Roof Lantern Installation
A glazed lantern is the most structural thing in the rooflight family, and the one where the measurements go wrong most often. Surveyed against the opening you have, not the one the brochure assumes.
A roof lantern is a glazed pitched structure sitting on a kerb above a flat roof. It is not a bigger rooflight. It carries its own weight, it carries snow and wind load, and it transfers all of that into a kerb and then into whatever is holding the kerb up. Everything that goes wrong with lanterns goes wrong at one of those junctions rather than in the glass.
That is why this page spends more time on the opening than on the unit. Choosing the lantern is the easy part.
What decides the size
Three things, in this order.
The structural opening. On a rear extension the lantern usually sits inside a flat roof formed between two walls, and the opening is bounded by trimmers on all four sides. Those trimmers have to carry the roof and the lantern. On a wider span, or where the opening eats most of the roof, that becomes a steel rather than timber, and the steel has to be designed before the glass is ordered because it changes the kerb position.
The room below. A lantern lights the area under it and about a metre past its edge in each direction. Beyond that the light falls away quickly. In a long kitchen-diner, two smaller lanterns down the length of the room give more usable light than one large one in the middle, and they usually cost less to support.
The roof it has to leave behind. A lantern needs a working margin of flat roof around the kerb for the covering to be dressed up and terminated. Squeezing the lantern out to the walls to gain glass leaves nowhere to detail the waterproofing, and that is the single most common cause of a lantern that leaks from the day it is fitted.
| Decision | The working guide | Why it matters |
|---|---|---|
| Margin of roof around the kerb | A clear, obviously deliberate margin on every side | Too little and it reads as a mistake rather than a choice |
| Plan shape | Rectangular generally sits better than square | A square lantern on a rectangular room fights the room |
| Pitch | Enough to shed water and dirt without dominating the roofline | Too shallow and it holds dirt; too steep and it is visible from the street |
| Position over the room | Centred on what the room is for, not on the roof | Over an island or a table it lights the thing you use. Centred on the roof it lights the floor |
| Number of units | Two smaller ones often beat one large one | Smaller units need less structure and give a more even spread |
| Glass | Solar control on south and west | Overhead glass takes more solar energy than any other glass in the building |
The kerb, and who builds it
The kerb is the upstand the lantern bolts to. It raises the frame above the water line so the roof covering can be dressed up its face and terminated under the lantern’s flange.
On most jobs the kerb is built by whoever builds the flat roof, and on a great many jobs that is not the same trade supplying the lantern. It is worth being direct about this at quotation stage, because a kerb built to the wrong height, out of square, or out of level is the defect that is most expensive to discover late. A made-to-size lantern will not sit on a kerb that is 20mm out of square, and the lantern cannot be adjusted.
Where we supply and fit, the kerb is measured and checked before anything is ordered. Where the kerb already exists, it is surveyed before we quote, and if it is not right we say so before you buy a unit that will not fit it.
Pitch
Twenty-five degrees comes up constantly, and it is a reasonable default rather than a rule. It is steep enough that rain carries dirt off rather than leaving it, shallow enough that the ridge does not dominate the roofline from the garden, and it keeps the overall height inside what permitted development usually allows on a rear extension.
Shallower than about fifteen degrees and self-cleaning glass stops working properly, because the coating relies on rainwater sheeting rather than pooling. Steeper than about thirty and the lantern starts to read as a conservatory roof, which is usually not the look anyone came for.
Glass
A lantern faces almost straight up, which is the hardest position glass gets asked to work in. It sees the most sun in summer and loses the most heat in winter, and both of those are worse than they would be in a wall.
Specification that works in this county: a low emissivity coating on the inner pane to hold heat in, a solar control coating on the outer to keep summer gain down, argon fill, and a warm edge spacer so condensation does not form first at the perimeter. Toughened outer, laminated inner is standard for anything overhead, and it is a safety requirement rather than an upgrade.
Solar control is the specification most often left out to save money and most often regretted. A south-facing lantern over a kitchen with no solar control makes the room unusable on a hot afternoon, and there is nothing to be done about it afterwards short of replacing the glass.
Frame and sightlines
Sightline is the width of aluminium you look at rather than through. Slimmer bars mean more glass and a lighter structure to look at, and they cost more because the aluminium has to do the same structural job with less section.
Powder coat colour is worth a moment on this coast. Anthracite grey is the default and looks right on most modern extensions. On the salt side, at Mersea, Brightlingsea and out toward Clacton, the coating specification matters more than the colour: a marine grade finish is the difference between a frame that still looks new in ten years and one that chalks in four.
Ventilation
An opening vent at the ridge is the most effective ventilation you can put in a room, because it sits at the highest point and lets the warmest air out by stack effect. It can be manual with a pole, or electric with a rain sensor. On a lantern above reach, electric is the only version anyone actually uses.
What the installation involves
- Survey. The opening, the kerb, the roof build up, the access and the fall are measured. Nothing is ordered before this.
- Fixed written price. Itemised: unit, labour, scaffold where needed, Building Control fee where notifiable.
- Lead time. Made-to-size lanterns are manufactured to order. Typically three to five weeks.
- Fit. Most single lanterns are a one to two day fit once the kerb is ready. The roof is left weathertight at the end of every day.
- Making good. The internal reveal is finished, and splayed where the depth allows it, because a splayed reveal throws noticeably more light into the room than a square one.
Where this sits in Colchester
Most lantern work here is on rear extensions in the post-war and inter-war suburbs, and on Victorian rear additions closer to the town centre. Inside the conservation areas, and through Dedham, Nayland and the Colne Valley villages, a lantern is often not the right answer at all: a flush conservation rooflight usually is, and the conservation officer will generally say so before you have finished the sentence. That conversation is worth having before a survey rather than after.
