Gloucester · Cheltenham · Stroud · the Cotswolds

Roof Lantern Installation

A roof lantern is the most structural member of the rooflight family and the one where the measurements go wrong most often.

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15 years in Gloucestershire
10-year workmanship guarantee
Building Control notification handled
The roof is never left open overnight

Tell us about the roof

We will come and look at it, and tell you honestly what it needs.

The enquiry form needs JavaScript. Email info@severnskylightsgloucester.co.uk with a photograph of the roof and rough measurements and you will get the same answer back.

What the job covers

The decisions that get made before anything is ordered.

Why installations fail

The unit is the small half of the job.

Almost every rooflight that leaks was detailed wrongly rather than manufactured wrongly. Four things decide whether yours is still dry in ten years, and you will never see any of them once the work is finished.

Upstand height. Measured from the finished surface, not the deck.
The fall. Designed at 1:40 so it is still 1:80 after deflection.
Where the laps sit. Dressed up the kerb and closed under the frame.
Where the covering terminates. Under the frame, never stopped short.

That is what the ten-year workmanship guarantee is written against.

A lantern is a triangle in section, so the rise at the ridge follows from the span and the pitch and cannot be chosen independently. That is what decides whether a lantern reads as generous or as a lid, and it is the calculation to do before anyone talks about frames.

How it runs

Two figures, and no surprises between them.

Send photographs A few pictures of the roof and the room, and rough measurements. An indicative range comes back by email.
The survey Forty minutes on site. Rafter centres, pitch off a digital level, the deck, the fall, and how a scaffold reaches the elevation.
A fixed price Written against what was measured, not against the drawing. It does not move unless you change what you asked for.
The installation Scaffold, opening, kerb, unit, weathering, reveal and making good. The roof is never left open overnight.
Notification and guarantee The Building Control notification is submitted and closed out as part of the job, and the workmanship carries ten years.

Asked before every job

The questions that come up first.

What is a roof lantern?

A roof lantern is a glazed pitched structure that stands up off a flat roof: several planes of glass rising from a horizontal base frame to a ridge or an apex, carried on aluminium or timber rafters, and bedded on a timber kerb built above the roof surface. It is the only member of the rooflight family that adds volume…

The full answer

How big can a roof lantern be?

Standard domestic lantern systems run comfortably to about 2 metres by 3 metres, and the ridge is what limits them: up to roughly 2.5 metres of ridge length in a plain aluminium extrusion, 3 to 4 metres with a steel core inside it, and beyond 4 metres it becomes an engineered structure with intermediate support rather than a catalogue product.…

The full answer

Do roof lanterns need planning permission?

Usually not. A roof lantern on a single storey rear extension or on an existing flat roof at the back of a house normally goes in under permitted development, with no planning application. Four things take that away: the overall height limit on the extension, which a lantern eats into by 400 to 600mm; designated land, meaning a conservation area,…

The full answer

Can a roof lantern be fitted to an existing extension?

Yes, and it is the most common lantern job in this county by a distance. A lantern goes into an existing flat roof the same way it goes into a new one: joists trimmed, kerb built and weathered, frame set, glass in, ceiling lined and plastered. What decides the job is never whether it can be done but what the…

The full answer

Do roof lanterns leak?

A correctly detailed roof lantern does not let water in, and where water does appear it is almost never through the glass or the lantern frame. Three things account for nearly every case: the kerb and the junction between it and the flat roof covering, standing water sitting against that kerb because the roof does not fall away from it,…

The full answer

How long does a roof lantern take to fit?

Three to four days on site for a straightforward lantern into a sound flat roof, plus a plasterer for a day and a drying period after that. The calendar answer is longer and it is the one that governs a date: a lantern is manufactured to your measured kerb dimension, so nothing is ordered until the kerb exists, and the…

The full answer

Can a roof lantern be opened?

Yes. One pane of a lantern can be built as an opening sash, hinged at the ridge or at the eaves and driven by a concealed actuator, and larger lanterns can carry two or more. Most lanterns that go in are fixed, because a fixed lantern is the simplest and driest version of the product, and ventilation is often better…

The full answer

Do roof lanterns make a kitchen too hot?

They can, and a kitchen is the hardest room in the house to get right because it is already the room that makes the most heat on its own. A lantern over a kitchen with plain double glazing, facing south or west, with a recirculating cooker hood and no way of venting at high level, will be uncomfortable from late…

The full answer

The technical detail

The whole job, from the kerb up.

A roof lantern is the most structural member of the rooflight family and the one where the measurements go wrong most often. It is a framed pitched structure, several planes of glass rising to a ridge or an apex, sitting on a timber kerb that has to be built square and level to the frame’s own base dimension before anybody orders glass. Severn builds the kerb, sets the lantern, weathers it into the flat roof and takes the internal finish through to plaster, on rear extensions and garden rooms across Gloucester, Cheltenham, Stroud, Tewkesbury and the Cotswold villages, with the Building Control notification submitted and closed out as part of the job. The technical detail sits on this page rather than behind a form because the sequence is what decides the outcome. Settle the kerb dimension, the pitch and the ridge span at survey and the rest is carpentry, lifting and glazing. Settle them after the frame has been delivered and something gets rebuilt at somebody’s expense.

