Replacing a Conservatory Roof with a Lantern

Chapter 1

Why a glass roof and a year-round room pull in opposite directions

A fully glazed or polycarbonate conservatory roof lets in a great deal of sky, but it also lets the weather straight into the room, which can leave the room comfortable only for a few weeks each spring and autumn.

The physics is simple. A conservatory roof is a large, shallow sheet of glazing facing almost straight up. In June the sun over Cambridge climbs to around 61 degrees above the horizon at midday, so a roof that faces the sky takes the full strength of it for hours. The heat lands on the floor, the furniture and the people, is absorbed, and is given back to the air, which collects under the ridge with nowhere to go. In January the same roof works the other way: even good double glazing loses heat far faster than an insulated roof, cold air slides down the inside of the panes, and the room needs a heater running to feel usable.

Older roofs add their own problems. Multiwall polycarbonate sheets yellow and cloud with age, drum loudly in rain and hail, and let in a flat, grey light. Early glazed roofs have heavy bars, tired seals and glass that fogs between the panes. A tired conservatory roof tends to become a room you walk through rather than a room you sit in.

Replacing the conservatory roof with a solid, insulated roof and a roof lantern changes the balance. The room keeps a generous source of daylight, placed where it does the most good, and gains a ceiling that holds the warmth in winter and blocks most of the summer sun. This guide explains how that upgrade works, what has to be checked first, how to size and glaze the lantern, and what the regulations ask.

Chapter 2

What changes when the glass moves to one place

The upgrade does not take the light away: it concentrates it. A lantern of the right size, set in an insulated roof, gives a steady, even top light over the part of the room you use, while the glazed walls still give you the view of the garden.

It helps to think about where daylight in a room actually comes from. The walls of a conservatory already admit a lot of light at eye level, and that side light is what gives the room its connection to the garden. The roof adds light from overhead, which is the most efficient daylight there is: a skylight or lantern of a given size lights a room far more evenly than a window of the same size in a wall, because it sees the brightest part of the sky and spreads the light down onto the floor and the table.

A fully glazed roof, though, gives you far more overhead light than a room needs, and gives it everywhere at once. The result is glare on screens, faded fabrics and a room that feels like the outdoors. When the roof becomes solid and the lantern takes over, the overhead light is gathered into one opening, with its own glass specification, and the rest of the ceiling becomes a calm, plastered surface that bounces a little of that light back down.

The feel of the room shifts in a few clear ways:

• The light becomes directional. It falls on a dining table, a kitchen island or a sofa, rather than washing over every surface.
• Glare drops sharply, because the bright sky is no longer overhead in every direction you look.
• Rain becomes quiet. An insulated solid roof, and a glazed lantern in place of polycarbonate, takes away the drumming.
• Winter mornings are warmer, because the room is losing heat through a small glazed area rather than the whole ceiling.
• The ceiling can carry lights, a fan or a speaker, which a glazed roof never could.

Cambridge is a good place for this kind of upgrade. The land is flat and the sky is big, so very little shades a roof. A lantern here sees the whole sky, which means a modest one does a lot of work, and it also means the glass has to be chosen for the sun it will actually get.

Chapter 3

Checking the conservatory can carry a solid roof

A conservatory was designed to hold up a light glazed roof, not a solid one with insulation, plasterboard and a lantern on top, so the first job is to check the frames, the walls and the base before any roof is ordered.

A solid roof weighs considerably more than the glazing it replaces. The load has to travel down through the frames or posts into the dwarf walls, and from there into the foundations. A typical conservatory from the 1990s or 2000s, on the 1930s semis of Cherry Hinton or the estate homes of Histon and Cambourne, has uPVC or aluminium window frames sitting on a low brick wall. Those frames were designed to hold glass in place, not to act as structural columns, so a solid roof normally needs a perimeter beam to spread the load, and sometimes new posts or piers inside the frame line.

At the survey these are the things we look at, from outside and from inside the room:

1. The dwarf walls: whether they are plumb, cracked or bowing, and how they were built.
2. The base and foundations, where they can be established: their depth and whether there is any sign of movement.
3. The frames: their material, their condition, and whether they are reinforced or can take a beam on top.
4. The junction with the house wall, where the new roof will meet the brickwork and the existing roof above.
5. The height available under first floor windows and eaves, which limits how high the new roof and lantern can go.
6. Drainage: where the new roof will shed its water, and whether the existing gutters and downpipes suit it.

