Roof Lantern Installation

Where a lantern earns its place on a roof

The honest test is the shape of the room below. A lantern adds height and direction to a ceiling, because the bars run up to a ridge and the eye follows them. In a wide rear extension, four metres or more across, that reads as a room with a top to it rather than a room with a bright rectangle in the ceiling.

In a narrow galley the same structure feels like a lid pressed down over your head, and a flush flat rooflight is the better call. The other input is what you see from the garden. A lantern is visible from outside and from upstairs windows, which is a feature on a period rear addition and a nuisance where a neighbour’s landing looks straight down onto it. Both of those are worth settling before any product conversation starts.

The parts of a lantern, named from the outside in

The vocabulary is small and it makes every later conversation quicker.

• Ridge: the horizontal member at the top where the sloping panes meet. It carries the glazing bars and, on many designs, houses the opening vent.
• Glazing bars: the sloping members running from ridge to eaves. Their thickness sets how heavy the lantern looks from below.
• Hip bars: the diagonal members on a hipped lantern where two slopes meet at a corner.
• Eaves beam: the perimeter frame that sits on the kerb and takes the load down into it.
• Kerb or upstand: the timber box built off the roof structure that the eaves beam lands on. Not part of the lantern, and not usually supplied with it.
• Sealed units: the glass panes, one per bay, plus one per hip triangle.

The distinction that catches people out is the last one in the kerb line. The lantern arrives as a frame and glass. The thing it stands on is built by whoever is doing the roof, which raises the question of who is responsible for the kerb on any given job.

What the roof has to be doing before a lantern goes on it

A lantern wants a flat roof, which in practice means anything under about ten degrees, and it wants that roof to be draining properly. Those two conditions do more to decide the outcome than any specification choice made later.

Draining properly means a fall of at least 1:80 as designed, delivering something close to that once deflection and construction tolerance have taken their share. It means outlets that are clear and positioned where the water is actually going. And it means no standing water sitting for two days after rain, because a roof that ponds will find whatever weakness exists in a kerb detail and exploit it every winter. Where a survey finds ponding, sorting the fall comes first and the glass goes into a roof that already drains.

The kerb, and the single most important dimension on the job

One hundred and fifty millimetres, measured from the finished roof surface to the top of the kerb where the lantern lands. That figure appears in manufacturers’ details across the industry and it is not arbitrary: it combines the depth of water that can stand on a flat roof with the height that wind driven rain can be pushed up a vertical face.

The kerb is normally built in treated timber, typically 100mm by 50mm or 150mm by 50mm studwork with an 18mm ply facing, insulated inside so it is not a cold ring around the opening, and set out to the manufacturer’s external frame dimension rather than to the glass size. Getting that setting out wrong by fifteen millimetres is the difference between a frame that drops on and a frame that sits proud on one corner. It is measured twice, from the actual delivered unit where possible, not from a brochure page.

Choosing the frame material without reading a specification sheet

Two materials cover nearly all domestic lanterns. Powder coated aluminium gives the slimmest bars, the longest unsupported spans and a finish that needs washing rather than attention. Timber, usually engineered laminated softwood or hardwood with an aluminium capping outside, gives a warmer internal face and suits a period room where painted joinery is already the language of the house.

The practical shortcut is to decide what the room is trying to be. A modern kitchen extension in Beaulieu Park or Great Notley almost always wants aluminium, because the point of the glazing is that it disappears. A rear addition on a Victorian terrace in Chelmsford or Colchester with existing sash windows often wants a timber interior. The full comparison, including thermal movement and upkeep over twenty years, sits on the page comparing aluminium and timber lanterns.

Pitch and ridge height, in one paragraph

Most lanterns are built somewhere between twenty and thirty degrees, with twenty five as the working default. Below about fifteen degrees run-off slows, dirt sits longer on the glass and rain noise gets louder because water strikes the pane closer to square on. Above thirty the structure starts to dominate a modest room and the permitted development height limit becomes a live constraint on a low extension roof.

Pitch and width are locked together. A wide lantern at a steep pitch produces a tall ridge, and on a single storey extension roof that can push the top of the glass close to the first floor windows behind it. The arithmetic and the trade-offs are worked through on the page about lantern pitch.

Specifying the glass so the room is usable in August

A lantern faces upward at the whole dome of the sky, which is why it out-performs a wall window for daylight and why it collects summer heat that a wall window never would. At the peak of a July afternoon the sun is high and its energy arrives almost perpendicular to a sloping pane.

The two numbers that decide how the room behaves are the U-value, which measures heat loss and wants to be low, and the g-value, which measures how much solar energy gets through and also wants to be low on anything facing south or west. A solar control coating with a g-value in the region of a quarter to a third takes the sting out of the afternoon while leaving the light looking neutral rather than tinted. The outer pane should be toughened and the inner laminated, because the glass is overhead. The detail is covered on the page about lantern glazing and solar control, and the wider thermal picture on energy efficient rooflights.

