Tar cracks when the coating becomes too brittle, too thick, poorly bonded, stressed by movement, or damaged by weathering and freeze-thaw cycles.
Cracking usually means the tar can no longer stretch or stay attached as the surface changes. Timber moves with moisture and temperature. Roofs expand and contract. Boats flex. Rope bends. If the tar film cannot move enough, it splits.
Age is one cause. As tar loses lighter fractions and the surface oxidises, the outer layer can become harder and less flexible. Sunlight, rain, salt, and dirt all accelerate surface change. A thin maintained coating may weather gradually, while a neglected coating may crack into islands.
Thickness is another cause. Heavy tar can form a firm skin over softer material. As the skin shrinks, heats, cools, or moves over an unstable base, cracks appear. Poor adhesion makes this worse because the coating has no strong grip on the surface beneath it.
A crack matters because it gives water an entry point. Once water reaches the underlying surface, it can lift the edges, deepen the split, and start a cycle of failure. Cracked tar should be read as a sign to inspect the material, not only the coating.