Roof Lantern Installation

A roof lantern is the one part of a kitchen extension people notice the moment they walk in, and the one part that shows up badly when it is specified without thought. Get it right and a single-storey rear extension stops feeling like a corridor with a table in it. Get it wrong and you have built a glasshouse over your hob, on the south-facing side of an estuary city, in July. This page sets out how structural glazed lanterns get installed on extensions across Southend-on-Sea and south Essex: the upstand they sit on, the glass that goes into them, what the regulations ask for, what the work costs, and the questions that separate a considered quote from a cheap one.

01What a roof lantern installation actually involves

A roof lantern is a raised, pitched glazed structure that sits on top of a flat roof. It has a ridge, glazing bars running down from that ridge to an eaves beam, and a perimeter frame that lands on a built-up kerb known as the upstand. That pitched shape is what separates it from a flat rooflight, which lies almost level with the roof deck and reads as a window in the ceiling rather than a piece of architecture. If you want the flatter, quieter option instead, that is covered on our flat roof skylight page.

Modern lanterns are almost all thermally broken aluminium. The bars are slim, structural and powder-coated, and the glass carries a good part of the load rather than sitting passively in a timber cage the way Victorian conservatory roofs did. That is why a lantern spanning three metres can be built with bars 50mm wide instead of 100mm. Less frame, more sky.

There are four separate things in play on any lantern job, and confusion between them is where most problems start.

• The structural opening. The hole in the flat roof deck, trimmed with joists or steel so the surrounding roof still carries its loads.
• The upstand. The insulated timber kerb built up around that opening, dressed with the roof covering, which lifts the glazing clear of standing water.
• The lantern frame. The aluminium system itself, made to millimetre dimensions taken off the finished upstand.
• The glazing units. The sealed panes, which are where the thermal and solar performance is decided.

On a new extension, the builder usually forms the opening and the upstand as part of the shell, and we arrive once the roof covering is on and watertight. On a retrofit, where a lantern is going into an existing flat roof that has been in place for years, the work is bigger: the deck has to be opened, the joists trimmed, the upstand built, and the roof covering cut back and re-dressed around it. Both are installations, and both end with a room that behaves completely differently.

The sequence matters more than people expect. A lantern cannot be ordered accurately until the upstand exists, because it is made to the measured size of that upstand rather than to the drawing. Order off the drawing and you find out on the day that the kerb is 14mm out across the diagonal and the frame will not sit square. So the honest order of events is: form the opening, build the upstand, weather it, survey it, then order. Manufacture usually runs three to five weeks for a standard aluminium lantern and longer for a bespoke colour or an unusual shape, which is worth building into the programme rather than discovering late.

One more thing that gets skipped in sales conversations. A lantern changes a room’s acoustics as well as its light. Two hundred kilograms of glass over a hard-floored kitchen makes it livelier, and rain on a lantern is audible in a way rain on a plasterboard ceiling is not. Most people like it. A few do not, and laminated inner panes take a noticeable edge off the sound while doing useful safety work at the same time.

02Which way the roof faces, and why it changes everything

Southend-on-Sea faces south across the Thames Estuary. For an English seaside settlement that is unusual, and it is the single most useful fact about glazing anything here. A house on the north side of a street running parallel to the seafront has its garden, and therefore its rear extension, pointing straight at the southern sky with the water beyond and very little in the way of mature tree cover to soften it.

Now add the geometry of a near-horizontal piece of glass. At this latitude the sun climbs to roughly 62 degrees above the horizon at midday in late June, and drops to around 15 degrees at midday in late December. A vertical window catches a low winter sun almost head-on and takes a high summer sun at a glancing angle. Glazing tilted towards the horizontal does exactly the opposite. It presents its full face to the June sun and shrugs off the December one. A roof lantern at 20 degrees of pitch is, thermally speaking, a solar collector aimed at the worst possible time of year.

This is why we survey orientation before we talk about products. On a site visit the questions are simple and the answers change the specification completely:

• Which compass direction does the extension roof face, and what is the pitch of the lantern going on it?
• What shades it, if anything, between eleven in the morning and five in the afternoon in July? A neighbouring two-storey wall on the west side is worth a great deal.
• Is the room used mostly in the morning or the evening? West-facing glazing hurts most at the end of the day, when a kitchen is actually being cooked in.
• What is the floor? A polished concrete or tiled floor in direct sun stores heat and gives it back all evening. A timber floor does not.
• How much other glazing is in the same room? A lantern above three metres of bifold doors is a very different heat budget from a lantern above a solid wall.

