Capacity problems
Valleys, gutters and outlets may cope with ordinary rainfall but overflow when the volume increases. Blockages, poor falls and narrow outlets matter more in those conditions.
Wind-driven rain
Strong wind can push water sideways under laps and around flashings that appear sound in vertical rain. Exposed roof edges and wall junctions are worth checking closely.
Low-pitched coverings
Where tiles or slates are used near their minimum pitch, heavy weather can expose weaknesses in lap and underlay details. This is particularly relevant on some extensions and porches.
Describe the conditions
When arranging an inspection, say that the leak is weather-dependent. “Only in heavy rain from the west” is far more useful than “roof leaks sometimes”.