Roof lantern pitch and how steep it should be

Which member the quoted pitch refers to

A lantern quotation gives one pitch figure and it almost always means the common rafter: the angle of the sloping members that run from the eaves beam up to the ridge, measured on the short axis of the lantern. That is the pitch of the two long faces, and it is the number that governs the height.

It is not the angle of the hips, and it is not the angle at which the glass on the hipped ends sits. Those are both shallower, for geometric reasons set out in a moment. Where a manufacturer quotes a range, say 15 to 40 degrees, it means the common rafter can be set anywhere in that band, and the frame is cut to suit at fabrication rather than adjusted on site. Some systems have a single fixed pitch, usually 25 degrees, with the corner castings machined for that angle and nothing else, which is a good reason to settle the pitch before choosing the system rather than after.

The arithmetic: rise from span and pitch

The rise of a lantern above its eaves beam is half the short span multiplied by the tangent of the pitch. That single line of trigonometry answers most of the questions people have about lantern height.

On a 1200mm wide lantern, half the span is 600mm. At 15 degrees the rise is 161mm. At 25 degrees it is 280mm. At 30 degrees, 346mm. At 40 degrees, 503mm. Widen the lantern to 2000mm and every one of those figures increases by two thirds: 268mm at 15 degrees, 466mm at 25, 839mm at 40.

The point that follows is that width and pitch multiply. A wide lantern at a modest pitch can be taller than a narrow one at a steep pitch, and the two decisions cannot be taken separately. Add the kerb to whatever the rise comes to, because the height that matters for consent and for how the thing looks from the garden is measured from the finished flat roof surface, not from the eaves beam. A 2000mm lantern at 25 degrees on a 150mm kerb stands about 620mm above the roof.

The hip is shallower than the rafter, and by how much

On any hipped roof, the hip runs diagonally, so it covers more horizontal distance to reach the same height, and it is therefore at a shallower angle than the common rafter. This is the fact that surprises people looking at a lantern drawing.

For a hip at 45 degrees in plan, which is the normal case on a rectangular lantern, the relationship is that the tangent of the hip angle equals the tangent of the rafter angle divided by the square root of two. A 25 degree common rafter therefore gives a hip at about 18.3 degrees. A 30 degree rafter gives a hip at 22.2. A 40 degree rafter gives a hip at 30.5.

Two consequences. The glass on the hipped ends sits at that shallower angle, so it drains more slowly, holds dirt longer and gets less benefit from a self cleaning coating than the glass on the long faces. And the hip rafter is a longer member than the common rafter for the same lantern, carrying triangular jack rafters on both sides, which is why the hip section is usually heavier than the common section in any decent system.

A pyramid’s true pitch is measured on the diagonal

A pyramid lantern has no ridge, four triangular faces meeting at an apex, and here the quoted pitch is the angle of the faces up from the eaves beam.

The diagonal members, which on a pyramid are all hips, sit at the shallower angle again. On a 1500 by 1500mm pyramid with 25 degree faces, the rise at the apex is 350mm and every one of the four hips lies at about 18.3 degrees. Since every pane on a pyramid is a triangle bounded by two hips, there is no glass anywhere on that lantern at the quoted 25 degrees except along the centre line of each face.

The practical effect is that pyramids drain and self clean less well than hipped lanterns of the same nominal pitch, and they are therefore usually specified a few degrees steeper. A pyramid also gains height fast, since the span is the full width in both directions, so on a 1800mm square pyramid at 30 degrees the apex is 520mm above the eaves beam and nearer 700mm above the roof surface. Over a stair void that height is welcome. On an extension already close to a permitted development limit it is the thing that breaches it.

Why dirt stays on shallow glass and washes off steep glass

Rain does two jobs on a lantern: it runs off, and it carries dirt with it. Both depend on the film of water moving fast enough to pick up and transport particles rather than simply wetting them and evaporating.

Below about 5 degrees the film barely moves, and the dust deposited in dry weather is wetted, spread and left in place, so the glass develops the grey bloom that looks like a permanent haze. Between 5 and 10 degrees it runs, slowly, and streaks form where the flow concentrates. From 15 degrees upwards water sheets properly and the glass stays visibly cleaner between cleans. Past 25 degrees the difference is marginal, so 15 to 20 degrees is where most of the benefit is already collected.

