The “zipper” pattern is caused by installing the shingles using the vertical racking method. When shingles are installed using this faster but substandard method, the shingle seal can be damaged. This practice prevents the shingles from properly adhering to the roof.
The vertical installation creates an expansion joint at the ends of the shingles. Through normal heating and cooling, the shingles will eventually pull apart along these vertical lines, creating the zipper effect seen here.
Vertical racking is a sub standard shingle installation practice, recommended against by the NRCA (National Roofing Contractors Association) and most manufacturers of asphalt shingles.
Nonetheless, it remains a popular method among installers less concerned about the longterm performance of their work. But, not at Blue Fox.