Garage roof leak

[PositronicRay]

Had a leak last yr in the flat roof, thought I'd fixed it by sloshing a load of stuff around purchased from Wickes.

Anyhoo the leak is back but what I cant understand is the tidemark on the ceiling. The edge is damp where it's discoloured (around 1cm) but the centre is dry? (tested with a multimeter.

The ceiling bows down in the centre from last yrs leak, I would have thought and moisture would have found its way to the centre. (I made a hole in the centre last yr to let the water drip through)

[PositronicRay]

Pheo said:

Probably only leaking when the rain is really bad meaning that it's progressing out by capillary action.

Thanks, something else to keep an eye on.