Approximately 20 years ago, roofing contractors and builders stopped leaving the proper space between the run of tiles in the roof valleys due to the high cost of labor, so drilling each tile to be suspended by a wire had to be changed. What we are left with is tiles that hold each other in place. If you look at a new home, before the tile roof is put down, you will see metal flashing in concave sections, that is a potential valley. Because the roofer can’t nail the tiles through the metal and to wire them is so expensive, the tiles are cut with a zero gap. Essentially the tiles hold each other in place by pressure. If the valleys are not cleaned for 12 to 14 years, your chances of having a roof leak is about 95%.