Alligatoring

Alligatoring is the cracking of a bituminous roof surface into a network of roughly rectangular or polygonal segments, so the surface looks like reptile skin. It is most associated with smooth-surfaced or inadequately protected built-up roofs, asphalt coatings, and flood coats that have been exposed to sunlight and heat for years. Related terms include crazing (fine surface cracks) and checking.

Causes

Asphalt and coal tar are kept flexible by lighter oils. Ultraviolet radiation, heat and oxygen gradually drive off and oxidise these oils, so the surface becomes hard and brittle. At the same time, daily heating and cooling makes the surface expand and contract. Once it can no longer stretch, it cracks into a pattern. Surfaces without enough gravel or reflective coating, or with too thick a layer of bitumen applied at once, are more vulnerable. Some coatings applied over incompatible substrates can also alligator as they cure.

Why it matters

Early alligatoring may be limited to the top surface. As the cracks deepen, they expose the underlying felts or reinforcement to weather and allow water to enter. Water in the cracks can then freeze, widening them further. Left alone, alligatoring is a step toward splits, leaks and wet insulation.

Identification

The pattern is visible to the eye, typically over large open areas rather than at isolated spots. Inspectors check crack depth: shallow surface crazing differs from cracking that reaches the plies. They also look for loss of surfacing aggregate, which often goes hand in hand with alligatoring on gravel-surfaced built-up roofs.

Treatment

Where the cracks are shallow and the membrane beneath is sound, cleaning and recoating, or restoring the gravel surfacing, can extend service life. Coatings and primers must be compatible with the existing bitumen. Deep cracking with wet insulation underneath usually means recover or replacement. Routine inspection, at least twice a year on low-slope roofs, helps catch the condition early.