A roof lantern is not fitted in one continuous push. The work moves through fixed hold points, and a crew will not cross one until the previous stage is finished and the conditions are right for the next. On a straight replacement with a sound kerb, the roof opens on Monday morning and is watertight again by Monday afternoon, with the lantern itself set and glazed on Tuesday. On a new opening with structural work, the same hold points apply, but spread across three to four days rather than two. What sets the pace is not labour. It is weather, cure times, and what the strip-back actually finds underneath the old covering.
This page goes through that sequence in the detail a homeowner planning around it actually needs: what happens each hour, where the job can stall, and how the opening is kept watertight if the lantern itself does not go on the same day it is meant to. For the full ten-stage overview and the published price table, see the main roof lantern installation page.
The programme, hour by hour
The table below sets out what typically happens on each day, for the two job types that cover almost every domestic lantern we fit. A straight replacement assumes the existing kerb is sound enough to reuse or repair rather than rebuild from scratch. A new opening assumes the structural work, whether trimmers or a ring beam, has already been designed by an engineer before the scaffold goes up.
| Stage | Straight replacement, sound kerb | New opening, structural work |
|---|---|---|
| Day 1, morning | Tower or scaffold up. Old lantern lifted off. Kerb exposed and inspected for rot and level. | Scaffold up. Existing covering stripped back to the deck. Opening marked to the engineer’s dimensions. |
| Day 1, midday | Kerb repaired, or rebuilt where the inspection finds rot, an undersized frame or missing insulation. | Deck cut. Trimmers or ring beam fixed to the engineer’s detail. Building Control informed if a site inspection is required. |
| Day 1, afternoon | Covering dressed up and over the kerb, laps mechanically terminated. Roof is watertight. | Insulated kerb built to a minimum 150mm above the finished roof at its lowest point, 200mm on exposed coastal sites. |
| Day 2 | Lantern frame set, squared and fixed. Glazing units lifted, gasketed and capped. Reveals started if plastering is in scope. | Covering dressed and terminated over the new kerb. Roof is watertight at the end of this day, whatever else remains outstanding. |
| Day 3 | Not required on a straight replacement. | Lantern frame set, squared, fixed and glazed, wind permitting. |
| Day 4 | Not required. | Capping, internal reveals, plaster where in scope, hose test and handover. |
Two things about that table matter more than the day count. First, the roof is watertight before the lantern frame goes anywhere near it. Second, the sequence never depends on a delivery arriving mid-job. The lantern is made to the surveyed opening and lead time is typically three to five weeks, so nothing starts on site until the unit is physically in the yard. That protects the programme from the single most common cause of a stalled job: a scaffold up and a hole cut, with the glazed unit still six weeks from the factory.
The weather-critical hold points
Three points in the sequence are governed by conditions on the day, not by the crew’s schedule. Get these wrong and the roof either leaks in a year or the glazing goes on unsafely. We do not skip them to keep a programme on track.
Membrane bonding temperature
Cold-applied adhesives used to bond EPDM and single-ply membranes to a kerb have a minimum application temperature, typically around 5°C at the substrate, below which the bond will not cure properly and can fail invisibly. A membrane that looks stuck but was applied on a frosty November morning is a leak waiting for its first heavy rain. On cold mornings we wait for the surface temperature to lift rather than force the bond, and on genuinely cold spells we sometimes shift a job by a day or two rather than compromise the one detail that does all the waterproofing work.
Adhesive and sealant cure before load
Once a membrane is bonded, it needs time to cure before anything is walked on it or loaded against it. Rushing this stage to get the kerb “finished” the same afternoon risks disturbing the bond before it has set, which shows up later as a soft blister or a lifted lap. We sequence the work so the membrane has had its cure window before the lantern frame is bedded onto it.
Wind and glazing
A large sealed unit behaves like a sail once it is off the ground. A 3.0 x 2.0m glazing pane on a lifter in gusty wind is a genuine safety risk to the crew and to the glass itself, and it is the one stage we will stop for without hesitation. In practice that means glazing does not go ahead in strong, gusty conditions, roughly a fresh breeze on the Beaufort scale and above. If the forecast turns on a glazing day, the kerb and frame work still goes ahead and the glazing is moved to the next suitable window, which is why the day count in the table above is typical rather than guaranteed.
The rule we do not bend. A hold point exists because skipping it produces a failure that shows up months later, not on the day. Every hold point above has cost us a day on site at some point. None of them has cost a client a leak.
How the opening stays watertight if the lantern is not on the same day
This is the detail most homeowners actually want to know, and it is worth being specific about it. A structural opening or a kerb rebuild is never left as bare deck overnight, and it is never left relying on a tarpaulin held down with a few bricks.