Pitch also decides where the dirt ends up. On any lantern, the bottom 100mm of each pane against the eaves gasket collects the run off from the whole pane above it, and that band goes grey first on a shallow lantern and stays cleaner on a steep one. It is worth looking at somebody else’s lantern from the garden before choosing a pitch, because the band shows from twenty metres away.

Self cleaning coatings and the angle they need to function

A self cleaning coating on the outer face relies on rain sheeting across a hydrophilic surface to carry away the organic dirt its photocatalytic layer has broken down. No flow, no cleaning.

Manufacturers are reasonably consistent that the coating needs a minimum of about 10 degrees to perform, with better results from 15 upwards. That has a direct bearing on the hips: at a 20 degree common rafter the hips are at about 14.5 degrees, and at a 15 degree rafter the hips are at only 10.7, right at the edge of where the coating does anything at all. On a lantern that cannot be reached for cleaning, and specifying a self cleaning coating because of that, the pitch has to be chosen to make the coating work rather than assumed to be irrelevant to it. The coating chemistry is set out further on the roof lantern glazing and solar control page.

Fifteen to twenty degrees: the long low lantern

A shallow lantern reads horizontally. On a wide single storey rear extension, particularly a full width one across a Victorian terrace, 15 to 20 degrees gives a lantern that sits down into the roof and looks like part of the building rather than an object on it.

What it costs: slower drainage and a dirtier appearance between cleans, hips down near the limit for a self cleaning coating, more rain noise because the drops strike closer to perpendicular, and less internal volume so the ceiling feels flat rather than lifted. What it gains: a low overall height, which matters when a permitted development limit is close, less glass area for the same footprint so slightly less solar gain, lighter panes, and a proportion that suits a long footprint. On a 3000mm long lantern only 1200mm wide, 25 degrees produces something that looks like a greenhouse ridge, and 15 degrees looks right.

Twenty five degrees, and why it became the default

Twenty five is the pitch most lanterns are made at, and it is not an arbitrary convention. It sits above the angle where drainage and self cleaning are compromised, with the hips still at a workable 18 degrees. It gives visible internal volume, a rise around a quarter of the span, which reads as a proper roof from below. It keeps the overall height inside about 450 to 550mm on domestic sizes, which usually keeps an extension inside its height limits. And it is steep enough that rain noise is noticeably lower than on a shallow unit.

It is also the pitch that most corner castings, ridge sections and hip connectors are tooled for, which means the widest choice of systems, the shortest lead times and the fewest special components. Choosing 25 degrees is a decision to be ordinary in a way that pays.

Thirty to forty degrees: volume, and the object on the roof

Steeper lanterns are specified for two reasons: internal drama, and winter light. Past 30 degrees the volume above the ceiling line becomes a real room feature, and the steeper faces catch the low December sun much more squarely, so a steep lantern brings noticeably more light deep into a room in midwinter.

The costs are all real. Overall height grows quickly, so consent gets harder. Glass area for the same footprint increases, at 40 degrees the sloping glass is about 30 per cent more than the plan area, which means more solar gain and more heat loss. Panes get longer and heavier, since a pane runs the full slope length. The frame becomes prominent from the garden and from a neighbour’s upstairs window. And a steep pyramid on a small footprint starts to look like a folly.

Where steep works: an orangery or a garden room with height to spare, a lantern over a stair void where the object is the volume, and a pitch matched deliberately to the pitch of the main roof behind it so the two read together, which is a good move on a Cotswold garden room where the main roof runs at 45 to 55 degrees.

Pitch against the permitted development height limit

Height is the limit that catches lanterns on extensions. A single storey rear extension built under permitted development must not exceed 4 metres in overall height, and within 2 metres of a boundary the eaves must not exceed 3 metres. A lantern adds its kerb plus its rise to the highest part of that roof.

The arithmetic is unforgiving because the roof is usually already close. Take an extension roof measured at 3.65 metres. A 1500mm wide lantern at 15 degrees on a 150mm kerb adds 351mm and the total is 4.00 metres, right on the line. The same lantern at 25 degrees adds 500mm and the total is 4.15, and permitted development has gone. Widen it to 2000mm at 25 degrees and it adds 616mm for a total of 4.27.

