How long does a flat rooflight installation take?

Four things move the number more than anything else, and they stack on top of each other rather than trading off.

• Is the opening already there. Swapping a failed unit for a new one, where the timber opening and the upstand are sound, is the quickest work on this list. The old unit comes out, the surround is checked and made good, the new unit goes in and gets sealed and flashed. Cutting a fresh opening into a deck that has never had a rooflight in it is a different job, because you are into joists, structure and a new upstand before any glass appears.
• The size and type of unit. A 600 by 900 mm rooflight is a two-person lift. A three or four metre lantern arrives in sections, needs setting out dead square, and every joint between the bars has to be built and sealed on the roof. More glass means more sealing, and sealing is the part you cannot rush.
• The kerb or upstand. Flat rooflights sit on a raised kerb so that water drains away from the glass rather than pooling against it. If the roof already has a sound kerb the unit lands straight on it. If one has to be built, framed, insulated and made watertight, that is often half a day on its own before the rooflight is anywhere near the roof.
• The roof covering. A single-ply membrane or a felt roof has to be cut back, dressed up the new kerb and re-terminated so the whole detail is continuous. On a warm-deck build-up that has to be done in the right order, insulation and vapour control included, and doing it properly takes longer than dropping a unit onto bare boards ever would.

None of these is a reason to hurry. A rooflight that leaks in year two costs far more time than the hour saved by skipping a detail on the day.

Take the common case: one flat rooflight, whether it opens or not, going into a rear extension where the opening either exists or is being cut into a straightforward timber deck. Here is roughly how that day runs.

1. Set out and protect, first half hour. The room below is sheeted, the position is marked from inside and out, and access onto the roof is set up. On a single-storey extension that is a step-up; on anything higher it means a tower or scaffold, which adds time at both ends of the job.
2. Form or check the opening, one to two hours. For a new opening the deck is cut, any joist that is in the way is trimmed and the timbers around the hole are doubled up so nothing is left unsupported. For a replacement this is where the old unit and any tired timber come out and the surround gets sorted.
3. Build the kerb, one to two hours. The upstand is framed to the right height, insulated to keep the Part L build-up continuous, and the roof covering is dressed up and over it so water is always shed away from the glass.
4. Set the unit and seal, one to two hours. The rooflight is bedded down, secured, and the covering is terminated against it. This is the slow, careful part. Every edge is sealed and the flashing is finished so the detail sheds water in every direction.
5. Finish inside, half an hour to an hour. The reveal is lined and trimmed so the opening looks square and deliberate from the room, and the site is cleared.

Add it up and a clean single unit lands in the four-to-six-hour range. That assumes dry weather, good access and no surprises in the roof structure once it is opened up.

Not every flat rooflight job is going to be a day, and it helps to know which before you plan your week around it. A large lantern is the obvious one: sections to assemble, a bigger opening to frame, and a lot more sealed joint per unit. A run of two, three or four rooflights across a wide extension multiplies the setting-out and the sealing even where each individual unit is small.

Structure adds days too. If the new opening cuts across load-bearing joists, or the span means a beam has to carry the trimmed timbers, that is a structural detail to design and build before the roof is watertight again. And if the existing roof covering is at the end of its life, it makes sense to sort the deck and the build-up properly while it is open rather than seal a new unit into a covering that will need lifting again soon. That is a covering job for a roofer, not part of setting the rooflight: replacing a tired unit is our work, while recovering a worn-out roof is theirs, and we set the new rooflight into whatever they leave us.

The install day is often the short part. Ahead of it sits the lead time on the unit itself, which is where the real waiting usually is. A stock-size rooflight can be a short order; a made-to-measure lantern or a specific glass specification is built to order and takes longer to arrive, sometimes several weeks. Getting the glass right at that stage is worth the wait, because the day the unit lands is the day the specification is settled for the next couple of decades.

New or enlarged rooflights are notifiable under Building Regulations, with Part L covering the thermal build-up and Part K the safety glazing. That notification runs in the background and does not add to your day on site, but it is part of the timeline from order to sign-off. Weather is the other variable nobody controls. Sealing a flat roof detail wants a dry window, so a settled forecast is worth waiting a day or two for rather than opening a roof up under cloud that is about to break.

People sometimes assume that asking for better glass adds days to the job. It does not add a single hour on the roof. A flat rooflight glazed with solar-control glass installs in exactly the same time as a clear one, because the work is the opening, the kerb and the seal, not the pane. The only place the specification shows up in the timeline is the lead time on the unit, and that is a wait before the crew arrives, not extra time once they are there.

It is worth getting right on a flat roof in particular. A flat rooflight points at the sky rather than at a compass direction, so it collects the high summer sun almost square on and behaves like a south-facing opening whatever way the house is turned. Along the estuary, where the southern horizon is open and light bounces off the water, a clear unit over a kitchen extension can turn a bright room into a hot one by mid-afternoon. Ordering the glass with a low G-value at the start costs you nothing in install time and saves you retrofitting shade later. That thinking sits at the centre of our flat roof skylight installation work, and it runs right through how we approach solar-control glazing generally.

If you want a firm timescale rather than a range, the two things that pin it down are the state of the roof where the unit is going and the size and type of unit you want. Tell us whether the opening exists or has to be cut, roughly how big the rooflight is, and whether the roof is single-storey or higher, and we can tell you whether you are looking at a day, two, or more. Our installation costs page sets the price against the same variables.

Ring 01702 898232 or request a quote, and we will look at the whole roof and the way it faces before we put a day in the diary, rather than just the hole you want cut in it.