At the end of any day where the lantern itself has not yet gone on, the sequence is the same. The insulated kerb is fully built. The permanent roof covering is dressed up and over the top edge and mechanically terminated, exactly as it would be on the finished job, not as a temporary measure. If the covering itself cannot be completed that day, for example because a membrane cure window has not passed, a breathable temporary membrane is fixed across the opening and battened down at every edge against wind uplift, checked by the crew before anyone leaves site. Either way, the roof sheds water on its own that evening. Nothing about the following morning’s forecast can catch us out, because the watertightness does not depend on the weather staying dry overnight.
On new openings with structural work, this is also the point where any bad news in the timber gets photographed and shown to you before it is built over. If the joists around the opening are wetter or more decayed than the survey suggested, that is a conversation we have that day, not a surprise on the invoice.
Where the programme can stall
Most lantern jobs run to the programme in the table above. The ones that do not, stall for a short list of reasons, and it is worth knowing them before you book time off work around the job.
- Rot found in the kerb. A replacement job assumes the existing kerb is sound. If the strip-back finds the timber has been wet for years, the job becomes a rebuild rather than a repair, which typically adds a day.
- The forecast on a glazing day. Covered above. It moves the glazing stage, not the whole job.
- A Building Control site visit. On a new opening, the local authority sometimes wants to inspect the structural work before it is covered over. We are not in a competent-person scheme, so this route through building notice or full plans applies to every retrofit, and the inspection slot is booked around the programme rather than the other way round.
- An engineer’s revision. If what the crew finds on site does not match the drawings, for example a joist that turns out to be smaller than assumed, the engineer needs to confirm the detail still works before the trimmers go in.
- Access. A blocked driveway, an unmoved car, or a locked side gate on the morning the scaffold is due can push the whole start back by half a day.
None of these are common. Most straight replacements run exactly to the two-day sequence. But a homeowner who understands why a job might run to three days, rather than assuming the crew is simply slow, has a much easier week.
What you need to have ready before we arrive
A short list, and most of it takes ten minutes.
- Clear access for a scaffold tower or full scaffold, including any gate or side passage it needs to come through.
- A parking suspension arranged in advance if the street requires one for a scaffold vehicle or skip.
- The room below cleared of anything that cannot get a light covering of dust, and breakables moved off any surface directly under the opening.
- Colour and glazing specification confirmed and signed off before the order goes in. This is fixed once ordered, because the lantern is made to that specification and changing it after manufacture starts is not possible within the lead time.
- A nearby power supply identified if you are having an electric opening vent fitted, so the electrician’s first fix can happen alongside the structural stages rather than afterwards.
- Someone available on the final day for the hose test and walkthrough.
The hose test and handover
Every installation ends the same way, regardless of how many days it took to get there. We hose test the completed lantern, running sustained water against the junctions most likely to show a fault first: the corners of the kerb, the ridge, and the glazing gaskets. We check from inside for any sign of ingress while the water is still running, not afterwards. Doing this before the scaffold comes down means any fault can be fixed immediately, without remobilising a crew for a second visit.
Handover includes a written record of the glazing specification and its U-value, the guarantee documentation, and a walkthrough of the actual detail rather than just the finished appearance from below. Every installation carries a 10-year workmanship guarantee. If anything about the fitting shows a fault within that period, it is put right.
For what all of this costs by job size, see our separate breakdown on roof lantern installation costs in Suffolk. If you are still deciding between a lantern and a flat rooflight before any of this becomes relevant, our comparison of roof lantern versus flat rooflight covers that decision. And if your kerb needs building from scratch rather than repairing, how a roof lantern upstand is built goes into that stage in more depth than this page does.
What happens if it rains halfway through the job?
Structural and kerb work can generally continue under light rain once the deck is protected, but membrane bonding stops, because a wet substrate will not take an adhesive properly. If the forecast is heavy rain on a day scheduled for weathering in, we sequence around it rather than bond a membrane that will fail. The opening is always left in a watertight state at the end of each day, whatever the weather does that night.
Can the roof be left open overnight in an emergency?
No. Every job we run finishes each day with the opening either permanently or temporarily weathered in and mechanically fixed down, never resting on a loose tarpaulin. If conditions genuinely prevent finishing a stage, the crew stays later or the following day’s work is rearranged rather than leaving the roof exposed.
How soon after the kerb is watertight does the lantern go on?
Usually the next working stage, often the same afternoon on a straight replacement or the following morning on a new opening, once any required cure time has passed. The lantern frame is not bedded onto a membrane that has not had time to set, which is why cure time occasionally adds a few hours to the programme.
Do you need a structural engineer’s sign-off before the glazing goes in?
For any opening involving a ring beam or a retrofit into an existing roof, yes. The trimmers or ring beam are built to the engineer’s calculation, and where Building Control requires a site inspection of the structural work, that happens before the covering closes it over. We work to the engineer’s design. We do not design structure ourselves.
If you want a fixed programme and a fixed price for your own roof before any of these stages are booked in, request a fixed quote and we will survey the roof, confirm which of the two programmes above applies, and tell you honestly if anything about your structure is likely to add a day.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.