27 sections 33 minute read

What a roof lantern is made of, part by part

Strip the marketing off a modern lantern and there are six components. The eaves beam or base ring is the horizontal frame that lands on the kerb, and it carries everything above it as well as resisting the outward push of the sloping rafters. The rafters run from that beam up to the ridge. On a hipped lantern there are hip rafters at the corners running diagonally, with shorter jack rafters between them. The ridge is a single extrusion along the top, and on anything over about two and a half metres long it contains a steel or a heavier section because deflection along its length is what limits the size. Between and over the rafters sit the glazing bars and gaskets, and the glass is either captured under an external cap or bonded structurally to the rafter top so the outside reads as glass with slim shadow lines rather than as a frame.

The sixth component is the one nobody looks at, which is the thermal break: a polyamide strip running the length of every aluminium section, separating the cold outer face from the warm inner face. Without it the inside of every rafter runs at close to outside temperature and streams with condensation in January, which is exactly what the aluminium lanterns of the 1980s did.

A lantern is a different product from a flat rooflight, which is one plane of glass on a shallow angle in a slim frame. Those are covered on the flat roof rooflight installation page, and both sit under rooflight and skylight services.

The kerb decides the lantern, and it goes up before the glass is ordered

The lantern does not weather the roof. The kerb does, and the lantern sits on top of a kerb that is already watertight in its own right.

The kerb is built in treated timber, commonly a 100 by 50 or 150 by 50 section depending on the height needed, standing at least 150mm above the finished surface of the waterproofing measured at the highest point of the fall. Not from the deck, and not from the low corner. The outer face is insulated with 25mm to 50mm of rigid PIR so the timber does not sit as a cold ring around the base of the glass, and the flat roof waterproofing is dressed up that face and over the top of the kerb, so the upstand becomes part of the roof instead of a joint arguing with it.

Two things then have to be true before the frame arrives. The kerb has to be square, checked on the diagonals rather than by eye, because a base ring is manufactured square and will not sit down onto a parallelogram. And the top has to be level along its whole length, checked with a straight edge and a level rather than assumed from the deck. A lantern set on a kerb that dips 5mm in the middle of a two metre run will not compress its gaskets evenly, and the place that shows is a slow mark at one rafter foot eighteen months later. Packing under a structurally bonded eaves beam is not a fix, it is a future problem, so the kerb top gets planed or shimmed and re-checked before the frame is unwrapped.

The kerb is built, weathered and checked square and level before the lantern is ordered, not after it is delivered.

External kerb size, internal aperture and sightline: three numbers that are not the same

This is where lantern orders go wrong, and it is worth being pedantic about it. There are at least three dimensions in play and suppliers quote different ones.

The structural opening is the hole cut in the flat roof deck between trimmed joists. The kerb size is measured over the outside of the finished kerb, and that is usually the number a lantern is manufactured to, because the base ring drops over or sits on the kerb top. On a 100mm kerb built around a 2000 by 1200 structural opening, the external kerb dimension is roughly 2200 by 1400 once the timber and the external insulation are counted, and a lantern ordered at 2000 by 1200 will be short by the thickness of both walls. The internal aperture, the daylight opening you actually see from the room, is smaller again once the plasterboard lining is on the inside face of the kerb.

Then sightline, which is how much aluminium you look through rather than past. A slim structurally glazed rafter shows about 50mm to 60mm on the outside. An older capped system shows closer to 90mm. Across a lantern with five rafters that difference is nearly 200mm of glass, and on a small lantern over a galley kitchen it is the difference between a glazed roof and a frame with windows in it.

Every one of those numbers goes on the survey sheet, and the order is placed off the measured kerb rather than off the drawing, because a drawing and a built kerb agree less often than people expect.

100mm
50mm
60mm
90mm

Rafter spacing, ridge spans and when a lantern needs a tie bar

Rafter spacing on a lantern is set by glass weight, not by appearance. Panes are commonly spaced between 500mm and 1000mm apart, and the reason is that a double glazed lantern pane weighs roughly 30 to 35 kilograms per square metre. Spread the rafters to 1200mm on a long lantern and each pane climbs past 80 kilograms, which stops being a two-person lift on a scaffold and becomes a mechanical one.

The ridge is the real span limit. Aluminium is stiff for its weight but it deflects, and a bare ridge extrusion running four metres between hips will sag visibly and open the gasket line at mid span. Manufacturers deal with that in three ways: a heavier ridge section, a steel core inserted inside the extrusion, or an intermediate support. Past roughly three metres of ridge length the reinforced option is normal and past four it is usually structural.