Hipped, gable and lean-to lanterns

Three geometries cover the domestic market and they suit different plans.

A hipped lantern is the safest choice on an open flat roof because water runs off all four sides and away. A lean-to needs a proper abutment detail where the top of the glass meets the wall, with a cover flashing chased into the brickwork rather than sealed against its face.

Forming the opening in a flat roof deck

Cutting a hole in a roof deck interrupts joists that were carrying load across the span, and that load has to be redirected. For openings up to about a metre and a half, trimming in timber is normally enough: the joists either side of the opening are doubled, trimmers go in at each end, and the whole thing is connected with joist hangers rather than skew nails.

Beyond that width, or where the opening runs across the span rather than with it, a steel is the usual answer and a structural engineer produces a calculation for it. That changes the shape of the project, adding a lead time for fabrication and a specific Building Control inspection. Finding out at design stage is routine. Finding out on the morning the lantern is delivered is not. The full account is on the page about structural openings and steel.

How a lantern is fixed down and weathered

The eaves beam is bedded onto the top of the kerb on a continuous bead of low modulus silicone or a butyl tape, depending on the manufacturer’s detail, and then mechanically fixed down through the frame into the kerb timber at the specified centres. The bedding is a seal against wind driven water, not a structural fixing. The screws do the holding down, and uplift on a lantern is a real force on an exposed Essex roof.

Weathering then happens in the right order and only in that order. The roof covering is dressed up the outside face of the kerb and terminated under the eaves beam or under a purpose made trim, so that any water reaching the kerb runs down the outside of the covering. A lantern sealed to the top of a covering that stops short of the kerb head will pass a hose test and fail in the third year.

Dressing the covering up to the kerb, covering by covering

The termination detail changes with what is on the roof, and the covering trade and the glazing trade have to agree it before either starts.

• Single ply membrane: welded up the kerb face and over the head, with a preformed corner gusset welded in at each angle. Corners are where this system is tested.
• Torch-on felt: a two layer upstand dressed over the kerb head, with the cap sheet lapped down over the underlayer and mitred at the corners.
• GRP: laminated continuously up the kerb and over the top, so the upstand and the field are one piece with no joint at the base.
• Lead or single ply on a warm deck: the insulation continues up the kerb so the timber is not a cold bridge, then the covering runs over it.

In every case the base of the kerb is where the covering has to be continuous, because that internal angle is the point that water reaches first and stays at longest.

Getting water away from a lantern

A lantern occupies a large part of a small roof, and everything that lands on the glass ends up on the covering around it. The area of roof left to carry that water is smaller than it was, and the falls have to still work.

Two situations cause trouble. The first is a lantern set close to a parapet, leaving a narrow channel between kerb and wall that catches leaves and closes itself. The second is a lantern positioned uphill of nothing, with the fall running from the lantern towards a wall rather than towards an outlet. Both are fixed at layout stage by moving the lantern a few hundred millimetres or by adding a second outlet. Both are expensive to correct after the roof is finished, which is why the position is agreed on the roof with a tape rather than on a drawing.

Cold surfaces, warm frames and where condensation shows up

Warm moist air rises, and a lantern is the highest point in the room. Over a hob or in a family kitchen that means the coldest surface in the room is directly above the biggest source of water vapour, which is why the first cold snap after installation often produces a call.

Three things govern how much appears. The frame needs a proper thermal break so the internal aluminium is not connected to the outside. The glass wants a warm edge spacer rather than an aluminium one, because the pane perimeter is where the first drops form. And the kerb has to be insulated on its inside face, since an uninsulated ply box is a cold ring sitting between a warm ceiling and cold air. Ventilation does the rest, which is a good argument for an opening vent even in a room that already has windows. The behaviour in a kitchen specifically is covered under lantern condensation.

The internal opening, and how the ceiling meets the glass

The reveal is the lining between the plasterboard ceiling and the underside of the lantern, and it decides how the light behaves once it is inside. A square reveal, with vertical sides, drops a hard edged rectangle onto the floor and leaves the ceiling around it dark. A splayed reveal, opening outward towards the room, spreads light across the ceiling and makes the opening read considerably larger than it is.

Whether the reveal is even possible depends on the depth of the roof build-up, which on a warm deck can be three hundred millimetres or more. That depth is an opportunity rather than a problem. It is also the moment to decide about services: any downlights, extract runs or track positions around the opening want setting out before the ceiling is boarded, because moving a cable in a closed ceiling around a lantern kerb means opening plasterboard that was finished last week.

