I have concrete roof tiles of the following design:
Water started dripping through the ceiling and I located the leak to the gap between 2 tiles after poking a hole in the ceiling (it was rotten anyway) and pulling away the felt.
I went on the roof and it struck me what a poor design the tiles were. Unlike flat clay tiles where the nth row fully overlaps the (n-2)th row, there is a considerable gap between those rows where it is down to the intermediate row to keep the water out. Also, unlike terracotta type tiles where the whole tile forms a deep and wide gulley for water to travel down and has a lip that comes right up and underneath the tile next to it, it simply has 2 pretty pathetic, thin, shallow looking gulleys that interlock with the tile next to it that IMO could easily get blocked up with moss or dirt and overflow; moss was actually bunging up the end of it, which I removed.
I completely cleaned out the interlocking in question by shoving a rod in the two gulleys, and ran a hose on the roof downwards and away to simulate rain, and the water was still dripping through, in fact water was gushing out of both gulleys. It had gone into the gap between the tiles, started gushing out the first gulley, overflowed into the second gulley and started gushing out of that, so clearly it wasn't going to take long to spill over the 2nd gulley.