Orientation also decides where you want the light rather than just how much. A north-facing lantern gives you flat, even, blue-tinged daylight that stays consistent from morning to evening, which is why artists have always wanted north light. A south-facing one gives you movement: bright pools of light that travel across the worktop through the day, strong contrast, and glare on a screen if someone works at the kitchen table. Neither is wrong. They are different rooms, and the glass specification is what lets you choose between them rather than accept whichever one the roof happens to hand you.

There is a winter side to this too, and it is genuinely positive. Overhead glazing is the most efficient daylight opening there is, roughly two to three times as effective as the same area of vertical window at getting light deep into a plan. In a south Essex January, when useful daylight lasts about eight hours, a lantern over the middle of a kitchen means the lights stay off through the afternoon. The job is to keep that benefit and lose the July penalty, and that is a glass problem, not a frame problem.

03The glass specification: U-value for winter, G-value for summer

Two numbers describe a glazing unit, and most quotes only mention one of them.

U-value is the rate of heat loss through the unit, in watts per square metre per degree of temperature difference, written W/m²K. Lower is better. It governs how much warmth leaks out on a cold night and whether the inner pane is cold enough to draw condensation. This is the number every installer quotes, because it is the one Building Regulations set a limit on.

G-value is the proportion of the sun’s energy that passes through the glass and ends up as heat in the room. It runs from 0 to 1, and lower means less heat gets in. Clear double glazing sits around 0.6, meaning roughly three fifths of the solar energy hitting it comes through. Good solar-control glass sits around 0.3. Almost nobody volunteers this figure unless you ask for it.

Put numbers on it. On a clear July afternoon, a near-horizontal surface in south Essex can be taking something in the region of 700 watts per square metre. A modest three-square-metre lantern in clear double glazing, at a G-value of 0.6, is therefore letting in roughly 1,260 watts of heat. That is more than a one-bar electric heater running full tilt above your head, with no thermostat and no off switch. Drop the G-value to 0.3 and you are at about 630 watts. Same room, same light levels to the eye, half the heat.

Here is how the common build-ups compare. These are typical industry figures rather than a specific product range, and the exact numbers vary by manufacturer, cavity width and gas fill.

The trade-off is honest and worth stating plainly: solar-control coatings cut light transmission along with heat, and they carry a faint tint. A neutral coating reads as a very slight grey when you look up through it, more noticeable at the edges than in the middle, and invisible within a week of living with it. Older-generation solar glass had a distinctly blue or bronze cast. Current neutral coatings do not, and if a supplier is offering something that visibly colours the sky, ask what generation of coating it is.

Self-cleaning glass deserves a word because it is oversold. The coating is photocatalytic: ultraviolet light breaks down organic dirt and rain then sheets off rather than beading, carrying the residue with it. It works, but it needs rain and it needs pitch. On a lantern at 20 degrees or more it keeps the glass respectable for years. On a very shallow pitch, or on a sheltered elevation, it does less. It is a reduction in cleaning, not an end to it, and it costs less than one visit from a company with the right ladders. There is more on the whole performance question on our energy-efficient skylight page.

One specification detail that is not optional: the inner pane of overhead glazing should be laminated. Toughened glass, if it ever breaks, breaks into thousands of blunt cubes, which is fine at waist height and not fine directly above a dining table. Laminated glass holds those pieces on an interlayer. Part K of the Building Regulations covers safety glazing, and any competent lantern specification will have laminated on the inside and toughened on the outside as standard.

04The upstand: the kerb that decides whether it works

Most lantern problems trace back to the upstand rather than to the lantern sitting on it. The upstand is unglamorous, it is buried under the roof covering, and it is the single most important piece of the assembly. It is also the part a homeowner has least visibility of, because by the time the glass arrives the kerb is already dressed and hidden.

The upstand is a built-up kerb around the structural opening, usually treated softwood, that raises the base of the glazing above the plane of the flat roof. It exists for three reasons. It keeps the frame clear of standing water, and flat roofs always hold some water somewhere. It gives the roof membrane a vertical surface to be dressed up and terminated against. And it lifts the glass above the insulation and roof build-up so the frame is not sitting in a cold, wet detail.

Height

The usable rule is 150mm minimum from the finished roof surface to the top of the upstand, measured at the highest point of the roof falls, and more is better on a roof with shallow falls or a parapet that could pond. Lantern manufacturers commonly quote 150mm as their requirement, and single-ply and EPDM membrane manufacturers want a similar dimension for a sound termination. Where somebody has built at 75mm to keep the lantern looking low from the garden, they have created a detail that will be tested every time it rains hard from the wrong direction.