Ventilation, and deciding about an opener before first fix

An opening vent at the highest point of a room does something no window can. Warm air collects at the ceiling and leaves through the vent, drawing cooler air in through windows lower down. That is the stack effect, and in a tall extension it shifts a real volume of air with no fan and no running cost.

The vent is usually a hinged section of the ridge or one full glazed bay on a slope, and it can be manual on a pole, electric on a chain or spindle actuator, or solar powered with its own panel and battery. Anything above about two and a half metres from the floor is irritating to operate manually within a month. The wiring decision matters more than the vent choice: a cable run at first fix costs almost nothing, and running it later means chasing a finished ceiling. The mechanisms and controls are compared under ridge vents and how they are operated, and the wider actuator picture under electric rooflight installation.

A lantern on a new extension against one on an existing roof

On a new build extension the lantern is easy, provided it is designed in. The kerb is built as part of the roof, the insulation and covering are detailed around it from the start, and the ceiling is boarded to a reveal that was set out on the drawing. The only real risk is a builder who forms the opening to the glass size instead of the frame size.

Retrofitting into an existing flat roof is a different piece of work. The covering has to be stripped back beyond the opening, the deck opened up, the joists trimmed, the kerb built, the insulation made continuous, and then a covering laid that ties into the surviving field without a weak joint at the junction. Where the existing covering is old felt near the end of its life, the sensible answer is often a new covering over the whole roof at the same time, because a new lantern in a tired roof puts the newest detail next to the oldest one.

What the Essex roof stock does to a lantern job

The county does not have one kind of roof, and the roof decides the job.

Post war and new town flat roofs across Harlow and Basildon are the natural home of the lantern, though many carry original felt build-ups with shallow or reversed falls that want sorting first. Victorian brick terraces in Chelmsford, Colchester and Maldon usually have a rear addition with a small flat roof, often at first floor, where a lantern lights a bathroom or a landing and where access is the awkward part. The interwar semis of the commuter belt tend to arrive with a modern rear extension already built, so the work is retrofit into a roof somebody else detailed. The weatherboarded villages around the Blackwater and Stock, and the timber framed centres of Thaxted, Coggeshall and Saffron Walden, bring visibility and consent questions rather than structural ones. Bungalows on Canvey sit on reclaimed land where movement history is worth asking about before a rigid frame is fixed across an opening. Local specifics are collected on the areas covered page.

Getting the lantern onto the roof

A lantern is delivered as a frame kit or as a factory assembled frame, plus glass in a crate. The glass is the awkward half. A single sloping pane on a three metre lantern can weigh over sixty kilograms and it is large, flexible and expensive.

Ground floor rear extensions reached through a garden or a side gate are straightforward with enough hands and a glass sucker. A first floor roof over a rear addition, or a garden with no side access where everything goes through the house, is where the method statement gets interesting: a scaffold with a loading bay, a glass manipulator, or on tight urban plots a lorry mounted crane over the roof of the house. That decision is made at survey, because it moves the price and the programme more than most people expect, and because a crane needs a road position and sometimes a permit from the highway authority.

The installation sequence, day by day

A straightforward retrofit lantern into a sound existing flat roof runs two to three days. One with a structural opening and a steel runs three to five, spread across two visits where the calculation drives a longer lead time.

The order is consistent. Protect the room below and strip the covering back beyond the opening. Open the deck and set out the opening. Trim the joists or install the beam, prop as required. Build the kerb, insulate it and check it for square and level with a diagonal measurement. Bring the covering up the kerb and terminate it. Land the eaves beam, fix it down, build up the bars and ridge. Glaze from the bottom bay upward. Fit the flashings and trims, then make the covering good across the field. Internally, board and set out the reveal, then plaster. At no point is the roof left open at the end of a working day, whatever the forecast says: the opening is decked and sheeted, or the covering is made watertight, before the van leaves.

Building Regulations, and the certificate at the end

Fitting a lantern into a roof is notifiable work under Building Regulations in England. Part L applies because a new glazed element changes the thermal performance of the envelope, Part A applies wherever the roof structure is altered, Part K applies where there is a risk of falling through, and Part B comes in where the opening interacts with an escape route.

The route is either a full plans or building notice application to the local authority, or an approved inspector, with inspections at the structural stage and at completion. Sunspire handles the notification as part of the job and hands over the completion certificate at the end. That certificate matters more than it feels like at the time. Unnotified structural alteration surfaces years later during a sale, when a buyer’s solicitor asks for paperwork that nobody generated and the answer holds up a chain.

Permitted development for a lantern on a house

Most lanterns on most houses fall under permitted development and need no planning application. The conditions are specific and they are worth reading rather than assuming. The glazing must not project more than one hundred and fifty millimetres beyond the plane of the existing roof slope, and no part of it may be higher than the highest part of the existing roof.

