Look at the wall above the bay
Cracked pointing, render or a sill can feed water onto the roof below.
Lead bays need room to expand
Splits often appear at folds or where previous patches have stiffened the sheet.
Useful evidence: keep one wide photograph that shows where the problem sits on the roof. Close-ups make more sense when the position is obvious.
Membrane bays still depend on sound edges
A perfect-looking centre section does not rule out a failed back upstand or corner.
Ready to get the fault inspected?
If you have photographs from safe ground, send one wide view as well as any close-up.
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