What causes shingles to bubble up?

Shingles bubble up, also called blistering, when small raised bumps or bubbles form on the shingle surface, often with granule loss on top of the bubble. This is different from curling or buckling. The primary cause is trapped moisture or air within the shingle or under the shingle that expands when heated by the sun. During manufacturing, if moisture is trapped in the asphalt or mat, it can vaporize and create blisters when the roof gets hot. More commonly, bubbling is caused by poor attic ventilation. Excessive heat and moisture buildup in the attic causes the shingle to heat from below, vaporizing any trapped moisture in the deck or shingle and forming bubbles. Installing shingles over a damp or wet deck, or installing on a very humid day, can trap moisture that later blisters. Another cause is using a non-vented or improperly vented roof system where moisture cannot escape. In some cases, blistering occurs when shingles are installed over an existing layer that has moisture underneath. While small blisters may not immediately cause leaks, they cause granule loss when they pop, exposing asphalt to UV and accelerating aging. If blistering is widespread, it indicates ventilation or installation issues and the affected shingles will have reduced lifespan. Improved ventilation and ensuring the deck is dry during installation helps prevent bubbling.