Sizing a roof lantern for the room below
The right size for a roof lantern is worked out from the room, not from the roof. The roof sets the ceiling on what is possible, and that side of it is covered in detail on the roof lantern installation page. This page does the other half: given a kitchen of a certain floor area, a certain ceiling height and a certain shape, how much glass overhead actually makes it feel like a glazed room rather than a room with a glazed hole in it. There is real arithmetic behind that, it is not difficult, and it explains why two lanterns of the same footprint can produce completely different results in two houses on the same Gloucester street.
Start from the floor area, not the roof plan
Most enquiries arrive with a lantern size already chosen, usually because it appeared in a photograph or a neighbour has one. The size that turns up that way is right about as often as a guessed shoe size.
The sequence that works runs the other way. Measure the room: length, width, ceiling height, and the position of any wall or full height unit that blocks light across it. Decide what the room is for, because a kitchen wants more daylight than a sitting room and a study wants it without glare. Work out the aperture area that delivers that. Only then check what the flat roof can carry, what the joist layout allows and what the falls need at the margins. Nine times in ten the room asks for less glass than people expect and wants it in a different place.
Daylight factor, and the number that separates a bright room from a lit one
Daylight is measured as a factor rather than an absolute, because the sky changes all day. Average daylight factor is the ratio of the illuminance inside a room to the illuminance outside under a standard overcast sky, expressed as a percentage. British daylight guidance has settled on rough thresholds: around 1 per cent for a living room, 1.5 per cent for a bedroom and 2 per cent for a kitchen, with anything above 5 per cent reading as a bright, sunlit space and anything under 0.5 per cent reading as a room that needs the lights on at two in the afternoon.
The factor depends on the glazed area, how much sky the glass can see, the transmittance of the glass, the total internal surface area of the room and how reflective those surfaces are. Every one of those is under somebody’s control, which is why the same lantern gives a different result in a room with dark grey units and a room with white walls.
Why overhead glass counts for more than the same area in a wall
A vertical window sees a half hemisphere of sky at best, and in a terrace it sees a wall, a fence and a slice of sky above them. Glass lying at 25 degrees in a flat roof sees almost the whole sky vault, with nothing in front of it.
The practical multiplier is between two and three. A square metre of lantern glass delivers roughly two to three times the daylight of a square metre of glass in a rear wall of the same room, and it delivers it into the middle of the floor rather than in a bright band along the back wall. That is the whole argument for a lantern over a wider set of doors, and it is why the glazed area needed overhead is smaller than people assume.
Glazed area against floor area: the ratios that work overhead
The rule of thumb worth carrying is 10 per cent. An aperture area of about a tenth of the floor area, glazed overhead and reasonably central, gives a bright, evenly lit room in this country. Vertical glazing needs closer to 20 per cent of floor area for the same result and still leaves the far corners dim.
That ratio bends in both directions. A north facing room with dark floors and dark units wants 12 to 15 per cent. A south or west facing extension with pale finishes will be comfortable at 8 per cent and difficult at 15, because the problem stops being daylight and starts being heat. Rooms that are also lit by a large set of doors need less overhead glass than a room with one small window. The arithmetic tells you where to start, not where to finish.
Aperture, footprint and the sloping glass are three different areas
Sizing conversations tangle because three areas get called the same thing. The footprint is the plan rectangle the lantern occupies on the flat roof, measured over the kerb. The aperture is the daylight opening you look up through from the room, smaller than the footprint by the thickness of the kerb and its internal lining on all four sides. The glass area is the sum of the sloping panes, which is larger than the footprint because the planes are tilted.
At 25 degrees the sloping glass adds about 10 per cent to the plan area. At 30 degrees it is about 15 per cent, and the hips add a little more because they lie at a shallower angle than the common rafters. Useful to know when a supplier quotes a glass area and a fabricator quotes a kerb size, because they are describing the same lantern with numbers that differ by a fifth.
Ceiling height changes the size you need
The higher the glass sits above the floor, the wider the pool of light it throws and the weaker that light is when it arrives. Under a diffuse overcast sky, useful light from an aperture spreads outwards roughly a metre for every metre it drops. A 1200 by 2000mm lantern in a 2.4 metre ceiling lights a pool on the floor around 3.5 by 4.5 metres, which covers a normal rear extension. Put the same lantern in a 3.2 metre ceiling in a converted double height space and the pool widens but the illuminance on the worktop falls noticeably.