At the bottom of the slope the problem reverses. Pitched rafters push outwards at their feet, and that thrust wants to spread the eaves beam and open the corners. A steel-reinforced eaves beam handles it on most sizes. On wide lanterns some systems use a tie bar across the short dimension instead, a slim aluminium or stainless rod at eaves level. It works, it is cheap, and it is visible from the room below for the next thirty years, which is worth knowing before a drawing is signed off rather than on the day it appears.

Frame materials against what they have to do in a roof

Four frame materials turn up in lantern enquiries and they behave very differently once they are lying at 25 degrees with sun on them all afternoon and rain on them all winter.

Frame	Typical sightline	How it behaves in a roof	Where it earns its place
Thermally broken aluminium, structurally glazed	50 to 60mm on the rafter	Dimensionally stable, no movement to speak of, powder coat lasts decades. The thermal break carries the whole thermal performance	Most modern extensions. The largest sizes, because it is the only material stiff enough at slim sections
Aluminium with external glazing caps	80 to 95mm	Same stability, but the cap sits proud and collects dirt along its upper edge, and the extra width shows	Smaller lanterns where cost matters more than sightline
Painted timber, usually engineered softwood or hardwood	60 to 100mm	Moves with moisture, needs a maintained paint film, and the ridge and rafter sections have to be deeper for the same span	Listed buildings and conservation area rear elevations where a painted timber frame is the condition of consent
Aluminium clad timber	60 to 80mm	Aluminium outside takes the weather, timber inside gives a warm internal face and a paintable finish. Heavier per metre than either alone	Where the room wants timber overhead and the roof wants aluminium outside

The internal face matters more on a lantern than on any other rooflight, because you spend the evening looking up at it from a sofa or a dining chair. An aluminium internal face in a dark colour reads as a strong grid. The same lantern in an off white internal finish almost disappears against a white ceiling, which is why dual colour frames exist.

Pitch: five degrees is the floor, twenty-five is where it starts to read as a lantern

Glass in a roof needs fall or it stays dirty. Five degrees is the practical minimum for water to shed and for a self-cleaning coating to do anything at all, and it is the figure below which dust deposited in dry weather is never washed off, so the glass looks grey within a season.

Lanterns are usually built much steeper than that. Typical rafter pitch runs from 20 to 30 degrees, with 25 as the common default, and the number is chosen for volume and proportion rather than drainage. A 25 degree pitch on a 2000 by 1200 lantern puts the ridge roughly 280mm above the eaves beam, so the whole assembly stands around 430mm above the flat roof surface once the kerb is counted. That total height is the figure that matters for planning, and it is dealt with further down.

Shallower pitches, around 15 degrees, make a lantern that is longer and lower and reads well on a wide single storey extension where a tall pyramid would look like a bus shelter. Steeper pitches, past 30 degrees, gain internal volume and throw more light deep into the room in winter when the sun is low, at the cost of a taller object on the roof and a bigger surface area of glass to heat.

30°
25°
280mm
430mm

Hipped, gable ended, pyramid, and the T over an L-shaped room

The shape is set by the plan of the opening and by what the room underneath is doing.

A hipped rectangular lantern is the standard: four sloping faces, a ridge along the long axis, hips down to the corners. It is the most forgiving shape, it sheds water in four directions, and it suits a rectangular opening in a rectangular flat roof.

A pyramid has no ridge at all, four triangular faces meeting at an apex, and it is the right answer over a square opening. It has more presence for its footprint than anything else, which is useful over a stair void or a small dining area and unhelpful over a kitchen where you wanted horizontal glass.

A gable ended lantern stops the two long slopes against vertical triangular glazed ends. It gives more headroom and volume than a hip, and it works where one end of the lantern runs up against the wall of the house so that end would never have been glazed anyway. Against a wall the vertical end has to be flashed into the brickwork properly, with a lead or a proprietary flashing chased into a bed joint rather than surface sealed.

An L or T configuration is two lanterns sharing a valley, and it is genuinely harder. The valley is the drainage weak point, it needs its own fall and a decent width, and it needs to discharge somewhere real. Over an L-shaped kitchen and dining space the honest alternative is often two separate lanterns with a metre of flat roof between them, which drains simply, weathers simply and costs less to set out.

Glass weight, and how it gets onto a roof in a Stroud valley

Daylight falling through a roof lantern into a kitchen below

Glass weighs about 2.5 kilograms per square metre for every millimetre of thickness. A lantern pane built as a 6mm toughened outer, a 16mm argon cavity and a 6.8mm laminated inner works out around 32 kilograms per square metre. A 1.2 by 1.6 metre pane is therefore about 60 kilograms of awkward, slippery, expensive rectangle that has to go up a scaffold and be placed accurately at arm’s length above head height. Triple glazing takes the same pane past 80 kilograms.

