An orangery roof lantern, and the ceiling built around it

The word orangery describes a proportion, not a product. The walls and piers carry the weight, the roof edge is solid and trimmed with a cornice or parapet, and the glass sits in the centre like a jewel in a setting.

A traditional orangery put its glass in the roof and in tall windows so that plants got light from above and from the side. The modern version keeps the same idea. The perimeter roof is a flat deck, built like any warm flat roof, and the lantern stands in the middle on its own kerb. The light arrives from above, the walls stay solid, and the room feels enclosed and lit at once. It is a different feeling from a fully glazed conservatory, which gets very bright and very hot and very cold by turns.

That is why the lantern and the roof have to be designed together. Every decision on one changes the other. A taller lantern needs a deeper kerb. A deeper kerb needs a thicker roof build-up or a taller upstand. A thick roof build-up changes the ceiling height inside. We work through these in the order they appear on a survey, and the chapters below follow that order.

If you are still deciding between the glass forms, our comparison of a roof lantern or a flat glass rooflight covers the choice, and the page on a lantern against a fully glazed roof explains why the solid perimeter is worth keeping. The full range of work sits under our roof lantern installation hub.

A garden room or outbuilding with a flat roof can take a lantern, provided the roof can carry the change. The survey starts from the structure, not from the lantern catalogue.

Many garden rooms were built with a light timber roof sized for the roof finish and a little snow. A lantern is heavier than the deck it replaces, and cutting the opening removes joists. The remaining joists have to pick up that load through trimmers, and on a garden room the span may be short enough for timber alone. On a wider room a steel or engineered timber beam around the opening can be needed. Our answer on whether a lantern needs a steel beam explains where that line falls, and the page on fitting a lantern into an existing flat roof takes the retrofit route step by step.

We look at four things before we talk about glass:

1. The direction and size of the existing joists, and what they bear on.
2. The condition of the deck and the current roof covering, which decides whether the perimeter can stay or must be stripped and relaid.
3. The height of the walls and any parapet, which sets the clear margin around the kerb.
4. The existing ceiling, and whether it is plasterboard on joists or a void that can be rebuilt.

On an older garden room it is often the case that the whole roof is renewed around the lantern. That sounds like more work. It is usually the simpler job, because one new warm roof with the lantern kerb built into it is easier to make watertight and well insulated than a new kerb cut into an old build-up.

Before anything is ordered, we look at the room from outside as well as in. The proportions that please in a drawing can sit awkwardly on a real lawn.

The ridge of the lantern has to clear the parapet or fascia by enough that the glass looks deliberate. On a low garden room, a tall lantern can look stacked on top. A lower pitch and a shorter span keep it in scale. Our page on how high a roof lantern sits shows how the height relates to the kerb and the width.

The house matters too. A lantern on a brick orangery behind a 1960s New Town semi wants a simple, slim frame. Against a tile-hung cottage it can carry a little more detail. The frame colour can echo the doors and windows below, and the page on what colour roof lantern frame looks best goes through the choices. The range of profiles and finishes is covered in our guide to aluminium roof lanterns.

In an orangery the flat ceiling around the lantern is not filler. It is a large surface with a job to do: it carries the insulation line, it holds the lights, and it absorbs sound that the glass would otherwise throw around the room.

A good perimeter ceiling is a clear band of flat plasterboard on every side of the opening, wide enough to read as a ceiling and never as trim. A narrow band makes the lantern look as though it is cutting the room in two. A deep one makes the room feel solid and calm, with the light dropping from the centre. The band also has to be wide enough for downlights, a blind track or a speaker, and those are positioned before the plasterboard goes up. Our page on lighting and blinds around a roof lantern covers the layout, and the answer on putting lights inside a lantern deals with the lantern itself.

The inside of the kerb is where the ceiling and the lantern meet. It is formed as a short return, or light well, from the ceiling line up to the underside of the lantern frame. That return can be plastered, painted and lit from behind. It is worth getting the depth right, because a deep return hides the frame and a shallow one exposes the whole kerb.

