Final answer:
The correct amount of overlap for asphalt shingles when installing flashing for a roof valley is 6 inches, to effectively channel water away and minimize the potential for leaks.
Step-by-step explanation:
When installing asphalt shingles near a roof valley, it is important to ensure that the shingles overlap properly to prevent water penetration and to ensure longevity of the roofing system. For a roof valley, the recommended overlap is typically greater than for normal shingle placement in order to provide additional protection against water infiltration. The correct amount of overlap is c. 6 inches. This extra overlap ensures that water is effectively channeled away and reduces the risk of leaks. Each roofing manufacturer may have specific recommendations, so it is always a good idea to consult the installation instructions provided with the roofing materials for the best practice.