Roof lantern installation in Exeter

A roof lantern is a small glazed roof in its own right. It has a ridge along the top, hips at each end and glazing bars running down to a base frame, which sits on a raised kerb called the upstand. Where a flat rooflight lies almost level with the roof, a lantern rises above it, usually as a hipped rectangle, sometimes as a square pyramid or an octagon over a round room.

Because the glass faces several directions at once, a lantern takes in light from the whole sky rather than from one slope. That is why it reads so differently from a flat rooflight of the same footprint. The pitched panes also shed rain and leaves toward the base frame, so the glass stays cleaner on a flat roof than a level pane does in a wet Devon winter.

There is a design reason too. A flat roofed extension can feel low, especially where the ceiling height has been kept modest to stay under a neighbour's window or inside permitted development limits. A lantern lifts the eye into a lit space above the ceiling line and gives the room a centre. Over a kitchen island, a dining table or the middle of a garden room, it becomes the thing the room is arranged around.

A lantern is not the right answer for every flat roof. A long, narrow extension, a roof that will be seen from bedrooms above, or a roof terrace that needs a walk-on panel may each be better served by a flat glass unit, and we go through that choice in our guide to a flat rooflight or a roof lantern. Our flat roof rooflights page covers the low profile option, and walk-on rooflights covers glass you can stand on.

The upstand, also called the kerb, is a raised box built around the opening in the flat roof. The lantern's base frame sits on top of it. It does three jobs: it lifts the glass clear of water lying on the roof, it gives the roof covering something to turn up against and finish under the lantern's base, and it carries insulation around the edge of the opening so the edge of the ceiling does not become a cold spot.

Upstands come in two forms. A timber upstand is built on site from treated timber, insulated, boarded and then wrapped in the roof covering, whether that is GRP fibreglass, EPDM rubber or a torch-on felt system. A prefabricated insulated kerb arrives ready made to the lantern's size and is fixed down and weathered in. Both work well. The choice usually depends on the roof covering, the lantern maker's instructions and whether the opening is a standard size.

A tidy upstand built square and true makes a crisp reveal possible.

Height matters on two counts. Too low, and the base frame sits close to standing water and wind-driven spray, which on an exposed roof in Dawlish or Exmouth is a real consideration. Too high, and the lantern looks perched, and the reveal inside becomes deeper than it needs to be. The right height depends on the roof falls, the depth of insulation and the lantern system, and it is one of the first things we settle at survey.

The upstand also decides a great deal about what you see from inside. Its inner face continues down through the roof deck and the joists to become the reveal, the plastered sides of the opening above your ceiling. A tidy upstand built square and true makes a crisp reveal possible. A skewed or uneven one shows through every coat of plaster. We go into heights, materials and insulation in more detail in roof lantern upstands and kerbs explained, and the coverings themselves in flat rooflights on GRP, EPDM and felt.

Between your ceiling and the base of the lantern there is a deep box: the depth of the joists, the insulation, the deck and the upstand stacked together. In a new extension built to current standards, that box can be deeper than many people expect. How it is lined and finished decides whether the lantern looks like part of the room or like a hole cut into it.

From the kitchen table you will see that reveal every day, so it is the part of the job we plan in writing before we start. There are several ways to form it, and the right one depends on the depth, the ceiling height and the style of the room.

Straight plastered reveal

The sides drop vertically from the lantern base to the ceiling, boarded with insulated plasterboard, beaded at the corner and skimmed. It is clean and calm, and it suits a shallow box or a lantern that already fills most of the ceiling.

Splayed reveal

The sides angle outward as they come down, so the opening is wider at the ceiling than at the glass. A splay spreads daylight further across the room and makes a deep box look shallower. It takes more careful framing, because each splayed face has to meet its neighbour in a true, sharp mitre at the corners.

Coffered or stepped reveal

The opening steps down in one or two tiers, like a framed recess, with a flat ledge at each step. Coffers suit tall rooms and orangeries, where a plain shaft would look too stark, and the ledges give a natural place for concealed lighting.

LED lines

A continuous LED strip tucked into a recess, a shadow gap or the ledge of a coffer lights the reveal after dark, so the lantern does not turn into a black rectangle at night. The channel and the cable route are formed while the reveal is being built, and the wiring itself is done by a qualified electrician.

