What is the purpose of lap nailing on shingles?

The purpose of lap nailing on shingles is to ensure double coverage and weather tightness. When shingles are installed, proper lap nailing involves placing nails in a way that overlaps adjacent shingles, thereby creating a secure and waterproof barrier. This overlapping technique is crucial in preventing water from penetrating beneath the shingles, especially during rain or snow conditions. By ensuring that shingles cover each other adequately, lap nailing significantly contributes to the roof's ability to protect the underlying structure from moisture damage. This method enhances the durability of the roofing system and helps maintain its effectiveness over time.