How big do valleys or pan flashings have to be?
This is another of “it depends” questions. The standard valley starts as a 24-inch wide sheet metal that then has ribs and channels bent into it. This generally provides an effective channel 1⁄2 inch deep and 20 inches wide under the tile. If the valley is exceptionally long, or another roof drains into the lower valley then a valley should be designed to accommodate the additional volume. There are a number of designs incorporating ribs that support the overlaying tile and allow for clear channels under the tile.
