How Long Does a Roof Lantern Take to Fit?

A lantern does not arrive and go straight onto the roof. The roof is prepared in a sequence, and each step depends on the one before it.

1. Opening. On a new lantern, the ceiling and roof structure are opened to the size on the survey sheet. Joists around the opening are trimmed and supported with new timbers.
2. Kerb. A timber or manufactured kerb is built around the opening, to the height and fall the lantern's maker requires.
3. Covering. The flat roof covering is dressed up and over the kerb, so that water cannot get behind it.
4. Lantern. The unit is lifted into place, fixed to the kerb and checked for level and square.
5. Finishing inside. The reveals are lined, plastered or taped and decorated to match the room.

Only the fourth step involves the lantern at all. The first three are roof work, and they take most of the time. The lantern then needs checking for square before it is fixed, since a unit that sits twisted on the kerb will not close properly along its vents.

Where a lantern is coming out and a new one is going back, the opening and the kerb often exist already. The old unit is lifted off, the kerb is checked, and the new lantern is set on it. If the kerb is the right size and sound, the job is largely the lift and the fixing.

If the kerb is the wrong size, too low or damaged, it is rebuilt before the new lantern goes on, which brings back the second and third steps of the sequence. The survey sheet records the kerb dimensions so that this is known before an order is placed.

A new lantern in a roof that has never had one is the longer job, because the structural opening is made first. That is a structural alteration, and it needs Building Regulations approval. The roof lantern installation page describes what is involved in each case.

Lanterns vary from compact units that two people can carry through the house to large glazed assemblies that need lifting equipment. The size, glass weight and frame material (aluminium or timber) decide which.

Large lanterns are sometimes delivered in sections and assembled on the roof. Others arrive complete. Access shapes this: a terrace with no rear access means everything goes through the house, and a lantern that cannot pass through a hall must be lifted over.

The access arrangement is settled at the survey, and the weight of the lantern is on the maker's data sheet. A crane or a hoist, if one is needed, is booked for a specific time, so the timetable has to line up with it. The page on roof lantern sizes shows the range.

Some of the elapsed time passes without anyone being on site.

• Manufacture. Lanterns are made to the survey sheet, so there is a gap between the order and delivery. The maker quotes it when the order is placed.
• Weather. A flat roof covering needs dry conditions to bond around the kerb. We plan the start for a dry, calm stretch and agree a new date if the forecast turns.
• Building Control. For a new opening, an inspector may want to see the structure before it is covered, and that visit has to be booked.
• Drying. Plaster inside needs time before it is painted.

The fit is also affected by what else is happening. If the lantern is part of an extension being built, it follows the builders' sequence. For other rooflight types, see the general timing answer.

A plain fixed lantern is the shortest version. Several choices lengthen the job, usually by adding a step that has to be finished before the next can start.

• Opening vents. A lantern with electric vents needs a power supply to the roof or ceiling, and wiring has to be run and tested before the finishing inside.
• Integrated blinds. Blinds are fitted at the end, once the lining is done, and may need their own power.
• A deeper light well. A thick roof build-up gives a deeper well below the lantern, which means more lining and more plaster to dry.
• Two lanterns on one roof. Each is its own opening and kerb, though they can share a scaffold or a lift.

Because these are chosen before the order, they show up in the quote's timetable and not as a surprise on site.

The written quote states the number of days on site for your lantern, the stages that fall in each, and any waiting built in for the order, an inspection or the weather. It is based on the survey readings, so a kerb that proved larger than expected or a covering that needed more work shows up there before you agree to anything.

If something that could not be seen at the survey appears once the roof is opened, such as a rotten joist end or a covering that will not take a new dressing, the remedy and its effect on the timetable are agreed with you before work carries on. Where the survey could see a problem in advance, it is listed on the survey sheet.