The first condition is the one that traps lanterns, because a lantern is by definition a raised structure. On a flat roof the relevant plane and the rule’s application depend on how the roof and the extension are read together, and on an extension that is itself permitted development the lantern is normally assessed as part of it. Rights are also commonly removed: by an Article 4 direction, by a planning condition on newer estates around Beaulieu Park and South Woodham Ferrers, or because the property is a flat rather than a house. The check takes a phone call to the local planning authority and is done before anything is ordered.

Conservation areas, and lanterns behind older Essex houses

Essex carries a high concentration of protected fabric for an English county, and conservation area boundaries cover the older cores of most market towns in it. A lantern on a rear extension, hidden from the street behind the main roof, is usually uncontroversial. One visible from a public vantage point is a design conversation.

Where the answer is that a raised structure will not sit comfortably, a flush unit in a slim dark frame reads more quietly, and that route is covered under conservation rooflight installation. On a listed building, listed building consent is a separate consent from planning permission and is needed for alterations affecting character, including work at the back. Neither route is a closed door. Both want establishing before the frame is drawn, because a lantern designed twice costs twice.

What moves the price of a lantern installation

Sunspire quotes in two stages: a range by email once the basics are known, then a fixed written figure after a survey has been on the roof. What moves the number between those two points is predictable, and knowing it lets you steer the design.

Size drives it, but in steps rather than smoothly, because past about two and a half metres in either direction the frame section changes and the panes become specials. Glass specification drives it, since solar control and laminated inner panes cost more than a basic double glazed make-up. Structure drives it hardest: a steel brings an engineer’s fee, a fabrication lead time and a Building Control inspection. Access drives it, and a crane or a loading bay scaffold is a line of its own. The condition of the existing covering drives it, because a lantern going into a roof that needs recovering is really two jobs. Indicative ranges by job type sit on the costs page.

Reading a lantern quotation line by line

A quote worth comparing against another quote is specific enough that you could order from it. It names the manufacturer and model, the external frame size, the pitch, the frame material and finish including the RAL reference inside and out, and the glass make-up with its U-value and g-value stated separately.

It then says who is doing what. Who forms the opening. Who builds and insulates the kerb. Who supplies and dresses the covering up it. Whether the structural calculation and the engineer’s fee are included. Whether the Building Control notification and certificate are included. Whether internal plastering and the reveal are in or out. What happens to the waste, and what access equipment is allowed for. A single line reading “supply and fit roof lantern” against one number is not a quotation. It is an invitation to a conversation on site, and that conversation is rarely held in your favour.

What the ten-year workmanship guarantee is covering

Two guarantees run alongside each other and they cover different things. The manufacturer warrants the frame and the sealed units, typically ten years or more on the glass against seal failure and longer on a powder coated finish. Sunspire’s ten year workmanship guarantee covers the installation: the kerb, the fixing down, the bedding, the covering termination and the flashings.

That second guarantee exists because of a simple fact about this trade. The parts of a lantern job that decide whether it stays dry are the parts nobody can see once it is finished. Upstand height. The fall of the roof around it. The laps of the flashings. Where the covering terminates. All four are inexpensive to get right on the day and awkward to correct afterwards, so the guarantee is placed exactly where the risk sits rather than on the brand of glass.

Living with a lantern, year to year

There is little to do, and that is the point of specifying it properly at the start. The glass wants washing once or twice a year, and a self cleaning outer coating reduces that rather than removing it: the coating breaks down organic dirt in daylight and relies on rain to rinse it, which works better on a steeper pitch than a shallow one.

The useful habit is autumn. Clear leaves and moss from the roof around the kerb, and check the outlet is running. Most water trouble around lanterns starts as a drainage path that quietly closed itself, and ten minutes with a gloved hand in October saves a long February. Check the vent runs through its full travel once a season, because an actuator that is never used stiffens. If misting appears between the panes rather than on the room face, that unit has reached the end of its life and the glass can be changed on its own, which is the same logic set out under skylight replacement.

Getting from an idea to a fixed price

The fastest start is three photographs and two measurements. A photograph of the flat roof from an upstairs window, one of the ceiling below from inside the room, and one of the outside of the house showing the extension against the main building. Then the room dimensions and, if you know it, the covering on the roof. That is normally enough for a sensible range by email.

If you are earlier than that, start with the roof rather than the product. What is the covering, which way does the fall run, what is under the deck, and which way does the room face. Those four answers narrow the field faster than any showroom visit, and they are the first four questions the survey will ask. If it turns out the roof is pitched rather than flat, the conversation moves to roof window installation instead, and if the extension is a side return with a terrace above, to walk-on glass. More background reading sits in the guides.