On a Quedgeley or Bishops Cleeve extension with a driveway and a level garden, that is two people, a scaffold with a proper loading bay and a set of vacuum lifting pads. On a Chalford or Rodborough cottage where the house is one storey at the front and three at the back, the lane is shared, there is a flight of steps and a retaining wall sits against the elevation, it is a different exercise. The glass may need to be carried through the house, which means a route measured at survey including the tightest door, the newel post on the stair and the ceiling height on the landing. Sometimes the answer is a mechanical lift or a glazing robot brought in for half a day, and where a lorry cannot get within fifty metres of the elevation the extra hands go in the price at survey rather than appearing later.

Pre-glazed panels are lifted as one unit and are heavier again. Most lanterns going into Gloucestershire gardens are glazed on the roof, pane by pane, for exactly this reason.

6mm
16mm
6.8mm

Glazing specification for glass that spends its life overhead

Overhead glass is specified differently from a vertical window, and two of the requirements are not optional.

The outer pane is toughened, so hail, a dropped tool or thermal stress breaks it into blunt granules rather than shards. The inner pane is laminated wherever there is habitable space below, which in a lantern is always, because laminated glass holds together on its interlayer if it breaks and stays in the frame instead of arriving on the dining table. A 6.8mm laminated inner is the common build and 8.8mm is used on larger panes.

Build up	Approximate weight	What it delivers	Worth specifying when
6mm toughened / 16mm argon / 6.8mm laminated, soft coat low emissivity	around 32 kg per m2	The standard lantern unit. Whole unit performance in the region of 1.1 to 1.4 W/m2K	The default on a north or east facing extension
As above with a solar control coating on the outer pane	around 32 kg per m2	Solar factor down from roughly 0.5 to between 0.25 and 0.35, with a slight blue or neutral tint	Any south or west facing lantern, and any lantern over a kitchen with an oven under it
Triple glazed, two cavities, laminated inner	around 42 kg per m2	Lower heat loss and a useful drop in rain noise. Heavier panes, closer rafter spacing	Where the room is used as a study or a bedroom and winter comfort governs
8.8mm acoustic laminated inner with an acoustic interlayer	around 35 kg per m2	Noticeably quieter under heavy rain and hail	Under a flight path, beside a main road, or where a shallow pitch amplifies drumming
Self cleaning coating added to the outer face	no change	A hydrophilic coating that breaks down organic dirt in daylight and sheets water off it	Any lantern that cannot be reached safely from a ladder or a window

Deeper numbers on coatings and heat loss sit on the energy efficient rooflights page.

Centre pane against whole unit U-value, and which figure to ask for

Lantern brochures quote heat loss in two ways and the gap between them is large enough to matter.

Centre pane U-value is the performance of the glass alone, measured away from the edges. It is a genuine number and it flatters the product, because it ignores the spacer bar at the perimeter of every pane and it ignores the aluminium rafters entirely. A good sealed unit will show 1.0 or better on that basis.

Whole unit U-value counts the glass, the spacer, the rafters, the ridge and the eaves beam as one assembly, in proportion to their areas. On a lantern with a lot of framing per square metre, that pulls the figure up considerably, and a unit quoting 1.0 centre pane may land at 1.4 or 1.6 as an assembly. Two things close the gap: a warm edge spacer, which is a polymer or stainless section instead of the old rolled aluminium box, and slim structurally glazed rafters that put more glass and less metal into the same footprint.

Ask for the whole unit figure, and ask what size it was calculated at, because the ratio of framing to glass changes with size and a manufacturer’s headline number is usually quoted on a large lantern where the framing is proportionally small.

Solar gain on a south or west facing lantern

A lantern is glass on four planes with nothing shading it, and in June the sun is high enough that the sloping faces take near perpendicular sun for hours. That is why the room under a lantern is glorious in April and difficult in July.

The number to look at is the g-value, the fraction of solar energy that gets through the glass. A plain low emissivity unit sits around 0.5, which means half the sun’s energy arrives in the room. A solar control coating on the outer pane brings that down to roughly 0.25 to 0.35. On a west facing extension in the Severn Vale, where the afternoon sun sits low over the river and comes in at the worst possible angle through the sloping glass, the difference between those two specifications is the difference between a room used all summer and a room the family avoids after two o’clock.

Ventilation does the other half of the work. Hot air collects at the ridge of a lantern by simple physics, and an opening vent at high level lets it out while a door or window at low level draws cooler air in. That stack effect moves a surprising volume of air through a room with a tall glazed roof and costs nothing to run.

Internal blinds fitted between the rafters are the last resort rather than the first. They work, but they trap heat against the glass, they collect dust on their upper face and they take away the reason the lantern went in.

The cold bridge at the head of the kerb, and where condensation collects

Nearly every complaint about a lantern dripping is a complaint about the kerb rather than the glass, and it is nearly always the same 40mm of construction.

