What would cause shingles to buckle?

Shingles buckle when they are pushed up into wavy or wrinkled ridges rather than lying flat, and the cause is almost always movement or moisture in the layers below the shingle rather than the shingle itself failing. The most common cause is moisture trapped in the roof deck. If plywood or OSB sheathing was wet, damp, or high in moisture content when shingles were installed, it later dries and shrinks or swells with humidity, pushing shingles up into buckles. Similarly, if roof underlayment or felt was not installed flat and had wrinkles or was installed over a damp deck, those wrinkles telegraph through the shingles over time as they heat in the sun. Another frequent cause is lack of proper spacing between plywood deck sheets. Plywood requires a 1/8 inch gap to allow expansion, and without it, sheets expand in heat and humidity and buckle upward. Poor attic ventilation contributes by trapping heat and moisture that warps the deck. Installing shingles over an existing layer during a layover can also cause buckling because the new shingles cannot lay flat over the uneven old surface. Structural movement or sagging rafters can also cause buckling patterns. Unlike curling which is shingle aging, buckling indicates a deck or underlayment issue that often requires removing shingles to correct the underlying problem.