The edges are what make a reveal look finished. A metal angle bead set on each external corner gives a straight, hard arris that will not chip when the room is decorated. A stop bead or a shadow gap bead where the plaster meets the lantern frame gives a clean junction instead of a cracked bead of filler. The reveal is lined with insulated board so its faces stay warm, which keeps condensation and mould off the plaster in winter.

In older homes we match the ceiling the lantern meets. A Georgian back addition with lime plaster gets lime around its reveal, and a 1930s kitchen with a gypsum skim gets gypsum. The ceiling finishes around a roof lantern guide shows each option in detail, and how a rooflight is finished inside covers the same craft for other rooflights.

A lantern into a new opening follows a set order, and most of it happens on the roof before anything changes inside. The room below stays usable for much of the job, with the dusty work kept to a short window.

It begins with a survey. We measure the roof, check the joist sizes and direction, lift a corner of the covering if needed, and look at the ceiling from below. That visit produces the written quote and the finish schedule. The lantern is then made to order, which takes a few weeks with most systems, and we book the start date once the frame is ready.

At the end of each day a temporary waterproof sheet is fixed over the hole, so rain does not reach the room below.

On the first day the roof covering is cut back around the marked opening and the room below is sheeted and dust screened. The joists are cut and trimmed with new timbers, or doubled and strapped where the engineer has asked for it, and the opening is framed square. At the end of each day a temporary waterproof sheet is fixed over the hole, so rain does not reach the room below.

Next comes the upstand. It is built or set on the trimmed opening, insulated and boarded, and the roof covering is dressed up its sides and over its top. On a GRP roof that means a laminated fibreglass turn-up. On EPDM it means a bonded rubber upstand, and on felt a torched cap sheet. The lantern's base frame is bedded and fixed to the upstand, the ridge and hip bars are assembled, the glass units are set in place and the external cappings go on. From that point the room is watertight under its new glass.

Then the inside. The reveal is framed to the shape in the finish schedule: straight, splayed or coffered. Any LED channel and cable route goes in at this stage. The faces are lined with insulated plasterboard, the corners beaded, and the reveal is skimmed or lime plastered to meet the existing ceiling. Where the ceiling around the opening was disturbed, it is made good so the joint does not show.

The plaster then needs time to dry before decoration. The reveal is handed over primed or painted, as agreed in the finish schedule, the dust screens come down and the room is cleaned. A typical new lantern takes a few days on site, plus drying time, and a like-for-like replacement on a sound upstand is usually quicker. Our answer on how much mess fitting a rooflight makes explains how the dust is contained.

If you would like a date for your survey, ask for a quote and tell us a little about the room.

Many lanterns fitted on extensions and conservatories in the 1990s and 2000s are now reaching the end of their useful life. Early uPVC lanterns and those glazed with polycarbonate sheet were often less well insulated than a modern system, and their seals and gaskets age.

The common signs that a lantern is due for replacement are:

• misting or droplets between the panes of the sealed units;
• glazing bars that have sagged or twisted, or cappings that have lifted;
• polycarbonate that has yellowed, gone brittle or turned cloudy;
• heavy condensation running down the bars on cold mornings;
• a room that loses heat fast in winter and overheats in summer.

Either way, the reveal inside is usually disturbed, so a replacement is the right time to improve it

If the frame is sound and only one or two sealed units have misted, replacing just the glass may be enough, and that work is not notifiable. Our guide to replacing just the glass in a rooflight explains when that makes sense, and the misted and failed rooflights page covers the wider question. Where the frame itself is failing, replacing the whole lantern gives you modern glass and a frame that meets current Part L standards. That replacement is notifiable, and we deal with the Building Control side. Our guide to Part L and replacement rooflights explains what the new unit must achieve.

A replacement starts with the existing upstand. If it is sound, square and the right size, a new lantern can often be made to sit on it, with the roof covering re-dressed around the base. If it is rotten, too low or the wrong size for the new system, it is rebuilt. Either way, the reveal inside is usually disturbed, so a replacement is the right time to improve it: insulate a cold reveal that has been growing mould, add a splay or a coffer, or tuck in an LED line. The finish schedule sets that out at quote stage.

How long a lantern lasts depends on its frame, glass and exposure, and our answer to how long do roof lanterns last looks at each. If your lantern is showing its age, send us a few photos with a quote request and we will arrange a survey.