Here’s a diagram i drew up. a cross section of the valley.
i have a cement slate tile roof. from pictures I’ve seen, debris gets caught under the tiles.
so i was thinking… what if I put a little cement in the gaps under the tiles (where i marked the red arrows above). perhaps that would stop debris from accumulating under the tile, and maybe stop rainwater from getting under the tile and into the house?
Any drawbacks to this plan?