Construction

A typical upstand is 100mm by 50mm or 150mm by 50mm treated timber, built off the trimmers around the opening, insulated on the outside face or within the frame, and sheathed with exterior-grade plywood ready for the roof covering. The insulation matters. An uninsulated timber kerb is a cold bridge running right around the perimeter of your lantern, and the reward for skipping it is a line of condensation on the plaster reveal every cold morning, which then gets blamed on the glass.

Squareness and level

Aluminium lanterns are manufactured to the millimetre. The upstand needs to be square within about 3mm across the diagonals and level within a similar tolerance, or the frame lands on a twist and the glazing gaskets never compress evenly. This is why we measure the built upstand rather than working from the architect’s dimensions, and why we would rather see the kerb before it is dressed than argue about it afterwards.

Weathering

The roof covering, whether that is EPDM, single-ply, GRP or felt, is dressed up the full height of the upstand and terminated under the lantern’s eaves. The lantern then sits on the top face, bedded on a continuous run of low-modulus silicone, and is mechanically secured down through the frame into the timber at the manufacturer’s stated centres. External cloaking trims cover the joint. Done properly, water reaching the base of the glass runs into the eaves gutter profile, out through weep holes and onto the roof covering, never touching the timber at all.

If a builder is forming the upstand as part of an extension, we are happy to issue the dimensions and details before they start, which costs nothing and saves the awkward conversation later. If there is no builder involved, the upstand is part of what we do.

05Sizing, spans and what the roof can carry

The instinct on a new extension is to make the lantern as big as the roof allows. It is usually the wrong instinct, for reasons that are practical rather than aesthetic.

Leave a meaningful margin of flat roof around the lantern. Three to four hundred millimetres on every side gives the roof covering somewhere to run, gives the falls somewhere to go, and gives anyone who ever has to work up there a place to stand. A lantern crowded to within 100mm of a parapet is a detail nobody can maintain and water cannot easily leave.

Proportion is the other half of it. A lantern that reads well is usually somewhere between a quarter and a half of the flat roof area, with its ridge running along the longer dimension of the opening. Run the ridge the short way and you end up with steep, stubby glazing bars and a shape that looks like a mistake from the garden. On a long, narrow extension, two or three smaller lanterns in a row often look better and perform better than one enormous one, because each has a shorter span and the roof between them keeps its structural continuity.

Weight

Glass is heavy and people consistently underestimate it. A sealed double-glazed unit with a toughened outer and a laminated inner runs around 25 to 30 kilograms per square metre. Triple glazing pushes towards 40. So a lantern measuring 2 metres by 3 metres carries roughly 150 to 180 kilograms of glass alone, before the aluminium. That load has to be carried by the eaves beam, transferred into the upstand, and then into trimmed structure around the opening.

Structure

Cutting a hole in a flat roof interrupts the joists. Those interrupted joists have to be supported by trimmers running perpendicular, and the trimmers have to carry both the roof loads they inherit and the lantern sitting on top. For a modest opening in a domestic flat roof, doubled timber trimmers usually handle it. For openings much over 2.5 metres in the direction across the joists, or where the roof is also carrying a parapet or a green roof build-up, this becomes a job for a structural engineer’s calculation rather than a rule of thumb. On a new extension the engineer has usually already sized it. On a retrofit into an existing roof, get it checked, because nobody wants to discover the answer through deflection.

Pitch

Most lanterns are built between 15 and 25 degrees. Below about 10 degrees, water sits and dirt stays; the glass looks tired within a season and self-cleaning coatings never get to work. Above about 30 degrees, the lantern starts to read as a conservatory roof and the height becomes noticeable from the garden and from neighbouring upper windows. Twenty degrees is a good default and looks right on almost every 1930s and post-war rear extension in this area.

Shape

Rectangular hipped lanterns cover the vast majority of jobs. Where the extension is an unusual shape, or the client wants a ridge that runs off-centre, a pyramid over a square kitchen, or a lantern that meets an existing wall on one side, that moves into bespoke territory and is worth reading our custom skylight design page on. Bespoke geometry is entirely doable. It just needs the survey done properly and the lead time respected.

06Planning permission and Building Control in south Essex

Two separate systems apply, and people routinely assume that clearing one clears the other. It does not.

Planning permission

Rooflights and lanterns on a house are generally permitted development, which means no planning application, provided they project no more than 150 millimetres beyond the plane of the existing roof slope and do not sit higher than the highest part of the roof. On a flat-roofed rear extension this needs care, because a lantern on a 150mm upstand plus a 300mm pitched frame plainly projects more than 150mm above the flat roof. In practice, lanterns on rear extensions are normally assessed as part of the extension itself rather than as a separate rooflight, and the extension’s own permitted development limits are what govern. Where the extension already has permission or falls within permitted development, the lantern usually rides along with it. Treat that as the general position rather than a guarantee. It costs nothing to put the question to the council in writing before the glass is ordered, and a written answer is worth more than an assumption when the house is sold years later.

The exceptions are the part worth reading twice:

• Flats and maisonettes have no permitted development rights at all. Any rooflight needs an application.
• Listed buildings need listed building consent regardless of size or position.
• Conservation areas restrict what can be done on roof slopes facing a highway, and local policy varies.
• Article 4 Directions withdraw permitted development rights entirely for the things they name. In this area, the Leigh Cliff and Leigh conservation areas carry an Article 4 Direction covering changes to roofing materials and windows, so work that would be permitted anywhere else needs a planning application there. Both conservation area appraisals were updated in March 2022.
• Clifftown, the Victorian estate behind the cliffs, and Leigh Old Town, with its fishing cottages and a core running from the thirteenth to the eighteenth century, are both conservation areas where roof alterations get looked at closely.

None of that is a reason not to do the work. It is a reason to ask the question early. The planning authority for the city is Southend-on-Sea City Council, and neighbouring jobs fall to Rochford, Castle Point, Basildon or Thurrock depending on where the boundary runs. Always confirm the position with the relevant authority for your address rather than relying on what a neighbour did in 2015.

Building Regulations

This one is not optional and it is not conditional. A new or enlarged rooflight is notifiable under the Building Regulations, every time. Two parts of the Approved Documents do the work:

• Part L covers thermal performance. New rooflights in an extension have to meet a limiting U-value, and any lantern from a serious manufacturer with a decent glazing specification will beat that comfortably. This is also where the case for solar control gets made, because overheating is assessed alongside heat loss.
• Part K covers safety glazing and protection from falling. In practice that means laminated inner panes overhead, and consideration of how the glass will ever be cleaned or replaced safely.

We make the Building Control notification to Southend-on-Sea City Council, or to whichever authority covers the address, on your behalf. You get the completion paperwork at the end, which is what a solicitor will ask for when the house is eventually sold. It is a small piece of administration that causes a disproportionate amount of trouble when it has been skipped, and it is not something we would ever leave with a customer to sort out later.

07Installation day, start to finish

A lantern going onto a prepared, weathered, square upstand is a one-day job for most domestic sizes. If the upstand has to be built, or the flat roof deck opened and trimmed, allow two to three days. Here is what the day actually looks like.

Before we arrive

The survey has already happened, the upstand has been measured and photographed, and the lantern has been manufactured to those measurements. Delivery is usually the morning of installation or the day before, and it needs somewhere flat, dry and out of the way. The glazing units arrive on edge in a stillage and stay that way until they go in.

Morning

Protection first. Dust sheets over worktops and floors below, and the opening covered from inside if the room is finished. Then the upstand is checked again: diagonals, levels, and the condition of the membrane termination. If the kerb is out of tolerance this is the moment it gets packed and corrected, not after the frame is down.

The eaves frame goes on first, dry-assembled and offered up to confirm it lands correctly on all four sides. Once we are satisfied, it is lifted off, a continuous bead of low-modulus silicone is run around the top of the upstand, and the frame is set down into it and mechanically secured through into the timber at the stated centres. Corners are sealed as they are assembled. The ridge and glazing bars then build up off the eaves frame.

Glazing

Units go in from the outside, usually bottom bay first, working towards the ridge. Anything much over about 60 kilograms needs two people and vacuum lifters, and on larger jobs three. Wind is the limiting factor rather than rain: a two-square-metre sealed unit is a sail, and above roughly 20 miles per hour it stops being sensible to lift one over a roof. That is the most common reason a lantern gets moved to the following day, and it is the right call every time.

Each unit sits on setting blocks, is compressed against the gaskets, and is then held by the pressure plates and capping profiles that clip over the bars. The capping is what gives a modern lantern its clean line, and it is also what keeps water out of the bar joints.

Afternoon

Ridge cap on, external cloaking trims around the base to cover the junction with the roof covering, and all the weep paths checked so water leaving the glazing rebates has somewhere to go. Any opening vent is wired or its manual operator adjusted and tested. If the lantern is motorised, that ties into the wider question of controls covered on our electric skylight page.

Internally, the reveal is squared and prepared. Plastering and decoration are usually a follow-on trade rather than part of the lantern installation, and it is worth agreeing who is doing that before the day rather than after.

