How long does a roof lantern take to fit?

Days on the roof, weeks in the diary

It helps to separate the two numbers, because installers quote whichever one flatters them.

Those stages do not simply add up, because some overlap and the plastering waits on a plasterer’s availability rather than on us. But if somebody tells you a lantern is a two-day job, they are describing one stage of six.

The kerb is a job before the lantern is a job

The single biggest difference between a lantern and a flush rooflight, in time as well as cost, is that a lantern needs a structure built for it first.

The kerb is carpentry: timber built off the deck or the joists, squared and levelled to within a few millimetres because the frame has to sit flat on it, insulated so the reveal is not a cold surface, then weathered so the covering runs up it and terminates properly. On a simple roof that is a day. Where the falls have to be corrected either side so water goes round the kerb rather than sitting against it, it is two.

It also has to be right rather than fast. A kerb top that is out of level by 5mm over two metres will show as an uneven gasket line for the life of the lantern, and there is no way to correct it once the frame is down.

Fitting the frame, then glazing it

The frame goes together on the kerb: eaves beams first, then the ridge, then the rafter and hip bars, squared and fixed as it goes. On a lantern up to about three metres that is a morning for two people.

Glazing follows. Each unit is lifted in, seated on its setting blocks, and the caps and gaskets fitted over it. This is the part that is weather-dependent and the part where pane weight decides the pace. Panes that two people can carry go in steadily. Panes that need a manipulator or a crane go in on the day the lift is booked, which pins the whole programme to a date that cannot easily move.

What a made-to-order frame does to the date

Standard sizes from a maker’s range can arrive in around two weeks. Anything made to your surveyed opening, which is most lanterns worth having, runs three to six weeks, and the glass is usually the long pole rather than the aluminium.

Two specification choices lengthen it noticeably. A non-standard powder coat finish adds time because the sections go out to be coated and come back. Solar control or acoustic laminate glass in a size that is not held in stock adds time at the processor. Neither is a reason to avoid them, but if you have a date in mind, decide the finish and the glass early rather than after the survey. Frame finishes and powder coating covers the finish options.

Nothing is ordered until the opening has been measured on site. A lantern ordered from a drawing arrives at a size the roof does not have, and the delay that causes is measured in weeks, not days.

Cutting the opening in an existing roof adds a stage

Fitting into an opening that already exists, on a replacement or a new extension built to receive the lantern, is the quick version. Cutting a new hole in a roof that has been there for thirty years is not.

The extra time goes on: tracing and diverting whatever runs in the ceiling void, which in a kitchen is downlights, extract ducting and often pipework; cutting the joists and installing trimmers or a beam; and dealing with whatever the deck turns out to be once it is open. If the boarding is soft, that has to be resolved before a kerb goes on it, and that is a variation nobody wants but everybody would rather have than a kerb built on rot.

Where an engineer’s beam is needed, add the design and the Building Control involvement to the front of the programme. That is typically another one to two weeks and it happens in parallel with the frame being made, so it rarely extends the overall date if it is identified at survey.

Making good inside is its own week

Once the lantern is watertight the roof team is finished, and the room is not.

The reveal has to be lined, boarded and plastered, splayed or square, and the junction with the existing ceiling made without a visible line. Plaster then needs to dry before it is painted, which is days rather than hours in a kitchen with the extract running. Where a beam has gone in, there is boxing and fire protection to do as well.

We build this into the programme rather than leaving it as the customer’s problem, and we say plainly which days the room is unusable. In a working kitchen that is usually the two days the opening is formed and the day the plaster goes on.

What the weather can and cannot delay

Rain does not stop a lantern installation in the way people expect. Building a kerb, forming an opening under a temporary cover, plastering inside: all of these proceed in ordinary British weather.

What genuinely stops is glazing in high wind. A large sealed unit being lifted onto a roof in a gust is dangerous, and we will stand a lift down rather than run it. That can move a day, occasionally two. Cold matters for some adhesives and membranes, which have minimum application temperatures, so a hard frost can move a weathering stage. Winter installations are entirely normal here and we cover the practicalities on fitting rooflights in winter.

The commitment that does not move is that the roof is never left open overnight. If the opening cannot be formed and closed the same day, it is not cut that day.

Two dates worth asking any installer for

Ask for the delivery date of the frame and glass, and ask for the number of working days on site after delivery. Those two numbers together tell you when your kitchen is back, and an installer who will not give both is guessing at one of them.

Ask also what happens if the frame arrives wrong. The honest answer is that it goes back and the process restarts, which is exactly why the survey happens before the order and not after. Our timings for the wider range of work are on how long a rooflight installation takes.

Fifteen years of this in Oxford, Building Control notified by us, a range by email and a fixed price after the survey, and ten years on the workmanship.