Where the structure is questionable, a structural engineer's calculations for the beam, the posts and the foundations are part of the Building Control submission. If the base or walls cannot take the load, the honest answer is that the conservatory needs rebuilding as an extension rather than a new roof. That is less common than people fear, but it is far better found at the survey than halfway through the job.

Chapter 4

Sizing and placing the lantern

A lantern of roughly 15 to 20 percent of the roof area gives a bright, balanced room; the exact size depends on the ceiling height, the orientation and what the room is used for.

The rule of thumb used across the trade is a lantern skylight of 15 to 20 percent of the internal roof area. Going up towards 25 percent gives a very bright room, but on a south or west facing roof it brings a real risk of overheating, and at that point the glass specification has to work much harder. A higher ceiling can take a slightly larger lantern, because the light has further to travel and spreads more before it reaches the floor. A low ceiling under an existing first floor window may need a smaller, lower profile unit.

So a typical 3.5 by 3 metre conservatory might take a lantern of around 2000 by 1000 mm. That still gives a strong overhead light, and it leaves enough solid roof for insulation to do its job.

Position matters as much as size. A lantern placed dead centre by default often ends up lighting the floor between the furniture. We start from how the room will be used. If it is becoming a dining room, the lantern goes over the table. If it opens onto a kitchen, a lantern over the island draws the eye through from the house. If it is a sitting room with a television, the lantern is kept away from the screen wall so the afternoon sun does not land on it. Our light planning guide explains how we map the sun patch through the day and the year before fixing a position.

The lantern itself sits on an upstand, a timber kerb built up from the new roof deck and insulated, so the glass is held clear of the roof covering and water runs away from it. The ridge normally runs along the longer side, and the orientation of the ridge affects which faces of the lantern take the most sun. On a small room a lower pitch lantern keeps the external profile modest, which also matters if the roof sits under a bedroom window.

Chapter 5

Glass for the way the room faces

The glass in the lantern decides whether the room stays comfortable on a July afternoon, so it is chosen for the orientation of the room, not picked from a brochure.

Conservatories were often built on the sunniest side of the house, facing a south or west garden, which is exactly why they overheat. The lantern for that room needs glass that lets in plenty of visible light while turning back a good share of the sun's heat. Solar control coatings do this, measured by the g-value: the lower the g-value, the less of the sun's energy passes through. Neutral solar control glass keeps the colour of the sky and the room natural; tinted glass cuts more heat but changes the colour of the light, which some people dislike in a kitchen.

A north facing conservatory is a different case. It rarely sees direct sun, so its main weakness was always winter cold rather than summer heat. Here a clear, high light transmission low-E unit is usually the right choice, because it lets through as much daylight as possible while holding heat in. East facing rooms get bright, low morning sun that can be glary over a breakfast table, and a light solar control coating helps without dulling the rest of the day.

Whatever the orientation, the inner pane of an overhead glazed unit should be laminated, so that in the unlikely event of breakage the glass stays in place. Self-cleaning coatings on the outer pane are worth considering on a lantern you cannot easily reach. Triple glazing improves the U-value further and can reduce condensation on very cold nights, at the cost of more weight and a little less light. If your existing roof is polycarbonate, our guide to replacing polycarbonate rooflights explains the difference moving to glass makes to light, noise and warmth.

Chapter 6

Building Regulations, planning and the paperwork

Swapping a glazed conservatory roof for a solid one is usually treated as building work that needs Building Regulations approval, while planning permission is often not needed; both depend on the details, so they are confirmed for each house.

Conservatories are generally exempt from Building Regulations when they are at ground level, under 30 square metres, separated from the house by external quality walls, doors or windows, have independent heating controls, and have glazing that meets the safety glazing rules. Guidance treats a conservatory as a room whose roof is largely glazed or translucent. Replace that roof with a solid, insulated one and the room no longer fits the definition, so the work is generally controlled and needs Building Control approval. Building Control will look at several things:

• Structure, under Part A: the beam, the frames or posts and the foundations must carry the new roof.
• Energy efficiency, under Part L: the new roof is normally expected to meet the standard for new roof elements, a U-value of around 0.15 W/m²K, and the lantern must meet the limiting value for rooflights of 2.2 W/m²K.
• Ventilation and condensation, under Part F and Part C: the roof build-up has to manage moisture, and the room needs adequate ventilation.
• Safety glazing under Part K, and fire spread under Part B where the roof is close to a boundary.