The top of the kerb is where three things meet: the flat roof insulation, the kerb timber, and the aluminium eaves beam. If the roof insulation stops at the face of the kerb and the kerb timber runs from a cold outside to a warm inside with nothing insulating it, that timber becomes a continuous cold strip around the whole base of the lantern. Warm humid air from the kitchen below rises, meets it, and condenses along the internal top edge of the kerb lining. It looks exactly like water coming in and it is water being made on the spot.

The detail that avoids it: rigid insulation continuous up the external face of the kerb and lapped tightly to the roof insulation so there is no gap, the internal face of the kerb lined and insulated too, and the vapour control layer taken up the inside of the kerb and taped to the eaves beam rather than trimmed short of it. A 20mm void of still air behind plasterboard at the top of a kerb is a cold bridge, and cold bridges are where black spotting appears in February.

The kerb lining itself is finished either as a vertical plastered reveal or as a splay. A splay throws more light out into the room and looks better from below. A vertical lining is easier to get airtight. On a kitchen ceiling with a lot of moisture in the air, airtightness wins.

A hipped roof lantern seen on a flat roof from outside

The structural opening: joists, trimmers and the beam over a knocked through wall

A lantern opening in a flat roof is formed the same way as any other trimmed opening, but the loads are higher because there is real weight sitting on the perimeter rather than around it.

Joists inside the footprint are cut, and doubled trimmers are fixed across the head and the foot of the opening, with trimming joists doubled at the sides. The cut joists hang off the trimmers on joist hangers, and the trimmers deliver that load plus the lantern’s own weight into the trimming joists either side, which are now carrying considerably more than they were sized for. Over one joist bay that is routine carpentry. Over a 1200mm or wider opening it is a calculation, and the answer is a deeper trimmer, a flitch plate, or a small steel. Where a calculation is needed it goes in with the Building Control application before the covering comes off.

The awkward version is a lantern that is wanted directly over the line of a wall that has already been taken out, which happens constantly on Gloucester terraces where the kitchen and the rear addition have been knocked into one. The beam over that opening is carrying the roof and sometimes a floor above, and it is often a steel with limited spare capacity and no drawings to prove it. A lantern kerb bearing onto or beside that beam changes its loading, so it gets checked rather than assumed, and on a job like that an engineer’s calculation is money well spent.

Where the flat roof covering stops and the kerb takes over

The junction between the roof covering and the kerb is the one detail that decides whether the lantern is dry, and it is finished differently for each waterproofing system.

On single ply, the membrane is dressed up the insulated kerb face and over the top, welded at the corners with purpose formed pieces rather than folded and hoped over, and terminated on the kerb top under the eaves beam. On EPDM, the same route, with the corners formed in uncured flashing tape and the internal angles bonded rather than stretched. On built up felt, an angle fillet at the base of the kerb stops the membrane cracking where it turns, and the cap sheet is dressed over the kerb top in one piece per side. On liquid applied systems, the reinforcement matting is taken continuously round the base with no joint at the change of plane, which is where thin coats fail.

Whatever the system, the same two rules apply. The waterproofing goes over the top of the kerb, not just up the face, so the eaves beam sits on a waterproof plane rather than on bare timber. And the falls in the surrounding roof take water away from the kerb rather than to it. A roof falling at a designed 1:40 will still be at 1:80 or better after deflection and normal timber movement. Where an existing roof already ponds against the position the customer wants a lantern, the deck needs re-falling first, and that is said at survey rather than discovered when the water sits there in November.

Positioning a lantern over an island, a table or a stair void

A roof lantern kerb meeting a tiled roof covering

A lantern is placed for the room, not for the roof, and then adjusted for what the roof will allow.

Over a kitchen island, the usual mistake is centring the lantern on the roof rather than on the island, which leaves the working surface half lit and the walkway brilliant. A lantern set over the island gives even light on the surface you use and it stops the pendant lights fighting a bright rectangle three feet to one side.

Over a dining table, the lantern wants to sit centrally on the table with a margin all round, because the frame reads as a ceiling feature and an off centre one is obvious for the life of the room. Get the two agreed together, table position and lantern position, before the kerb is set out.

Over a stair void or a double height space, a pyramid or a square hipped lantern works better than a long rectangle, and it throws light down through both storeys. That is also the position where an opening vent has to be electrically operated, because nothing four metres above a landing is going to be opened by hand. That is covered under electric rooflight installation.

Against the constraint list: the kerb has to clear the abutment wall by enough to weather into it, usually 150mm of flat roof at minimum, and it has to clear the roof outlet so the fall to that outlet still works. Soil vent pipes, extract terminals and flues already coming through the flat roof are all on the survey drawing before a position is agreed.

How large a lantern a flat roof will actually carry

People arrive with a size in mind and the roof usually has an opinion. The useful measure is not the lantern’s own dimension but the flat roof it has to sit in, because the surrounding roof has to keep draining.