So pitch, width and the existing roof height are one calculation rather than three decisions, and it is done at survey off a measured height rather than a drawing. Where the numbers are tight, the ways out are a shallower pitch, a narrower lantern, a flat glazed unit instead, or a planning application. The consent detail sits on the hub page and the general planning position on do roof lanterns need planning permission.

Pitch, pane length and pane weight

Pane length runs up the slope, so a steeper lantern has longer panes for the same footprint, and a longer pane is a heavier pane.

On a 1400mm wide lantern the slope length of each long face is roughly 725mm at 15 degrees, 772mm at 25 and 913mm at 40. With rafters at 800mm centres and standard double glazing at about 32 kilograms per square metre, that is a pane of 18.5 kilograms at 15 degrees, 19.8 at 25 and 23.4 at 40. Modest differences on a small lantern. Scale it up to a 2400mm wide lantern with rafters at 1000mm centres and the same comparison runs from 39.7 kilograms at 15 degrees to 50 at 40, and with triple glazing to 66.

Two people on a scaffold can place a 40 kilogram pane accurately above head height. At 60 or 70 the answer is vacuum lifting pads and more hands, or a mechanical lift. Steepening a large lantern for the sake of appearance can therefore change how the glass gets to the roof, which changes the day and the price, and on a Chalford or a Rodborough site where the glass has to come through the house it can change what is possible at all.

Sun angles over a sloping plane at this latitude

Gloucester sits at about 51.9 degrees north, so the noon sun stands at roughly 61 degrees above the horizon at midsummer and about 15 degrees at midwinter. Pitch decides how squarely that arrives.

Energy passing through glass falls off with the cosine of the angle between the beam and the perpendicular to the surface. At midsummer noon, on a south facing 15 degree slope, the beam is about 44 degrees off perpendicular and roughly 72 per cent of it goes through. Steepen the same slope to 40 degrees and the beam is only 19 degrees off perpendicular and about 95 per cent goes through. So a steep south facing lantern face takes more summer sun, not less, and the north facing slope of that same lantern takes almost none.

Winter reverses it in the useful direction. At midwinter noon with the sun at 15 degrees, a 15 degree slope receives the beam 60 degrees off perpendicular and passes half of it, while a 40 degree slope receives it 35 degrees off and passes over 80 per cent. That is the real argument for a steeper lantern in a room used in winter: more of the low December sun, deeper into the room. The trade is more of the June sun as well, which is what the solar control coating is for.

Rain noise, hail and the energy that arrives perpendicular

Rain noise on glass depends on how much of a drop’s momentum is delivered into the pane rather than along it, and that is a cosine relationship too. A drop falling vertically onto a 5 degree pane delivers almost all of its energy into the glass. The same drop on a 30 degree pane delivers about 87 per cent, and a good part of what is left goes into sliding it down the slope.

The audible difference is bigger than those percentages suggest, because a shallow pane also holds a thicker film of water that transmits the impacts, and because a shallow pane is closer to a drum skin in the way it flexes. In practice, moving from 5 degrees to 25 makes heavy rain noticeably quieter, and the laminated inner pane does more again.

Hail is the same mechanism at higher energy, and Gloucestershire gets hard hail off the Severn in spring squalls. A steeper pane sheds it and a shallow one takes the full impact, which is one of several reasons the outer pane is toughened rather than annealed. Noise on flatter glazing is dealt with on are roof lanterns noisy in rain.

Snow: sliding, sitting and the load case

Snow slides off glass more readily than off most roof coverings, because glass is smooth, and the angle at which it starts to move is somewhere around 20 to 25 degrees depending on how wet the snow is.

Below that it sits, which has two consequences. It is a load, and a shallow lantern retains snow while the steep one beside it has shed it, so the structural case for the shallow one is worse. And it blocks the light entirely for as long as it lies there, which on a lantern that is the only daylight into a kitchen is a real inconvenience in a cold week.

Above about 25 degrees snow tends to slide, and that raises a question about where it goes. Snow sliding off a lantern lands on the flat roof around it, which is fine, unless the lantern sits close to a roof outlet where a mass of sliding snow can block the drainage at the worst moment. On the higher ground in this county, Painswick, Birdlip, Cleeve Hill and the wolds villages, where the snow load is genuinely higher than in the vale, that is worth thinking about at the setting out stage. The load figures are in the structural openings and steel page.