Kerb height is part of that measurement. A lantern on a 150mm kerb with a 300mm internal lining sits nearly half a metre higher than the ceiling plane, and on a deep warm deck build up the kerb can be taller again. That extra height also lengthens the reveal, which throws away light if the reveal is boxed vertically and returns some of it if the reveal is splayed.
Room depth, and the far corner that stays dim
A single lantern lights a pool. It does not light a long room evenly, and the mistake that follows is buying a bigger lantern to reach the far end when what the room needed was a second aperture.
Industrial rooflight design has a spacing rule that transfers well to houses: apertures spaced no further apart than one and a half times their height above the working surface give even light with no dark bands between them. On a 2.4 metre ceiling above a worktop at 900mm that is a spacing of about two and a quarter metres between centres. In a room six metres deep, two modest lanterns at that spacing beat one large one for evenness, and each one is lighter, cheaper to lift and simpler to weather.
Pane modules: a lantern is sized in whole panes
Lanterns are not made to any dimension you like. The rafter spacing is set by pane weight, commonly 500 to 1000mm between rafters, and the ridge length has to divide into a whole number of panes with the hips taking up the ends.
So the practical size steps are coarse. Ask for a 2100mm long lantern and the fabricator will build it as three panes at 700 or as four at 525, and the difference is visible from the sofa for the next thirty years. Asking for the pane layout at quotation stage, not just the overall size, is how you avoid a five bar grid over a small kitchen. Sightlines and bar widths are covered on the roof lantern sightlines page.
Landing the footprint on whole joist bays
Flat roof joists usually run at 400 or 600mm centres. A lantern footprint that lands neatly so that the trimming joists fall on existing joist positions needs one pair of doubled trimmers and nothing else. A footprint 80mm wider, so that a third joist has to be cut and the load carried further, can turn routine carpentry into an engineer’s calculation and a Building Control submission.
That is worth 80mm of somebody’s ambition almost every time. The joist positions are recorded at survey, the footprint is nudged to suit, and the structural side of it is set out on the structural openings and steel page.
Proportion: the plan ratio that stops a lantern looking stubby
Below a certain size a hipped lantern stops reading as a lantern. The hips eat into each end, the ridge shortens to almost nothing, and what is left is a glass pyramid with a flat top.
The ratio to watch is length against width. At 2:1 or longer, a hipped lantern has a proper ridge and reads correctly. At 1.3:1 the ridge is barely there. Below about 1000mm wide the hips consume the whole plan and a pyramid is the honest answer, or a flat glazed unit instead, described on the flat roof rooflight installation page. Height matters too: at 25 degrees a 1400mm wide lantern rises about 330mm at the ridge, and a lantern much taller than a third of its width starts to look like a garden structure on the roof.
What the frame does to the ceiling as an object
From below, a lantern is a grid on the ceiling, and its scale is judged against the room rather than against the roof. A 1200 by 2000mm lantern in a 24 square metre kitchen occupies a tenth of the ceiling and reads as a feature. The same lantern in a 12 square metre galley occupies a fifth, and every glazing bar is in your eyeline from the sink.
Two things soften that. A pale internal frame colour merges the grid into a white ceiling, and fewer, wider panes give more glass and fewer lines. Colour choices sit on the frame finishes and powder coating page.
