Hi, yes that's the neighbouring house in the background. You can see that their bottom tile slopes down properly but mine is sat on a horizontal facia.
I wonder how long it's been leaking. I bet it's made a mess of the cavity insulation. Any other problems this could have caused if its been leaking for years?
Your tiles will be letting water in at that pitch when it is raining heavy, the felt is then supposed to take that water into your gutter, but if it is the old type of felt then it will have rotted by now.
Try sealing all possible leaks with sealant. I also think that the rain gutter might not be able to heavy rain waters try reinforcing your rain gutter.
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