Flat roof area	Realistic lantern footprint	What governs it	Common outcome
Narrow rear addition, around 1.8 to 2.4m wide	Up to about 1000 by 1500mm	Margin needed for falls each side and to weather into the house wall	Often better served by a flat rooflight than a lantern, because a lantern this size looks stubby
Single storey rear extension, 3m by 4m	1200 by 2000mm, hipped	Joist trimming across one or two bays, plus a clear route to the existing outlet	The most common Gloucestershire lantern by some distance
Wide rear extension, 4m by 6m	1500 by 2500mm, or two smaller lanterns	Ridge length and the point at which a reinforced ridge becomes structural	Two lanterns often beat one, for lifting, for drainage and for the ceiling below
Garden room or orangery, freestanding	2000 by 3000mm and upwards	Ridge reinforcement, pane weight, and how glass reaches the roof	Steel cored ridge, mechanical lifting, and a longer programme
Square void over a stair or a hall	1200 by 1200mm to 1800 by 1800mm	Trimming in two directions and the position of the ceiling binder below	Pyramid, with an electrically operated vent

The margin around a lantern matters visually as well as technically. A lantern that leaves only 200mm of roof at one side reads as a mistake from the garden, and it gives the roofer nowhere to form a decent upstand. Around 400mm to 600mm of flat roof all round is where a lantern starts to look deliberate.

Opening vents, actuators, and reaching glass that is four metres up

Most lanterns going in are fixed, and a fixed lantern is the simplest and driest version. Where ventilation is wanted, there are two routes.

An opening vent within the lantern replaces one pane with a hinged or a lifting sash, driven by a chain or a spindle actuator hidden in the rafter. It sits at the top of the volume where the hot air is, which is where you want to vent from, and with a rain sensor it closes itself when the weather turns. The wiring is run before the ceiling is boarded, which means the decision has to be made at survey rather than at second fix.

A separate opening rooflight elsewhere in the flat roof is the alternative, and on a wide extension it often gives better cross ventilation than a vent in the lantern itself, because air comes in low at one end of the room and leaves high at the other.

Reach is the other half of this. Lantern glass on a rear extension is typically two and a half to three metres above the garden and four or more metres above an internal floor if the room is open to the roof. Cleaning it from a ladder leaning on aluminium is a poor idea, so the practical specifications are a self cleaning outer coating, an opening vent that can be tilted for access to the pane next to it, or accepting that a pole and brush from the garden will be the arrangement. It is a decision worth making on purpose, because the alternative is a lantern that looks grey after three winters and cannot be got at.

Rain noise, hail and drumming on a shallow lantern

Nobody asks about rain noise until they have lived under glass, and then it is the first thing they mention. A single sheet of toughened glass at a shallow angle is a drum skin, and heavy rain on it is loud enough to interrupt a conversation.

Three things reduce it. A laminated inner pane is the biggest single improvement: the plastic interlayer damps the vibration, and a 6.8mm laminated inner is quieter than an 8mm toughened one of the same weight. An acoustic interlayer takes that further for a modest addition to the glass specification. And pitch helps, because rain striking glass at 25 degrees delivers less energy perpendicular to the surface than rain striking it at 5 degrees, which is one of the quiet arguments for a lantern over a shallow flat unit on a room where people sit and talk.

Hail is the same problem at higher energy, and it is the reason the outer pane is toughened rather than annealed. Gloucestershire gets hard hail off the Severn in spring squalls, and a toughened outer takes it without marking.

6.8mm
8mm
25°
5°

Severn Vale rear extensions, and homeowners who have seen water inside

Gloucester’s Victorian and Edwardian terraces, Longford, Tewkesbury and the vale villages nearly all share the same rear geometry: red brick, a two storey addition one room wide, and a flat roofed kitchen extension across the back of it at some point in the last forty years. That flat roof is where a lantern goes, and it is usually the darkest room in the house that sits under it.

Two Severn Vale specifics change the job. The first is that a lot of these flat roofs were built in the 1970s and 1980s on 100 by 50 joists with no real fall, three layers of felt and no insulation to speak of. A lantern is a good moment to find that out, because the deck often needs re-falling and the roof re-insulating anyway, and doing it while the scaffold is up costs a fraction of doing it as a separate exercise later. The survey opens up a small area and looks, rather than guessing from below.

The second is flood history. People along the Severn and the Avon watched water come into their houses, most memorably in 2007, and they are entirely reasonable to be sceptical about anyone cheerful about cutting a hole in a roof. The answer to that scepticism is the perimeter detailing set out above, in writing, plus a sequence where nothing is ever open at the end of a day, plus the ten year workmanship guarantee that sits behind the four details the workmanship controls.

Cotswold stone garden rooms, and the lantern behind a stone parapet

Lanterns in the wolds villages are rarely cut into an old roof. Cotswold stone slate is laid in diminishing courses on torched battens at 45 to 55 degrees, and a pitched lantern has no business in a roof like that: the right product there is a flush conservation rooflight, described on the conservation rooflight installation page.