Building Control may also look at how the room relates to the house. If the doors between the conservatory and the house are staying, the room can remain thermally separate. If they are coming out to make one larger space, the glazed walls become part of the heated house, and the overall performance of the new room may need to be shown to be acceptable. We handle the Building Control notification where the work is notifiable, and our guide to Building Control for replacement rooflights sets out how that process works.

Planning is usually simpler. Replacing the roof of an existing conservatory without making the room bigger or noticeably taller is often within permitted development, but this depends on the house. A listed building needs listed building consent for almost any change. In a conservation area, or where an Article 4 direction removes permitted development rights, or where the original permission for the house carries conditions, the position can be different. Cambridge and South Cambridgeshire are covered by Greater Cambridge Shared Planning; homes further out fall under East Cambridgeshire, Huntingdonshire or Uttlesford. We check the position for your address at the survey and, where there is any doubt, the planning authority can confirm it before work starts.

Chapter 7

How the work runs, from strip-out to plastered ceiling

The work follows a clear sequence: take off the old roof, fit the structure, build and insulate the new roof, fit the lantern, then finish the room, with the room kept covered at each stage.

The old glazing or polycarbonate comes off first, and the room is protected with temporary sheeting while the new structure goes in. The perimeter beam is fitted on top of the frames or new posts, and the roof joists or rafters are set from the beam to the house wall. A deck goes on, followed by insulation, a vapour control layer and the roof covering, which for a flat or low pitch roof is typically a single ply membrane, EPDM or GRP. The lantern kerb is built into the deck at the planned position, insulated and dressed with the roof covering so the junction is watertight, and the lantern is fitted on top.

Inside, the underside of the roof is boarded and plastered, the reveal around the lantern is lined and finished, and any lighting, fans or speakers go in before the ceiling is closed. New gutters and downpipes are fitted where the roof edge has changed. The whole roof usually takes a matter of days rather than weeks, although plaster drying time and the weather both affect the finish date, and the programme is agreed at the quote so you know what to expect each day.

Access depends on the height of the roof and what surrounds it; our answer on whether replacing a rooflight needs scaffolding explains the options. The old glazing and panels are removed from site, with glass recycled where the local facilities allow, as our answer on what happens to the old rooflight describes.

Chapter 8

Other ways to upgrade a conservatory roof

A solid roof with a lantern is not the only upgrade, and the right answer depends on how you want the room to feel, how it faces and what the structure will take.

The simplest option is a new glazed roof with modern solar control glass. It keeps the fully open feel of a conservatory and is lighter than a solid roof, so the structure rarely needs strengthening. It will be far better than old polycarbonate, but it is still a very large area of glass overhead, and a south facing room will still get warm in summer.

A pitched tiled roof, with the slopes finished inside as a vaulted ceiling, suits a room that should look like part of the house. The daylight then comes through roof windows set in the slopes, which work well on a pitch but give a more directional light than a lantern. On a low or flat roof, one or two flat rooflights can replace the lantern, giving a clean, minimal look from inside and a lower profile outside, which can matter under a bedroom window.

The solid roof and lantern suits rooms meant to be used as kitchens, dining rooms or family rooms: rooms where a strong pool of light over one area matters more than glass everywhere. If you already have a lantern elsewhere that has reached the end of its life, our guide to replacing a failed roof lantern covers that case. For what drives the price of each option, from the structure to the glass, see our page on skylight and rooflight costs. The wider picture of renewing older roof glazing is on our misted and failed rooflight replacement page.

Chapter 9

Questions people ask

Will the room be darker with a solid roof?

It will have less light overhead in total, but it rarely feels darker in use, because the glazed walls still give plenty of side light and the lantern puts strong top light where you sit. What drops away is the glare and the heat. A well placed lantern of 15 to 20 percent of the roof area gives a bright, comfortable room through the day.

Can we keep the existing conservatory frames?

Often, yes, if they are in good condition and the survey shows they can carry a beam, or if new posts take the load inside the frame line. If the frames themselves are tired, replacing them at the same time gives better glass in the walls and a stronger base for the roof.

Does the new roof need to match the house?

Not necessarily. A flat roof with a lantern is a clean, contemporary look that sits well against gault brick or render. In a conservation area, or on a house where the rear is visible from the street, a finish that suits the house and a modest lantern profile make approval simpler.

Will the work disturb the rest of the house?

The work is mostly confined to the conservatory itself, and the house stays in use throughout. The junction with the house wall is the one place where both sides are involved. Our answer on whether replacing a rooflight will damage the ceiling explains how making good inside is handled. To see where the light would land in your own room, book a light survey through our quote form or message us on WhatsApp.