So last November, we sprung a leak from our roof (let's ignore the "warranty" it came with in 2014).
A trek up (thank god for bungalow) trying to match the underside trail to above singled out the valley which is formed by the inverted "L" where two runs meet (the 90 degree part). Because the flashing was obviously not replaced in 2014 (let's not go there) I quickly lashed some plastic sheeting under the tiles and flashing as a temporary fix.
This lasted perfectly until we had the beast from the East. As the snow/ ice was melting, we had a slight drip again. A quick trip up top revealed compacted ice in the valley and plastic. Removing that, and refitting the plastic seemed to do the trick.
Come Friday, we had quite a prolonged downpour, and returned home to find a few drips on the floor. However, it wasn't actually leaking anymore. And yesterday, despite some heavy downpours ... no leak.
In advance of the biblical deluge threatened for later today/tomorrow, I have just crawled all over the roof with a hose, and sprayed everywhere - including *up* the tiling and flashing (i.e. against gravity) but no leak.
I don't believe in woo, so am suspecting there is a wind/rain interaction which somehow blew water over the layer I've fitted.
That said, I *did* see a hole in the flashing. About 1mm in size. That's now plugged with some emergency epoxy plumbers putty. It's the first definite fault I've seen.
So, as subject ... fingers crossed for later. Although I'm hoping that sods law will kick in, and having done all that clambering (my legs are wobbly as hell) it won't rain at all. Like when I fitted the guttering :)