Blinds, and the pitch that makes them work or sag

Internal blinds in a lantern run up the slope in tracks between the rafters, and gravity is the whole problem. On a steep pane a blind runs and retracts cleanly because the fabric is closer to hanging. On a shallow pane the fabric wants to sag away from the glass in the middle of its run, and the fix is a tensioned wire or a track with a deeper guide, which is more hardware in view.

Pleated and honeycomb blinds are the usual choice, held in side tracks under tension, and manufacturers generally quote them from around 15 degrees upward. Below that the sag becomes hard to control across a long pane. A steep lantern also gives a blind somewhere to stack when it is retracted, tucked up near the ridge where it is barely visible, while a shallow one leaves the stack in the middle of the view.

Worth noting that a blind tight against the glass traps heat in the cavity between the two, which raises the pane temperature and, where a shadow line falls across a pane, contributes to thermal stress. Shading outside the glass, or a coating in it, does the job better. Blinds inside a unit are covered on the electric blinds fitted inside the rooflight page.

Pitch on a gable ended lantern and against a house wall

A gable ended lantern replaces one or both hips with a vertical glazed triangle, and that changes the pitch conversation in two ways.

Because there is no hip at that end, no glass is sitting at the shallow hip angle, so drainage and self cleaning are better across the whole lantern. And because the vertical end adds height without adding slope, the volume gained for a given pitch is greater, which means a gable ended lantern at 20 degrees can feel like a hipped one at 28.

Where the gable end runs up against the wall of the house, which is the usual reason for choosing one, the pitch has to leave the flashing detail room to work. The vertical end is flashed into the brickwork with lead or a proprietary flashing chased into a bed joint, and the higher the lantern, the further up the wall that chase sits, which on a two storey rear elevation can put it awkwardly close to a first floor cill. Measuring the available height from the flat roof surface to the underside of that cill, at survey, decides the maximum pitch as effectively as any aesthetic judgement.

Where the roof or the parapet fixes the pitch for you

Three Gloucestershire situations take the decision out of the customer’s hands.

On a Cotswold garden room or a rear addition behind a coped stone parapet in Painswick, Bibury, Northleach or Chipping Campden, the lantern has to stay below the top of the coping to keep the benefit of being invisible from the lane, which is exactly what makes a conservation officer look on it kindly. A 600mm parapet above the flat roof gives a height budget, the kerb takes 150 to 200mm of it, and the pitch follows from what is left over the width wanted. That frequently lands at 15 to 20 degrees rather than 25.

On a Cheltenham rear extension behind a listed or conservation area frontage, the accepted specification is often a slim painted timber frame in a dark colour, and timber sections are deeper for the same span, so the pitch is set as much by the ridge depth as by the geometry. Those elevations are covered on the aluminium and timber lanterns compared page.

On a narrow Victorian rear addition in Gloucester, Tredworth, Kingsholm or Longford, the roof is only 2.4 metres or so wide, the lantern is around 1000mm, and at 25 degrees it rises only 233mm. The instinct is to steepen it to make it look like something, and 35 degrees on a 1000mm width gives 350mm, which starts to read as a small ridge. That works, and it is one of the few cases where a steeper pitch on a small lantern is the right call. That house type is dealt with on the Victorian rear addition page.

Fixing the pitch before the frame is made

The pitch is cut into the frame at fabrication. Corner castings, hip connectors, the ridge and every rafter end are machined to the chosen angle, so it cannot be adjusted once the sections arrive, and a change of pitch after ordering is a new frame rather than a modification.

What settles it at survey: the measured overall height of the existing roof and how much headroom is left under any limit; the width of the lantern, since rise is driven by width as much as by angle; which way the lantern faces and whether winter light or summer heat is the greater concern; whether the glass can be reached for cleaning, and therefore whether a self cleaning coating has to be made to work; the parapet or the first floor cill above, where there is one; and the frame material, because timber sections want a little more depth and a little more pitch to look right.

Send a photograph of the flat roof from an upstairs window and one of the room below to info@severnskylightsgloucester.co.uk, with the rough width of the roof, and the first reply can put real numbers against two or three pitch options. The shortest version of this answer sits on what pitch does a roof lantern need, and other written answers are under guides.