Building the kerb a roof lantern sits on

Who builds it

Usually whoever builds the flat roof, which on an extension is the main contractor or the roofer. The lantern supplier then arrives to fit a made to size unit onto whatever has been built.

That division of responsibility is where jobs go wrong. A lantern manufactured to a stated size will not sit on a kerb that is 20mm out of square, and the lantern cannot be adjusted. At that point somebody has to rebuild a kerb or reorder a lantern, and the argument about which is nobody’s idea of a good day.

Where we supply and fit, we measure and check the kerb before ordering. Where the kerb already exists, we survey it before quoting, and if it is not right we say so before you have bought a unit that will not fit it.

Square, level, and the right size

Three requirements, all obvious and all regularly missed.

Square. Measure both diagonals; they must match. A parallelogram kerb will not take a rectangular lantern.

Level along the top. The roof beneath falls, but the top of the kerb is level, because the lantern sits on it. That means the kerb is deeper on the downslope side. Building a kerb of constant height on a falling roof produces a lantern that sits at an angle.

The right size. To the manufacturer’s stated dimension, which may be an internal or an external kerb dimension depending on the system. Getting that wrong by one wall thickness is a common and expensive error.

Height

The working figure is around 150mm above the finished roof surface at the lowest point, measured from the finished surface rather than the deck. Too low and the covering has nowhere to terminate. Too high and the lantern sits up like a chimney.

Insulation

On a warm roof, the insulation sits above the deck, and the kerb interrupts it. Unless the outer face of the kerb is insulated, it becomes a cold bridge running the entire perimeter of the lantern, and it will lose more heat than the glass does.

It is cheap, it is invisible once complete, and it is skipped constantly.

Vapour control

The vapour control layer under the insulation has to be carried up the inside of the kerb and sealed to the lantern frame. If it is not, warm moist air from the kitchen enters the roof build up at the perimeter, and the resulting condensation is invariably diagnosed as a leaking lantern.

More on the product is on the roof lantern installation page.

The part that decides whether a lantern lasts

The glass on a roof lantern is a manufactured component with a known performance and a long life. The kerb is built on site, it is invisible once the covering is dressed over it, and it is where nearly every problem with a lantern originates.

It also does more than a flat rooflight kerb does, because it carries the structure of the lantern as well as weathering the opening.

Setting the height from the finished level

At least 150mm above the finished roof surface, and there is a good argument for more on a roof with marginal falls.

Every flat roof holds some water. A heavy shower, a blocked outlet or a slight ponding area raises that level considerably, and a perimeter seal designed to shed water is not designed to be submerged.

On a green roof the upstand has to clear the growing medium as well, so on a sedum roof with 80mm of substrate you are looking at 230mm or more, plus a vegetation free margin around it.

Square, level and true

More demanding than on a flat rooflight, because a lantern is a rigid framed structure manufactured to your dimensions.

A kerb that is out of square by a few millimetres will not take the frame evenly. The lantern will still go on, and it will sit stressed, with the seal loaded unevenly from day one and the glazing bars slightly out of true.

That is not a snagging item you can adjust out afterwards. It is why the exact opening dimensions come from the survey after the structural design is settled, and why the lantern is manufactured to those dimensions rather than to a drawing.

Fall on the deck around it

The deck must fall away from the kerb, not toward it.

A roof with an adequate overall fall can still pond against a lantern if the kerb interrupts the drainage path and creates a dead area behind it. Where a kerb sits across the direction of fall, the detail needs to let water get around it rather than back up against it.

Where the existing roof has no fall, correcting it locally around the kerb using tapered insulation or firrings is part of the job rather than an optional extra. On an older roof that correction may extend beyond the immediate area of the opening.

Look at the roof after rain before anybody quotes. Where the water goes, and where it does not, tells you most of what you need.

Insulation on the outer face

The item that appears on no quotation and the one that decides how the room feels.

A lantern kerb has a long perimeter. An uninsulated timber kerb punches straight through the roof insulation around all of it, presenting a substantial area of poorly performing envelope at a U-value several times worse than the glazing above.

Rigid insulation on the outer face, continuous around all four sides, taken up to meet the underside of the lantern frame and lapped properly into the main roof insulation so there is no gap at the base.

It is a sheet of board and an hour, it disappears under the covering, and that is precisely why it gets skipped. Ask directly whether it is included; it is the fastest way to tell who has detailed the job.

