a small kitchen with glare and heat. Orientation changes the balance. A south-facing room gets strong light and warm afternoons, and may call for solar control glass or an opening section. A north-facing room usually welcomes the full size.
Height of the room is another limit. The lantern's glazed part rises above the ceiling line, so the ceiling beneath needs a lining that leads the eye up into the glass, usually with a splayed or square reveal. Deeper reveals cut glare but also light; shallow ones pass more light but need neat plastering. The plasterer makes the reveals after the lantern is in, so their depth should be agreed when the opening is set out.
Mark the proposed opening on the plan and look at it from where people will sit. For the wider comparison with a flat panel in the same position, see our lantern or flat rooflight guide.
Upstand height above the finished roof
The upstand is the height the kerb stands above the finished roof surface. It matters for two reasons: water and appearance. The kerb has to stand clear of any water on the roof, and the lantern base sits on top of it, so the finished height sets how the lantern looks from the garden.
As a working figure the upstand is commonly at least 150mm over the top of the final roof layer, to the maker's instructions. The maker's guidance for that particular unit overrides any rule of thumb, and it may ask for more where the roof is warm, the covering thick or the fall steep. 'Finished roof surface' is the phrase to hold on to. It means the top of the final waterproof layer, after insulation, boards and membrane have all been added, not the top of the deck.
That is the trap. On a warm roof, insulation sits above the deck, so the finished surface is a good deal higher than the structural timber. A kerb that was cut to a height measured from the deck can end up too low once the build-up goes on. The cure is to work out the roof build-up first and give the kerb height as a figure above the finished surface, with the build-up thickness stated alongside.
- State the roof covering (EPDM, GRP, felt or a single-ply membrane) because each is dressed up the kerb differently.
- State the insulation depth so the finished surface can be calculated.
- Give the upstand as a height above the finished surface, in millimetres.
- Allow for a fall in the roof, since the kerb height is measured at the low side.
A tall upstand makes the lantern look raised from the garden; a low one sits neat but leaves less margin for water. The order of work is the same either way: the roofer completes the membrane up the kerb and the lantern is fitted afterwards, as set out in our note on who builds the kerb.
Standard sizes or made to measure
There are two ways to buy a lantern: from a maker's range of set sizes, or built to an exact specification. Both are sound. The choice affects the lead time and the freedom you have with the opening.
A set size comes from a published range. The maker lists the lantern's external dimensions, the kerb it needs and the loading it will bear, and the builder frames the opening to those numbers. It suits a project where the room has not yet been drawn in detail, and it can be quicker where the maker holds stock or can run the size through a standard process.
Made to measure means the lantern is built to the dimensions you give. It lets the lantern be placed to the exact centre of a dining table, to follow the garden doors or to fit a roof with an awkward shape. The cost of that freedom is time, because the unit is made after the order, and the date is given by the maker. The written quote states that wait, and the order is checked against the programme.
Here the wedge in our own method applies. The sizes are fixed early. Once the choice is made, we send the kerb and opening dimensions for that exact unit to the builder or architect, in writing, before the roof is built. The order is then placed counted back from the day the roof can take glass, and homeowner and builder are told the firm delivery date. Neither route should leave the roof deck open, waiting for glass.
Once ordered, a made-to-measure lantern cannot be altered, so build the kerb only after the maker's drawing has been checked against the plan. Our cost page, how much a roof lantern costs to fit, and the frame comparison, aluminium or uPVC, cover what else shapes the specification.
Structural openings and the steelwork
A lantern is heavier than a flat panel. The frame, the glazing bars and the glass all weigh more, and the kerb has to carry that load into the roof structure. For that reason the structural design of the opening belongs to whoever designs the roof, which is usually the architect's structural engineer or the builder's engineer.
In a flat roof the opening is framed in joists, with trimmers on each side that carry the joists cut or displaced by the hole. In a larger extension, steel beams may span the room, and the opening is placed between them so the lantern's weight lands on structure.
Several things need to be known by the engineer: the size and weight of the lantern from the maker's data, the position of the opening, the roof build-up and the finish of the ceiling below. The maker supplies the weight and any loading limits, and the engineer's calculation turns that into joist sizes and fixings. This is also the information the building control inspector will want to see, and the regulations side is covered in our Building Regulations guide.
Where steelwork is needed, it is ordered by the builder, and its delivery date can be a cause of delay. That should be sequenced alongside the lantern's lead time. A lantern is no use if the beam it depends on has not arrived, and a beam in position with no lantern leaves a roof exposed to weather. A sensible programme lists the steel, the kerb, the roof covering, the lantern and the plastering as separate steps with dates.
What to send your builder
Everything above comes down to a short document the builder can build from, and it takes less time to write than to argue over a mismatched kerb afterwards.
We prepare it as part of the quote. The quote lists the unit, glazing, lead time and what the builder must supply, and a supporting sheet gives the dimensions. If you are sending your own, include the following.
- The exact unit. Maker, range, frame material and glazing, so the drawing relates to a real product.
- Internal and external kerb sizes. Both numbers, labelled, with the position of the opening on the roof plan.
- Upstand height. Given above the finished roof surface, with the roof build-up stated.
- Roof covering. EPDM, GRP, felt or another membrane, since the roofer needs to plan the dressing.
- Weight and loading. From the maker, for the engineer.
- Delivery date. The confirmed date of the lantern, so the roof can be programmed around it.
Send it to the builder and, if there is one, the architect and the engineer, at the same time, so all three work from the same sheet. If the kerb is going to be built by the builder, they should confirm it back in writing once it is in place, with the sizes measured on site. We then check those figures before the glass is ordered or released.
Two further questions come up often. Whether a lantern can go on a flat roof that already exists is covered in our answer on existing flat roofs. And whether it needs planning permission depends on height and position, which the answer at lantern planning permission explains. When you are ready, our roof lanterns page shows the range, and a survey will turn all of the above into one set of numbers.
Smaller sizing questions
- Can I change the kerb size after the lantern is ordered?
- A made-to-measure lantern is built to the sizes given at the order, so changing the kerb afterwards usually means a new unit. Check the maker's drawing against the plan and the built kerb before the order is confirmed.
- Who measures the kerb once it is built?
- The builder confirms the built sizes in writing, and we check them against the drawing before the glass is released. Two people measuring the same kerb catches most slips while they are still cheap to correct.
