Wilford's history records modern brick-built houses replacing old thatched cottages, and that change sets the scene for the roofs we meet today. Brick houses mean tiled or slate roofs on timber frames, with rafters at predictable spacing, and that makes a roof window straightforward to fit if the position works.
Later development added the Silverdale housing estate in the late 1950s, on land from the former Wilwell Farm at the south-western edge of Wilford. Houses of that period usually have tiled roofs of moderate pitch and a loft space that was designed for storage, with trussed rafters or traditional cut roofs. In a trussed roof the webs between the rafters limit where an opening can go. A cut roof is more forgiving. Which of the two you have becomes clear once someone is up in the loft, and the position is only promised after that.
The pitch of the roof decides the headroom in the loft, and the headroom decides whether a bedroom is possible at all. A steeply pitched roof gives more usable floor under the slope. A shallow one gives less. Where headroom is short, a window placed lower in the slope or a dormer-style approach may be considered, though that falls outside what a simple roof window does.
The covering decides the flashing. Each of clay, concrete and slate wants a kit of its own, and where a roof has been patched the material around the opening may be replaced locally, so the new window is set in healthy roof. All of that is written into the quote beforehand.