A solid, heavy ceiling also works for quiet. Plasterboard on a proper fixing, with the void above it insulated, damps the room. Glass alone reflects. The mix of a large soft ceiling and a small area of well-specified glass is one reason an orangery can feel hushed even when it has a lantern in it.

The perimeter of an orangery roof is a warm flat roof, with the insulation above the deck and the structure kept on the warm side. That is the build that suits a room you want to use in January.

From the inside outwards, the make-up runs like this:

• Plasterboard ceiling, fixed to the joists.
• A vapour control layer, taped and lapped, which stops warm moist air from reaching cold surfaces.
• The structural deck, usually plywood or OSB laid to falls.
• Rigid insulation boards above the deck, in layers with staggered joints.
• The waterproof membrane, turned up the kerb and sealed.

The thickness of insulation is set by the room's thermal performance. For a new flat roof, the fabric standard in Approved Document L is a U-value of 0.15 W/m2K or better for the roof element, and the glazed lantern has its own limiting value. We set the insulation depth to meet the roof standard with the kerb height in mind, because the thicker the boards, the higher the kerb must stand above the deck. Our page on energy efficient rooflights explains the glass side, and the answer on whether lanterns are energy efficient enough for Part L gives the numbers that apply to the frame and the panes.

The kerb itself needs insulation too. A bare timber or aluminium upstand in the middle of a well-insulated roof is a cold bridge, a path for heat to leave the room. The kerb is lined on its inner and outer faces so that the insulation above the deck runs up to meet the lantern frame. The boards are cut tight, the joints are taped, and the vapour layer is carried up into the kerb. This detail is where an orangery roof is most often let down, and it is also where condensation starts. Our answer on why a lantern gets condensation explains the mechanism.

The kerb is the joint between solid roof and glass. It sets the height, it carries the load, it holds the waterproofing and it decides how the room looks from the garden.

A kerb on a warm roof has to do four things at once. It must be level, so the lantern frame sits without strain. It must be tall enough for the membrane to be turned up and sealed, with a clear margin above the finished surface. It must carry the lantern weight down into the structure. And it must be insulated so that the ceiling line and the lantern share one continuous thermal layer.

A flat roof is laid to a fall, for example 1:80 at completion, so that water reaches the outlet without standing against the upstand. The kerb interrupts that fall, so its position matters. We keep it clear of the low edge and away from outlets, and on a long orangery we set the lantern slightly towards the high side so water has a clear run on the other. The detail is covered in our page on roof lantern kerbs and upstands, and the full sequence of work is set out in how a roof lantern is installed, step by step.

An orangery lantern is sized against the room under it and the solid margin around it. The margin is what makes it an orangery.

As a rule we want the flat ceiling around the opening to read clearly on all four sides, and in a garden room that usually puts the lantern at something like half to two thirds of the room's width. The size guide for extensions goes deeper on measurements, with all three figures that a quote should state, in our roof lantern size guide. The table below gives starting points for the common shapes of orangery, so you can see how the choices relate.

The shape of the lantern itself is a matter for the room as much as the roof. Our answer on which shape suits a long, narrow room shows how it plays out, and the page on glazing bars and how many you need explains how the bar count grows with the footprint. If the room sits between standard sizes, a made-to-measure lantern can follow the ceiling instead of a catalogue.

A lantern brings in a lot of light, and on a south or west facing orangery it can bring in a lot of heat. The solid perimeter helps, because it keeps the glazed area modest, but the glass still has to be chosen for the room.

Part O of the Building Regulations deals with overheating in new dwellings, and Approved Document L limits the glazed area of an extension. A large lantern above wide glazed doors can push the total past the limit, and the design then has to show performance elsewhere. The glass itself can carry a solar control coating that cuts heat without turning the room grey, and the frame can carry opening vents to let hot air out at the top. Our page on orientation and the summer sun goes through the reasons, and the answer on whether a lantern makes a room too hot gives the practical side. Vents are covered in can a lantern have opening vents.

