Mark Ormesher
Hi if it's a lean to it could be the pitch of your roof we come across loads of these problems lately exspeciley with this weather lately if you push a tile up an see the top of the tile under neath if that's wet or the batten is wet there your problem , if it's dry check the pointing on the gables if the mortar work got not mortar missing an your lead work in place still it has to be the sky light cheers Mark