What a lantern costs, and what moves it
The unit is usually the smaller half of the figure, which surprises people. Five things move the number, in roughly descending order.
Whether the opening exists. Fitting into a sound existing kerb is the cheapest version of this job by a wide margin. Forming a new opening in a finished roof means opening the covering, altering structure, building a kerb, dressing the covering back and finishing the ceiling below. That is a week of work in an occupied house rather than a day on a building site.
Structure. A wide opening usually needs a steel, designed by an engineer, supplied, installed and bearing onto padstones. On a large lantern the steel frequently costs more than the lantern.
Access. A single storey flat roof reached from a tower adds nothing. Anything higher, or over a conservatory or a fragile roof, needs scaffold, and scaffold is a real line rather than a rounding error.
Glass. Solar control on anything facing south or west is worth the money. Triple glazing usually is not.
Size, but less than you expect. Doubling the glass area does not double the price, because a large part of the cost is labour, access and structure rather than glass.
Two smaller against one large
Always worth pricing both. Two lanterns often light a long room better, and two modest openings may need no steel where one wide opening certainly would. That single difference can make the two-unit option both better and cheaper, and it is not obvious until somebody prices it.
How the job runs
- Indicative range by email. Free, usually same day, from a description and photographs of the roof and the room.
- Survey. Free. The opening, the roof build up, the fall, the structure, the access and what sits in the ceiling below.
- Structural design where a steel is involved, because the steel position sets the kerb position and the kerb sets the lantern size.
- Fixed written price, itemised, including Building Control where notifiable.
- Kerb built and measured. Only then is the lantern ordered.
- Manufacture, typically three to five weeks for a made to size unit.
- Fit, usually one to two days once the kerb is ready.
- Making good, reveal formed and splayed where the depth allows.
That order exists because every step in it can move the one before. Ordering a made to size lantern from a drawing rather than from a measured kerb is the single most expensive mistake available on this job, and it is not recoverable: the unit cannot be trimmed.
Living with one
Cleaning
The pitch sheds most dirt with rain, which is one of a lantern’s real advantages over a flat rooflight. What it does not shed is bird mess, tree sap and leaf staining. On a single storey extension a water fed pole from the ground reaches most lanterns, and twice a year is plenty for most. Self cleaning glass is worth having where the lantern is genuinely hard to reach.
The inside is the awkward face. Dust settles on the inner surface of sloped glass, and cooking in a kitchen adds a fine greasy film that dust sticks to. An extendable microfibre pole once or twice a year is the practical answer.
Do not walk on it. A lantern is not a walk-on unit and the panes are not rated for it.
Noise in rain
Audible, and most people find it pleasant. Against a polycarbonate conservatory roof a glass lantern is dramatically quieter; against the insulated roof it replaced it is louder, because the whole point is that there is nothing between you and the sky.
A laminated inner pane damps it, and on a properly specified lantern you have that anyway for safety. Asymmetric glass, where the two panes are different thicknesses, cuts it noticeably more and costs very little. Over a bedroom it is worth specifying from the start, because it cannot be changed later without changing the glass.
Condensation
A lantern over a kitchen sits in the wettest air in the house at the highest point of the room, so it will show condensation on a cold morning if the detailing is poor. A warm edge spacer, an insulated kerb and a vapour control layer sealed to the frame are what prevent it, and all three are decided during construction rather than afterwards.
What goes wrong, and why
Four causes account for nearly every lantern we are called back to, and only one of them is the lantern.
The kerb was too low. The covering has nowhere to terminate under the flange, so water gets behind it. The most common cause we find.
There was no roof margin. The lantern was pushed out towards the walls to gain glass, leaving nowhere to detail the waterproofing. The detail becomes impossible rather than merely difficult.
Water ponds against the upstand. Inadequate fall, or a position where water arrives slowly.
It is not a leak at all. Condensation from an uninsulated kerb, which appears in cold dry weather and is fixed by detailing rather than by a new lantern.
Where we are called to an existing lantern, we establish which of the four before quoting a remedy. Replacing a sound lantern to fix a kerb problem is money spent on the wrong thing, and the problem will still be there.
From quote to daylight, with no surprises
Fixed quote
A clear, fixed written price, online or by phone.
Survey
A specialist confirms the right glazing spec. Flexible times.
Install
Clean, weather-tight fitting that protects your property.
Guarantee
We tidy up and back the work with a written guarantee.
Frequently asked questions
How much does a roof lantern cost in Colchester?
The unit is usually the smaller half of the figure. The variables that move it most are the size, whether the opening needs steel, whether scaffold is required for access, and the glass specification. A fixed written price is given after survey and it does not move afterwards.
Do I need planning permission for a roof lantern?
On a rear extension it is usually permitted development, provided the lantern does not project more than 150mm above the existing roof plane and the extension itself is within limits. In a conservation area, on a listed building, or where an Article 4 direction applies, that changes. We check before quoting rather than after.
Does a roof lantern need a steel beam?
Sometimes. It depends on the span of the opening and what is carrying the roof around it. On a modest opening in a new flat roof, timber trimmers are usually enough. On a wide opening, or where the lantern takes up most of the roof, a steel is likely and it needs designing before the lantern is ordered because it sets the kerb position.
Will a roof lantern make the kitchen too hot?
Without solar control glass, a south-facing lantern over a kitchen will overheat on a summer afternoon. With it specified properly, it does not. This is the single specification most often dropped to save money and the one most often regretted, because the only fix afterwards is new glass.
How long does it take to fit?
Most single lanterns are a one to two day fit once the kerb is ready and square. The lead time before that is the manufacturing, typically three to five weeks for a made-to-size unit.
Helpful reading on this
Costs, comparisons and answers to the questions we’re asked most.
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