I have 3 Windows in my bedroom. One slider with 2 picture Windows on the sides.
The slider window casing leaks from the top whenever there is heavy rain. It does not leaks right away or constantly. Leaks starts after a few minutes of heavy rain and stops when it's slows to a drizzle or light rain.
We just had new windows installed but nothing changed. Still leaks the same way.
We have a tile roof and there is nothing under the tiles. Basically you can see the bottom of the tiles from the attic. The attic is almost inaccessible where the leak is.