Guide · Roof Lantern Installation
Sizing a roof lantern for the room below
The starting figure is glazing area at somewhere between fifteen and twenty per cent of the floor area of the room underneath. On a four by five metre kitchen extension that is three to four square metres of glass, which lands close to a 2000mm by 1500mm lantern. That number is a beginning, not an answer, because half a dozen things about the roof, the room and the structure will move it in one direction or the other before it is ordered.
This page works through those corrections in the order a survey meets them. It is written around the rear extensions, side returns and knocked-through kitchens that make up most lantern work across Chelmsford, Braintree, Maldon and the commuter towns either side of the A12, because the footprints repeat and so do the sizing mistakes.
Start with the floor area, then correct it
Measure the room, multiply length by width, and take between fifteen and twenty per cent of it as a target glazing area. Use the higher end where the room is deep, has one external wall, or faces north. Use the lower end where the room already has bifolds or a wide set of french doors feeding it, because those are doing work the roof does not need to repeat.
Then apply the corrections. A room with a dark floor and dark units needs more glass than the same room in pale oak, because reflectance inside the room does a surprising amount of the distribution. A room that is genuinely open to a hallway or a stairwell is bigger than it measures. And a room with a tree or a two storey neighbour close to the south boundary is losing light the calculation does not know about.
Why the roof, not the room, usually sets the maximum
In practice the roof runs out before the daylight target does. A lantern has to sit on a kerb, the kerb has to sit clear of the roof edges, and the roof covering has to run all the way around it with room to drain.
The working figure is a minimum of 300mm of flat roof between the outside face of the kerb and any parapet, upstand or abutment, and 400mm is more comfortable where water is being pushed toward an outlet on that side. On a 3.5 metre deep extension that leaves about 2.7 metres of usable depth, and the lantern then has to fit inside it with the falls taken into account. This is the single most common reason a customer’s first sketch shrinks. The kerb itself, and who builds it, is covered on the kerb page.
Three different sizes, and which one a supplier is quoting
A lantern has three numbers attached to it and confusion between them is expensive. The external frame size is the outside of the aluminium base ring. The kerb size is the outside of the timber upstand the lantern sits on, which is usually the same as the external frame size or a few millimetres under it. The structural opening is the hole in the roof deck, which is smaller again because the kerb sits on the deck rather than in it.
Manufacturers quote to different conventions, and a 2000 by 1000 lantern from one supplier does not always sit on the same kerb as a 2000 by 1000 from another. The number that matters on site is the kerb size, because that is what the carpenter builds. Every order Sunspire places names the kerb size in writing and the external frame size alongside it.
If a quote gives one size and does not say whether it is the frame, the kerb or the opening, it is not yet a size.
Standard sizes, bespoke sizes and the step between them
Most manufacturers hold a grid of stock sizes, typically in 500mm increments from around 1000 by 1000 up to 3000 by 2000. Those sizes carry the shortest lead times and the lowest cost per square metre, because the extrusions are cut to jigs that are already set.
Bespoke sizing is available from nearly every serious manufacturer and it is made to the millimetre. The cost step is smaller than people expect, often in the region of ten to fifteen per cent, and the lead time step is usually two to three weeks. Bespoke is worth it when the roof dictates an awkward number. Forcing an extension roof to suit a stock lantern is the wrong way round: the roof is already built, and the glass is the part that can be made to fit.
Proportion, and matching the lantern shape to the room
Area is only half of sizing. Shape decides how the light lands. A lantern that echoes the plan of the room reads as intentional. A square lantern in a long thin room leaves both ends dark and looks marooned.
The useful discipline is to keep the lantern’s aspect ratio within touching distance of the room’s. A 6 metre by 3 metre kitchen wants something around 2:1, so 2400 by 1200 or 3000 by 1500 rather than 2000 by 2000. Where the room is close to square, a square lantern is genuinely the right call and the hipped form it produces, with four sloping faces meeting at a short ridge, is the most balanced shape a lantern makes.
One lantern or two down a long extension
Beyond about five metres of run, one lantern struggles. Either it becomes so long that the ridge dominates the ceiling, or it stays a sensible length and leaves a dark half.
Two smaller lanterns, spaced with a solid band of ceiling between them, usually beat one long one. The light arrives in two places rather than one, the structure is simpler because each opening is shorter, and the pair reads as rhythm rather than as a rooflight that got away. The band between them wants to be at least 600mm so it is legible as ceiling rather than as an accident. The one caution is cost: two kerbs, two sets of flashing and two units is more work than one.
Room depth, and how far daylight actually reaches
Daylight from an opening in a roof spreads more evenly than daylight from a window, but it is not infinite. The rough working rule is that a rooflight lights a zone about one and a half times its own width, measured out from each edge at ceiling level, before the illuminance drops away sharply.
That gives a direct sizing test. Stand the plan of the lantern on the plan of the room, add 1.5 times its width to each side, and see whether the shaded area covers the parts of the room you actually use. A worktop run along the back wall of a deep extension is frequently outside that zone when the lantern has been centred on the room, and moving the lantern back is a better fix than making it bigger.
Ceiling height, and the height a lantern adds
A lantern adds volume as well as light. A 1500mm wide lantern at 25 degrees adds roughly 350mm at the ridge, and at 30 degrees closer to 430mm. In a room with a 2.4 metre ceiling that is a real gain and it is felt immediately.