Handover

We hose the lantern down and check it from inside while the water is running, which is a far better test than waiting for weather. Then it is a clean-up, a walk through the operation of any vents, and the paperwork: the manufacturer’s glazing warranty, our 10-year workmanship guarantee, and the Building Control notification we have already lodged on your behalf. Over fifteen years of installing skylights and rooflights along this stretch of the estuary means we have seen most of the ways a flat roof can surprise you, and the survey is where we would rather find them.

08What a roof lantern costs in south Essex

Prices vary with size, glass specification, frame colour, access and how much of the substrate work is included. The ranges below are general market guidance for aluminium lanterns supplied and installed onto a prepared upstand, not a quotation. A survey is what turns a range into a number.

What moves the number within those ranges:

• Glass specification. Moving from clear double to a good solar-control unit typically adds a few hundred pounds on a mid-size lantern. Against a room that becomes unusable on hot afternoons, that is the cheapest part of the whole job.
• Frame colour. Anthracite grey and white are usually stock. A bespoke colour, or a different colour inside and out, adds cost and lead time.
• Opening vents. A manual opener adds modestly; a motorised vent with a rain sensor and a remote adds more, plus an electrician.
• Access. A lantern that can be reached off a garden with a tower is straightforward. One over a rear extension only reachable through the house, or one needing scaffolding, costs more in time before anyone touches the glass.
• Size of individual panes. Cost per square metre climbs once units get big enough to need mechanical lifting equipment.

Two things are worth spending on and one is not. Spend on the glass, because you cannot change it later without taking the lantern apart. Spend on the upstand, because it is buried and it determines whether the assembly works. Do not spend on decorative Georgian-style bars unless you genuinely want them, because they cost money and they take away the thing you paid for, which is sky. There is a broader breakdown across all our work on the costs page.

09Choosing an installer, and the questions worth asking

Roof lantern quotes are difficult to compare because they are rarely written to the same scope. One includes the upstand, one assumes it exists. One quotes clear glass, one quotes solar control. One includes the Building Control notification, one leaves it with you and you find out at the point of sale. Asking the same questions of everybody is how you get comparable answers.

Ten questions to put to anyone quoting

1. What is the G-value of the glass you have quoted? If the answer is a shrug, or only a U-value comes back, the solar side has not been considered. On a south-facing extension in this city, that is the number that decides whether you enjoy the room in August.
2. What is the U-value, and is that the centre-pane figure or the whole-unit figure? Centre-pane numbers always look better. The whole-unit number including the frame is the honest one.
3. Is the inner pane laminated? It should be, for overhead glazing.
4. Does your price include building the upstand, or does it assume one is already there, square and weathered? This is the single biggest cause of quotes that are not comparable.
5. Who makes the Building Control notification, and when do I get the completion certificate?
6. Will you measure the finished upstand before ordering, or order off the drawing? The right answer is measure.
7. What is the lead time from survey to installation? Three to five weeks is normal for standard aluminium. Anything much shorter usually means stock sizes rather than made to measure.
8. What guarantee covers the workmanship, as distinct from the manufacturer’s product warranty? They are two different things and both should be stated.
9. How will this be cleaned in five years? If the answer requires scaffolding every time, that changes the case for a self-cleaning coating.
10. Have you looked at which way this roof faces? If nobody has stood in the garden with a compass, the specification is a guess.

What a good quote contains

A quotation you can actually rely on names the lantern system and manufacturer, gives the finished external upstand dimensions it has been priced against, states the glazing build-up with both performance figures, says explicitly what substrate work is and is not included, sets out the vent arrangement, states who notifies Building Control, and gives a realistic lead time. If it is one line and a price, you are being asked to trust rather than to compare.

Warning signs

Be careful with a quote that has not involved a site visit, because a lantern is made to measure and the measurements come off a physical kerb. Be careful with a price that seems to ignore the upstand entirely. Be careful with pressure to sign quickly for a discount that expires, which is a double-glazing sales habit that has no place on a structural glazing job. And be careful with anyone who describes solar control as unnecessary on a south-facing extension here, because they have either not thought about it or they do not want to price it.

One honest boundary on our side. We install and replace skylights, rooflights and lanterns. If your existing lantern is sound and the problem is a leaking flat roof around it, a roofer is the right person to call, not us. If the unit itself has failed, misted, or is a single-glazed relic losing heat all winter, taking it out and installing a properly specified replacement is exactly our work, and it is usually a one-day job onto the existing upstand if that upstand is sound.

We cover Southend-on-Sea and the surrounding area, from the seafront and the 1930s semi belt through to Leigh, Rochford, Shoeburyness and further into south Essex. Send us the details of your extension and we will come and look at the roof. Phone 01702 898232, or use the quote form and tell us which way the extension faces. That is the first thing we will ask anyway.

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