An older roof carries its history. Tiles or slates have been patched, ridges rebedded, and a chimney stack may have been taken down and capped. None of that stops a rooflight going in, but it changes the approach. Before a quote is written, the covering around the planned opening gets a close look to see whether it is sound enough to be cut into and flashed against. If a handful of slates or tiles have gone brittle, it makes sense to swap those for matching ones within the same visit, so the new unit is bedded into good roof rather than a tired patch.
The slope you choose matters. A rear slope tends to be hidden from passers-by and is often kinder to the look of the house. The front slope can catch more sun on some plots, but it is the one a neighbour, or the council, is more likely to care about. The slope that serves the spot best is chosen first, then tested against the planning position in the next chapter.
Newer roofs in the village are a different job. A modern concrete-tiled roof on a recent house is usually a larger plain surface with a regular rafter layout, and a roof window made for that pitch fits neatly between the rafters with a ready-made flashing kit. For a single straightforward unit the work on the roof usually takes one or two days, and it suits a loft bedroom or a landing in a newer house just as well.
Whatever the roof, an opening needs sound structure around it. Cutting a rafter without carrying its load round the hole weakens the roof, so the opening is framed with timber trimmers and a header, and those are insulated so the ceiling beside the opening does not become a cold strip.