Blinds are the last layer. A garden room that doubles as an evening sitting room wants a way to dim the glass, and blinds fitted under a lantern need space in the light well that has to be planned before the plasterboard is closed. The answer on blackout blinds explains the options.

Rain on a lantern can be loud. An orangery has an advantage here, because most of its roof is solid and only the centre is glass.

The noise comes from the glass and the frame, not from the perimeter. The choice that matters is the glazing build: a laminated inner pane, a sensible thickness on each leaf, a cavity that is not a thin sliver, and seals that stay firm. The soft ceiling around the lantern then absorbs what the glass passes on. We describe the choice by specification, and it goes into the written quiet spec that comes with every quote. Our page on the quiet spec shows the format.

In the Crawley area the noise outside might be aircraft on the Gatwick routes, the A23 and M23, the Brighton main line or simply a wet November. We listen for whichever applies at the survey and build the glass and upstand around it. The answer on whether lanterns are noisier in heavy rain gives the physics in detail, the guide to glazing builds for a quieter room shows the choices side by side, and why the inner pane is laminated covers the safety reason that also helps with sound.

A self-cleaning coat on the outer pane is worth a mention for a garden room under trees. It keeps the glass clear between cleans, and our page on self-cleaning glass explains how it works and what it does not do.

A garden room that becomes an orangery sits on the edge of several rules. Some concern planning, some concern Building Regulations, and they do not always point the same way.

Planning first. Whether the lantern needs permission depends on the height above the roof, the position of the building and whether it is an extension to the house or a separate outbuilding. Our answer on whether a lantern needs planning permission lays out the usual questions, and the Planning Portal at planningportal.co.uk is the official place to check the rules for your own property. If the house is in a conservation area, that changes the position, and we check it for the address.

Then the regulations. The work is installed to current Building Regulations: Part L for energy, Part F for ventilation and Part B where the roof is near a boundary, with safety glazing over people as good practice. Where the work is notifiable, we handle the Building Control notification. Snow load on the lantern and the roof is an engineering point rather than a planning one, and our page on snow load on a roof lantern explains how it is dealt with.

The order of work follows the roof. The deck and insulation go down first, the kerb is built and lined, the lantern is craned or lifted into place, the membrane is dressed up the kerb and the flashings are fitted. Inside, the ceiling is boarded, the light well is formed and the room is made good. Our installation process page has the full sequence, and the guarantee page sets out the 10-year workmanship cover.

The lantern is only part of the figure. The roof build, the structure and the interior finish often add up to more than the glass.

The lantern price depends on size, frame, glass and options such as vents and self-cleaning coats. The roof around it depends on the state of the garden room and the insulation depth. The interior depends on the light well, the lights and the blinds. Our page on what affects the cost of a roof lantern goes through each driver, and the costs page gives the wider market picture. We give a written quote after the survey, and every quote comes with a quiet spec.

Can I add a lantern to a garden room I already have?

Usually yes, if the structure can take the change. The joists around the opening are trimmed, and a beam may be needed on wider rooms. The roof covering often has to be renewed around the lantern so the membrane and the kerb are one continuous build. The survey decides, and the written quote states what is needed.

Will an orangery lantern make the room too hot?

It can, on a south or west aspect, if the glass is chosen badly. A solar control coating, vents at the top and a blind under the lantern keep the room comfortable. The solid perimeter helps, because the glazed area stays modest. We write the choice into the quote so you can see why it was made.

How thick does the roof around the lantern need to be?

It depends on the insulation needed to reach the Part L standard for the roof, plus the deck, the ceiling and the falls. That total sets the kerb height. On most garden rooms the roof ends up noticeably thicker than the old one, and the ceiling height inside is checked before work starts.

Is the lantern going to be noisy in rain?

It is quieter when the glass is built for it. A laminated inner pane, sound seals and a soft ceiling around the opening all help. The solid perimeter means only the centre of the roof is glass. We describe the build in the quiet spec.

When you are ready to talk it through, get a quote or message us on WhatsApp.