Where lanterns do go in around Painswick, Bibury, Northleach and Chipping Campden is on new work: a garden room, a kitchen extension, an orangery, or the flat roof behind a stone parapet on a rear addition. The parapet is the interesting bit. A lantern set behind a coped stone parapet is almost invisible from the lane, which is exactly why a conservation officer will look at it more kindly than a rooflight on a visible slope. It also means the parapet gutter behind the coping is the roof’s only drainage, so the lantern position has to leave that gutter its full width and its falls, and it must not obstruct the outlet or the chute through the parapet.

Stone also brings a matching conversation. Where a kerb is built off a new stone wall, the lantern sits on a timber kerb behind the stone rather than bearing on the coping, and the lead or single ply is taken up behind the coping and into a chase. Getting that wrong puts water into the top of a stone wall, and stone that stays wet through a Cotswold winter spalls.

Cheltenham rear extensions inside a conservation area

Central Cheltenham is stucco, sash windows and Welsh slate, much of it listed and nearly all of it in a conservation area, and lanterns go on the rear extensions rather than on the historic roofs. Lansdown, Montpellier, Pittville and the Bath Road terraces have deep rear gardens and a long history of Victorian and later additions across the back, which is where the glass goes.

Three things shape the job there. Conservation area status controls the appearance of the building and often removes permitted development rights through an Article 4 direction, so the assumption is an application rather than no application. On a listed building, listed building consent is needed for alterations affecting character whether or not they are visible from the street, and that includes what happens to an internal ceiling. And the accepted answer on those rear elevations is frequently a painted timber or timber cored lantern in a dark colour with slim sections, rather than a wide silver aluminium frame, which is a specification decision to take before ordering.

The sensible order of operations is pre-application advice with photographs of the rear elevation and a sketch of exactly what is proposed, before anyone orders a frame with a twelve week lead time. Officers in Cheltenham have seen a great many glazed rear extensions, and they will usually indicate early what will be acceptable, which is a great deal cheaper than a refusal with a scaffold standing.

Building Regulations that bear on a lantern specifically

Forming a lantern opening in a roof is notifiable work, and several parts of the regulations touch it at once.

Structure. Trimming joists and loading a beam over a previously removed wall are both structural. Where the span or the load needs it, an engineer’s calculation goes in with the application.
Conservation of fuel and power. Current guidance quotes rooflights against a limiting standard of 2.2 W/m2K assessed in the vertical plane, and any current lantern is comfortably inside that. The check that matters more in practice is insulation continuity around the kerb, which is inspected as part of the same requirement.
Safety glazing. A toughened outer pane and a laminated inner pane in overhead glazing above habitable space, which on a lantern is every pane.
Ventilation. Purge ventilation to a habitable room means an openable area of at least one twentieth of the floor area. Enlarging a kitchen and roofing it in glass changes that floor area, so the openable provision is counted across the whole room rather than assumed from the doors that were already there.
Fire safety. A lantern is not an escape opening, so where an extension alters escape from an upper storey, the escape route is dealt with separately. Where a lantern sits within a metre of a boundary the roof construction and its surface spread of flame come into it.
Electrical work. Any mains wiring for an actuator or a rain sensor is certified under the electrical part.

The notification is submitted, the inspections are booked, and the completion certificate comes back in the homeowner’s name. Keep it in the house file: it is the paperwork a buyer’s solicitor asks for, and finding it missing on the day of a sale is a worse problem than dealing with it now.

Permitted development for a lantern on a rear extension

A lantern on a single storey rear extension is usually assessed as part of that extension rather than as an alteration to the roof of the house, and that changes which limit catches you.

The limit that catches people is height. A single storey rear extension under permitted development must not exceed 4 metres in overall height, and within 2 metres of a boundary the eaves height must not exceed 3 metres. A lantern adds its kerb plus its pitched height to the roof, commonly 400mm to 550mm on a domestic size. An extension roof already sitting at 3.7 metres is inside the limit. Put a 450mm lantern on it and the highest part of the extension is 4.15 metres, and permitted development has been lost for the sake of a detail nobody checked. That figure is worked out at survey, from a measured height rather than from a drawing.

Where the lantern is going into the roof of the original house rather than an extension, a different limit applies: an alteration to the roof must not project more than 150mm beyond the plane of the existing roof slope. A pitched lantern on a pitched roof will not achieve that, which is one of the reasons lanterns live on flat roofs and roof windows live in slopes. The roof window installation page covers the in-plane option.

Flats and maisonettes have no permitted development rights for roof alterations at all. Article 4 directions remove rights street by street, and both Gloucester and Cheltenham have areas covered by them. The local planning authority is the only body that can confirm the position for a specific address, and a written confirmation before work starts is worth the fee if a sale is anywhere on the horizon.

