Can a small roof leak cause serious damage?
Yes. A small leak can repeatedly wet insulation, plasterboard and roof timbers, encourage mould and eventually contribute to decay. The visible stain may be much smaller than the area affected inside the roof, which is why early investigation is generally cheaper than waiting for the leak to become severe.
How a small leak grows
The volume of water is only part of the picture. Repeated wetting keeps timber, plaster and insulation damp for long periods, and materials that never fully dry behave very differently from ones that get a single soaking.
Damp insulation also stops performing, so a small persistent leak can affect comfort and heating cost long before the ceiling shows a mark.
Why early repairs stay small
Dealing with the entry point early usually means one localised repair. Once water has been tracking for months, the same job can involve timber, plaster, decoration and sometimes insulation as well.
If you have noticed a stain that comes and goes with the weather, that pattern is itself useful evidence — mention it when you book, as it helps narrow the search.