Aperture size against room and use
| Room and use | Floor area | Aperture for a bright result | Sloping glass at 25 degrees | Typical footprint that delivers it |
|---|---|---|---|---|
| Galley kitchen in a narrow rear addition | about 12 m2 | 1.2 m2 | around 1.4 m2 | 800 by 1500mm, or a flat glazed unit instead |
| Single storey rear extension, kitchen and eating | about 18 m2 | 1.8 m2 | around 2.0 m2 | 1000 by 2000mm hipped |
| Wider rear extension, kitchen and dining | about 24 m2 | 2.4 m2 | around 2.7 m2 | 1200 by 2000mm hipped, the commonest size in the county |
| Open plan kitchen and living, one space | about 30 m2 | 3.0 m2 | around 3.3 m2 | 1500 by 2500mm, or two at 1200 by 1500mm |
| Full width rear extension across a Victorian terrace | about 40 m2 | 4.0 m2 | around 4.4 m2 | Two lanterns, spaced for evenness rather than one large one |
| Hall or stair void, circulation only | about 9 m2 | 0.7 m2 | around 0.8 m2 | 1000 by 1000mm pyramid |
| Study or home office, glare matters | about 14 m2 | 1.1 m2 | around 1.2 m2 | 1000 by 1200mm with solar control glass |
These are starting points from the daylight arithmetic, not sizes to order from. Orientation, finishes, existing windows and the joist layout all move them.
Bigger glass means more heat, and the number rises with area
Solar gain scales directly with glazed area, so every square metre added for daylight also adds a fixed quantity of summer heat. Doubling the aperture in a west facing Severn Vale extension roughly doubles the afternoon gain, and the point at which a room becomes uncomfortable arrives well before the point at which it becomes too bright.
That is the real ceiling on size in most houses. Past about 12 per cent of floor area, the specification has to work harder: solar control coatings, an opening vent at high level, and shading that is outside the glass rather than inside it. The glass side is covered on the roof lantern glazing and solar control page, and the venting on the ridge vents page. Part L also caps the total glazed area in a new extension as a proportion of its floor area, which is dealt with under energy efficient rooflights.
Sizing around a knocked through Gloucester terrace kitchen
The most common version of this job in the city is a Victorian or Edwardian terrace off Barton Street, Tredworth, Kingsholm or Longford where the rear addition and the old kitchen have been knocked into one and a flat roofed extension runs across the back. The room that results is L shaped, deep, and lit from one end by doors.
The arithmetic there is not a single number. The old rear addition leg of the L is typically 2.4 metres wide and gets no light at all from the doors, so it needs its own aperture, and 2.4 metres of width leaves room for a lantern about 1000mm wide once the falls and the weathering into the house wall have their margin. The new leg is wider and shallower and is already half lit. Two apertures of different sizes in the two legs beat one large lantern in the middle, which lights the junction and leaves both ends dim. Lanterns on that house type are covered further on the Victorian rear addition page.
Warden Hill, Quedgeley and the extension that is already at its height limit
On the interwar semis around Warden Hill and Charlton Kings, and the estates at Quedgeley, Abbeydale and Bishops Cleeve, the flat roofed rear extension is often built as high as the first floor windows will allow. The lantern then has to fit inside a height budget as well as a plan one.
Size and pitch are linked, so a wide lantern at 25 degrees can breach a permitted development height limit that a narrower one at the same pitch clears comfortably. A 2500mm wide lantern rises about 580mm at the ridge; a 1500mm wide one rises about 350mm. Where the extension roof is already close to four metres overall, the sizing exercise becomes a three way trade between width, pitch and consent, and the pitch side of it is set out on the roof lantern pitch page.
Furniture, extract hoods and where the size gets trimmed
The last cut to a size is usually made by something inside the room. An island extract hood rising to the ceiling takes up plan space the lantern wanted. A pendant light over a table has to sit clear of the eaves beam. Full height units along one wall block light across the room and shift the aperture away from centre, which is dealt with on the positioning a lantern over a kitchen island page.
Get the kitchen layout drawing and the lantern setting out on the same sheet before the kerb is built. Cheaper by a wide margin than discovering the clash after the frame has been made.
What gets recorded at survey to fix the size
For sizing specifically, the survey sheet carries the room length, width and ceiling height, the finished floor level and the underside of the existing ceiling, the joist direction, depth and centres taken from a small opening rather than guessed, the position of every full height obstruction inside the room, the existing glazed areas and which way they face, the flat roof dimensions from gutter to house wall, the position of the outlet and anything already coming through the deck, and the overall height of the extension roof measured rather than scaled.
From that, one footprint, one pane layout and one aperture dimension go on the order. Send photographs of the room and of the flat roof from an upstairs window to info@severnskylightsgloucester.co.uk and the first reply will already have the arithmetic in it. Coverage across the county is listed on the areas pages, and other written answers sit under guides.
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