The reverse case matters too. In a room with a high vaulted ceiling already, a lantern of modest size can disappear into the volume and deliver less than the same unit would in a standard height room, because the light has further to fall and more surface to cover. Where the ceiling is already generous, size up. How steep the slope should be is worked through on the pitch page.
Sizing against what holds the roof up
Every millimetre added to a lantern is a millimetre taken out of a roof that was carrying load. Up to about 1500mm across the span, timber trimming with doubled joists either side and trimmers top and bottom will normally do it. Past that, and particularly where the long axis of the lantern runs across the joists rather than with it, the load has to be picked up.
This is the point where sizing stops being a daylight question and becomes an engineering one, and it is worth knowing before a size is fixed rather than after. The thresholds, the calculations and what a steel does to the programme are set out on the structural openings page.
Overglazing, and the summer consequence
The correction that pushes size down is solar gain. A room glazed at twenty five per cent of floor area with a south facing aspect and a clear glass lantern will be uncomfortable from June to September, and no amount of opening the doors fixes it, because the heat is arriving through the glass rather than through the air.
Building Regulations put a ceiling on this too. Part L limits the total area of windows and rooflights in a new extension, conventionally to twenty five per cent of the new floor area plus the area of any openings closed up, unless the whole thing is justified by calculation. Where a design wants to sit near that limit, the glass specification carries the load, which is the subject of the glazing page.
Sizing around what is already in the ceiling void
The survey looks up as well as down. A soil vent pipe rising through the flat roof, a boiler flue, a bathroom extract duct, a run of downlight cabling or a concealed gutter all occupy the plan, and none of them appear on the customer’s sketch.
Moving services is possible and sometimes routine, but it changes the price and occasionally the size. A 2000mm lantern that clears an existing SVP by 100mm is a better outcome than a 2200mm one that needs the pipe rerouted through a ceiling that is already finished. Establishing this early is why the survey looks in the void rather than only at the roof.
Typical Essex extension footprints, and what fits them
The stock repeats, and so do the sizes that suit it.
| Extension type | Typical footprint | Lantern that usually fits |
|---|---|---|
| Victorian rear addition infill | 2.0m to 2.5m wide, 3m to 5m long | 1500 by 1000, or two at 1000 by 1000 |
| Interwar semi, full width rear | 5m to 6m wide, 3m deep | 2500 by 1500 |
| Post war estate, wraparound | L shaped, 6m by 4m plus return | 2000 by 1500 in the main run |
| New build garden room | 4m by 3m | 2000 by 1200 |
| Bungalow flat roof link | 3m by 3m | 1500 by 1500 hipped |
Sizing a lantern for a side return
The side return infill is its own problem, and it is common on the Victorian terraces around Moulsham Street and in the older parts of Colchester and Brentwood. The strip is narrow, often 1.8 to 2.4 metres, and one long side is a party wall.
The 300mm border still applies against the party wall parapet, and the flashing there is more demanding than on a free edge. That usually caps a lantern at around 1200mm wide in a 2 metre return. Length is where the gain is, so a long narrow lantern of 2400 or 3000 by 1000 is often the honest specification. The particular constraints of that house type are covered on the Victorian rear addition page.
Sizing a replacement into an existing opening
Where a lantern is being swapped rather than installed new, the opening is already there and the sizing exercise reverses. The measurement that governs is the outside of the existing kerb, taken at both ends and in the middle, because timber kerbs built fifteen years ago are rarely square.
A new lantern can be ordered to sit on an existing kerb almost every time, provided the kerb is sound and level. Where the old unit was a dome or a pyramid on a much lower upstand, the kerb often wants rebuilding to current height, and that is the moment to reconsider size, since the carpentry is open anyway. The wider question of swapping an aged unit is covered under replacement.
How size changes the number of glazing bars
Lanterns are made from panes, and panes have a maximum practical size before weight and deflection force a bar in. Most systems land a rafter bar every 600mm to 1000mm along the ridge, so a 3000mm long lantern carries four or five bays a side and a 1500mm one carries two.
This matters at the sizing stage because a size chosen without reference to the bar module can produce an awkward set of panes, with three wide bays and a narrow one at the end. Asking the manufacturer for the bay layout at quotation, not at delivery, avoids it. How thick those bars look from underneath is a separate question and is covered on the sightlines page.
Getting a sensible size range by email
Four things settle a size well enough for a range: the internal dimensions of the room, the dimensions of the flat roof it sits under, which way the extension faces, and whether the roof is existing or still to be built. A photograph of the roof from an upstairs window and one of the ceiling from inside covers most of the rest.
From that, Sunspire will come back with a size range and an indicative figure by email. The survey then fixes it, because the difference between a sketch and a tape measure on a kerb is usually thirty or forty millimetres, and thirty or forty millimetres is the difference between a lantern that lands and one that has to go back. Costs are set out under costs, and the coverage across the county under areas.
Keep reading
More on roof lantern installation
Roof lantern glazing and solar control
Read the guide →Structural openings and steel for a roof lantern
Read the guide →Roof lantern ridge vents and how they are operated
Read the guide →Roof lantern pitch and how steep it should be
Read the guide →Roof lantern kerbs and who builds them
Read the guide →
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