The vapour control layer

Insulation deals with conduction. The vapour control layer deals with air movement, and air leakage around an opening loses more heat than conduction through a small area of glass.

On a warm roof, where the insulation sits above the deck, the vapour control layer sits below it and has to be carried up the kerb and sealed to the lantern frame. On a cold roof it sits at ceiling level and is sealed around the opening.

Either way it has to be continuous. A cut edge left loose is an air path, and it carries moisture into the construction where it condenses out of sight and eventually shows as wet insulation or decayed timber.

Dressing the covering

The roof membrane is dressed up the full height of the kerb and terminated properly, before the lantern sits on it.

That sequence matters. Where the covering was dressed after the lantern was set, or where it stops short of the top, water gets behind it. It is invisible from above once finished, which is why it survives inspection and fails in year three.

The termination detail varies with the covering: felt, single ply, liquid applied and GRP each have their own, and compatibility between the membrane and the lantern materials is worth confirming rather than assuming.

The structural job the kerb is doing

A lantern is heavy and everything it weighs passes through the kerb into the structure below.

On a new extension the opening is designed in and the joists either side are doubled up. On a retrofit, cutting joists means a designed trimmer and beyond a certain span steel, calculated by a structural engineer rather than judged on site.

Which way the joists run relative to the opening changes that more than the size of the lantern does. An opening set within the span rather than cutting across it frequently avoids steel entirely, and on a rectangular lantern that sometimes means simply rotating the design.

Deflection matters as much as strength. A structure that sags slightly is fine for a roof and not fine for a rigid glazed frame sitting on it.

What it should look like when it is finished

Nothing. That is the point.

From outside, a covering dressed neatly up to the base of the lantern frame with no visible upstand detail beyond a clean termination. From inside, a plastered reveal and a lantern that sits square.

Every decision described above is invisible, which is why they need naming on a quotation rather than assuming.

The lines to ask for

Kerb height. Whether the deck falls away from it and whether any correction to the falls is included. Whether the outer face is insulated. Whether the vapour control layer is being sealed to the frame. And the structural design, with confirmation of whether an engineer’s fee and any steel are in the price or excluded.

Five lines. They account for most of the difference between two lantern quotations, and none of them is about the glass.

Why we will not price a lantern without a survey

Because the fall on the existing roof, the direction of the joists and the state of the covering are the three things that decide the cost, and none of them is visible in a photograph taken from the ground.

An indicative range by email costs nothing and tells you whether the project is viable. The fixed price comes after somebody has been on the roof.

Three requirements, all of which have to be met

Square and level. The lantern is made to the finished kerb dimensions and the tolerance is small. Measure both diagonals and check the top edge with a level along each side.

A lantern on an out of square kerb does not sit down evenly. The gap gets taken up in sealant, and sealant carrying a structural gap is a temporary arrangement from the day it is built.

High enough. Enough to clear any water the roof can hold, including with a blocked outlet, and on a ballasted or green roof enough to clear the finished surface by a real margin.

Measure the required height from the finished level, not from the deck. That is how kerbs end up too low.

Insulated. On the outer face, continuous with the roof insulation, running up to meet the underside of the lantern frame with no break.

Without it the internal face runs cold, sits at the top of a warm humid room, and produces condensation and staining every winter. It is the most common defect on lantern installations and it gets reported as a leak.

The construction

Usually a timber box built up from the deck, fixed to the trimmers around the opening, with insulation on the outer face and the waterproofing dressed up it.

Insulated kerb systems are available as manufactured units, arriving square with the insulation built in. On a job where the kerb is being built anyway the cost difference is modest, and the risk of the insulation being forgotten disappears.

Sealing the vapour layer at the kerb

Carried up the inside face of the kerb and taped to the lantern frame, continuously.

Where it is not, warm moist air from the room enters the construction and condenses inside it, wetting timber where nobody can see. The symptom is damp in the ceiling around the opening in cold weather rather than after rain.

The sequence

Structural design and steel. Opening formed. Kerb built to the steel, square, level and insulated. Waterproofing dressed up the kerb and terminated. Survey to the finished kerb. Then order the lantern.

Ordering from a drawing before the kerb exists produces a unit that is a few millimetres out, discovered on the day, with weeks of lead time to correct.