Starter strip shingles are a special first row of shingles installed along the edges of a roof, underneath the visible first course. They have a strip of adhesive positioned to seal down the bottom edge of the shingles above them and they leave no gaps where the shingle seams fall. This locks the roof's edge against wind uplift and blocks water from getting under the first row, which is why proper starter is essential at the eaves and rakes.
The edges of a roof, where the wind first catches and where water wants to run back under, are the most vulnerable spots on the whole surface. Starter strip shingles are made specifically to protect those edges. Installed as a hidden first layer along the eaves and up the rakes, they provide a continuous bed of sealant exactly under the bottom edge of the first visible row of shingles.
That sealant placement is the whole point. A regular field shingle has its adhesive higher up, designed to bond to the shingle below it. The first row has nothing below it, so without starter, its bottom edge is unsealed and free to flap. Starter strip puts adhesive right at that bottom edge and offsets the seams so there is no straight path for water to sneak under. The result is a sealed, wind-resistant perimeter instead of a loose, leaky one.
For a homeowner, starter strip is another of those hidden steps that a corner-cutting roofer can skip to save money, sometimes by cutting up regular shingles instead of using proper starter. It does not show from the ground, but it is a major factor in whether the roof's edges survive a windstorm. A quality installation uses real starter strip at every edge, and it is reasonable to confirm that on your estimate.
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