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Frame finishes, and a colour decision that is harder than it looks

Aluminium lanterns are polyester powder coated to a RAL reference, and the coating is durable enough that colour is a design decision rather than a maintenance one. Three choices come up on every job.

Outside colour. Anthracite grey, RAL 7016, is the current default and it works because it reads as a shadow line from the garden rather than as a frame. Black, RAL 9005, is crisper and shows dust more. White is right on a rendered or a painted rear elevation and wrong on Cotswold stone.

Inside colour. This is the one people regret. A dark internal frame overhead reads as a heavy grid across the ceiling, particularly in a north facing room where there is no direct sun on it. The same lantern with an off white or a light grey internal face merges into a white ceiling and the glass becomes the thing you notice. Dual colour frames cost a little more and are almost always the right answer.

Finish texture. A matt or a fine texture powder coat hides handling marks and looks better close up than a gloss, and on a lantern you are close up to the internal face every day.

On a listed rear elevation or in a conservation area, the colour is often a condition rather than a preference, and a dark painted timber or a heritage RAL is what gets consented.

The installation sequence over the days it takes

A lantern is a longer job than a roof window and the sequence is built around never leaving the roof open.

Scaffold goes up first, with a loading bay if glass is being lifted, and inside the room floors, units and furniture are sheeted before anything is cut. The flat roof is opened and the joists trimmed, the deck and the falls made good, and the kerb built, insulated and weathered into the covering. At that point the roof is whole again with a watertight kerb standing on it, and it can safely stay like that overnight or over a weekend if the weather turns.

The frame goes on next: eaves beam bedded on the kerb top in low modulus silicone and mechanically fixed down, hips and rafters set, ridge fitted, everything checked for square and for level before a single pane goes in. Glass follows pane by pane, gaskets and caps or structural tapes fitted, and the whole assembly is water tested rather than assumed.

Then the inside. Kerb lining insulated and boarded, vapour layer taped to the eaves beam, reveal formed vertical or splayed, plaster, and any wiring for an actuator terminated. On a straightforward 1200 by 2000 lantern into an existing sound flat roof, that is commonly three to four days on site plus the plasterer. Where the deck needs re-falling, where a beam needs checking, or where the glass has to come through the house on a valley site, it is longer, and the survey says which before a date is agreed.

An opening is never started unless it can be closed and made watertight the same day.

What drives the price of a lantern

No figures are published on this site, and the reason is that a 1000 by 1500 aluminium lantern on a sound flat roof with a driveway to park on, and a 2000 by 3000 timber cored lantern on a Cotswold garden room reached by a footpath, are not the same job at any price. A published number would be wrong for both.

What actually moves it:

Footprint and ridge length, and whether the ridge needs reinforcement or an intermediate support.
Frame material and sightline, with slim structurally glazed aluminium and timber cored frames at the involved end.
Glass specification: double or triple, solar control, acoustic interlayer, self cleaning coating.
Whether the existing flat roof deck, its falls and its insulation are sound, or need re-forming first.
Structural work: a designed trimmer, a flitch, a steel, or a check on an existing beam over a removed wall.
How glass reaches the roof: scaffold and two people, a route through the house, or mechanical lifting on a valley site.
Fixed or vented, and whether an actuator needs mains wiring run before the ceiling closes.
How far the internal finish is taken, and whether kitchen units and worktops are in the way.
Planning and conservation requirements, including any pre-application work and a specified frame colour or material.

Then two figures, in that order. First an indicative range by email from photographs and a rough size, free, back within a day or two, so nobody spends an afternoon on a survey for a project that was never going to work financially. Second, after a survey, a fixed price itemised so it is clear what is frame, what is glass, what is labour, what is scaffold and what is Building Control. That figure holds unless the scope changes at the customer’s request, or unless something structural is uncovered that was written down as a risk in the survey notes beforehand. The costs page sets the process out in more detail.

Photographs that make the first reply useful

Email info@severnskylightsgloucester.co.uk. For a lantern specifically, five things turn a vague first answer into a useful one: a photograph of the flat roof from an upstairs window looking down, which shows the falls, the outlet and anything already coming through the deck; a photograph of the room from inside with the ceiling in frame; the rough size of the flat roof, gutter to house wall and side to side; roughly where the glass is wanted, over the island, over the table, or central; and the address, because in this county the postcode already tells you a great deal about the roof and the conservation status.

From there it is the range by email, a survey at a time that suits, the fixed price, then a date. Severn covers Gloucester and thirty miles by road around it, listed town by town on the areas pages. Where a lantern turns out to be the wrong product for the roof, the alternatives are a made to size flat unit, a walk-on rooflight where a terrace sits over the room, a sun tunnel into a windowless space, or a skylight replacement where a unit already in the roof has reached the end of its life. Written answers to individual questions sit under guides, and about explains how the company works.

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