Valley flashing is used at the intersection of two roof planes. The point at which two pitched roofs join is known as a valley and is where most shingle materials rely on a piece of metal for a watertight connection to each other. This one is known as valley flashing. This type of flashing varies according to the individual house. On asphalt roofs, the valley flashing is often omitted in lieu of lapped shingles or a rolled mineral roofing valley. In either case, apply metal flashing under the asphalt material, because the valley always deteriorates more quickly than the adjacent field of shingles, and it's difficult to repair the valley without harming affected shingles. Depending on the details of your house, various types of flashing can be used. Valley flashings are usually categorized as open or closed. The former is visible when completed, the latter is not. The main difference is that for open valleys, long sheets of 16 minimum oz. copper are cleaved to the sheathing and underpayment before the shingles are applied. Adjacent sheets of copper are lapped a minimum of 8. The sheets are nailed at the top only with copper or bronze nails. Closed valleys are constructed during shingle installation by inserting copper flashing squares between successive layers of shingles. These flashing squares are folded on the diagonal. If slate or tile is used for the roof covering, it is recommended that 20 oz. plain or lead coated copper be used for valley flashing.