PeterC was thinking very hard :
On my shed's roof, the moss had slightly pushed up the overlaps so that
rain was blown under by strong winds from the wrong direction.

Squirt some good frame sealant into the inside (uphill) joint edges.
With a similar problem, that is what I did and it has proven to be very
effective.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk