Roof Installation and Zippering

[@salvadorlopez8867]

So that only affects the corners? If that were true wouldn’t it eventually unseal the entire shingle? Not just the corner?

[@KnightsSolutions]

Thermal expansion/contraction wouldn’t necessarily unseal the entire shingle. If the entire shingle is unsealed but not damaged (creased/torn), that’s usually a defective/inadequate cohesive sealant strip on the underside of the shingle. The reason it’s the only the corners unsealed in this situation is because of the “racking” pattern of installation, which has essentially broken the slopes into “sections” the length of the shingle. During each hot/cold cycle, the “sections” are being pulled apart ever so slightly which results in the corners of a section becoming unsealed/detached from the adjacent section. With a proper installation pattern, slopes are not broken down into sections which helps minimize the effects of expansion/contraction.