Short Answer
Starter material is installed along roof edges beneath/at the first course of shingles. It contributes to proper sealing and wind resistance.
Detailed Explanation
Starter strip is a specialized shingle or roll product installed along the bottom edge of the roof before the first row of field shingles. It provides a sealed, wind-resistant foundation for the first course of shingles, which is the most vulnerable to wind uplift. Without a proper starter strip, the first row of shingles has nothing to seal to, and the cut edges of the shingles are exposed — making them susceptible to wind damage and water intrusion. Some contractors try to save money by cutting regular shingles to make a starter course, but purpose-designed starter products provide better wind resistance and sealing performance.
Why It Matters
Starter strip is critical for wind resistance at the roof's most vulnerable edge. Using cut shingles instead of purpose-designed starter is a common cost-cutting measure that compromises performance.