Short answer
Early, and before the roof structure is closed in. A rooflight in a new extension should be sized and ordered while the builder is still setting out the joists, because the opening is framed with the roof and the unit takes weeks to arrive. The kerb goes up with the carpentry, the membrane is laid around it, and the glass is fitted last. Done in that order, the opening is right first time and nothing has to be cut back later.
Size the opening before the joists go up
The best moment to talk about a rooflight in a new extension is when the drawings are still being turned into a frame. Joists are cut to length and laid at set centres, and the opening for a rooflight is simply a gap in that layout with a doubled joist either side and a header across the ends. Framed with the roof, that costs the carpenter a few minutes.
Added afterwards, the same opening means cutting through a finished deck, trimming joists that are already carrying load and, often, taking up a covering that has just been laid. So the sizes matter early. A rooflight for a new extension is ordered to a structural opening, and the structural opening is set by the unit, not the other way round.
Kerb construction with the carpentry
A flat rooflight sits on a kerb, a timber upstand built around the opening and fixed to the trimmers. In a new build the carpenter can make it as part of the roof, with the height set to the roof insulation and the covering that is going on. The right height is the one that keeps the finished upstand clear of the roof surface when everything is laid.
The kerb also carries the fall. Manufacturers specify a minimum pitch for flat glass, and a kerb built at the framing stage can give it without any extra work. If you are weighing options for the roof, flat rooflight sizes helps set the opening, and flat rooflight upstands shows the finished detail.
Membrane order and when the glass goes on
Once the kerb is built, the roof covering goes on. The membrane is laid across the deck, taken up the outside of the kerb and dressed over the top edge. That is a job for the roofer, and it is best done once, with the kerb in place, so the upstand is continuous and has no joins cut in later.
The glass should come last. A rooflight is fragile on a building site, with trades moving ladders and plasterboard past it. Keeping the unit in its packaging until the roof is clear and the scaffold is coming down protects it. A temporary board over the kerb keeps the opening closed in the meantime, so the fitting day is a short visit. A skylight for a single storey extension is usually one of the lightest tasks left on the programme at that point, because the structure, kerb and covering are all done.
- Joists framed with the opening, doubled trimmers either side
- Kerb built and fixed by the carpenter
- Membrane laid up and over the kerb by the roofer
- Glass unit fitted once the roof is clear of trades
Order lead time against the build programme
Rooflights are made to order. Standard sizes may come from stock, but anything built to your opening is made after the order is placed, and the wait is measured in weeks. The simplest way to avoid holding up the build is to place the order when the opening is fixed, not when the roof is nearly done. Our page on skylight lead times explains what drives the wait.
On a typical extension programme, the roof structure goes up early, the roof covering follows, and glazing and plastering come after. The rooflight order needs to land between those points. The expected delivery goes in the written plan, beside the date the builder needs the kerb finished, so the two schedules line up.
| Build stage | Rooflight task |
|---|---|
| Drawings and setting out | Survey, size and unit agreed |
| Roof structure | Opening framed, order placed |
| Roof covering | Kerb built, membrane dressed up it |
| Closing in | Unit delivered and stored |
| Final roof visit | Glass fitted, finishing checked |
Who notifies Building Control for what
A new extension is notifiable building work in its own right, and your builder or architect usually deals with the notice for the whole project. A rooflight that is part of the original design is covered by that notice, because it appears on the plans and the inspector looks at the roof as one piece. Whichever route applies, the unit itself is held to the Part L 2021 limit for rooflights, which Table 4.2 puts at 2.2 W/m²K.
If the rooflight is added after the extension has been signed off, it is a separate piece of work, and we handle the notification for it where it is notifiable. The job plan states which of the two applies to your job, so there is no doubt about whose paperwork it is. The reasoning behind both routes sits in the rooflight Building Regulations guide.
Working with your builder
Most of the friction on a roof comes from two trades working to different information. A short call at the start of the build avoids it. We give your builder the opening dimensions, the kerb height and the delivery date in writing, and the builder tells us when the roof will be ready. The rooflight in a new extension then becomes one more line on the programme, not a surprise.
The free survey and written quote cover this at the drawing stage, with a 10-year workmanship guarantee on our part of the work. For the unit choices themselves, the overview of flat roof rooflight installation is the next stop, and the question of how many to fit is taken up in how many rooflights an extension needs.
If your builder is already on site, a short conversation early on saves the most time. Share the unit size and kerb height with them before the roof carpentry starts, and ask who will supply the temporary cover for the opening. Agreeing those two points in writing means the carpenter, the roofer and the glazing team all work from the same figures.