What happens after rain
A true roof leak often responds to weather. The patch enlarges, darkens or drips after rainfall. Wind direction may make a difference too.
Condensation has its own pattern
Condensation is usually worse in cold conditions and in rooms producing moisture. Mould at corners, around window heads or behind wardrobes points more strongly in that direction than a single brown-edged stain.
The loft can contain both
A cold roof space with poor ventilation may have condensation on nails and underlay, while a separate small roof leak exists elsewhere. Finding one does not always rule out the other.
Keep the diagnosis simple
Before paying for roof work, establish whether the damp changes with rain. Before paying for ventilation work, make sure there is no wet timber or obvious defect above. Basic observations prevent a lot of crossed wires.