alligatoring

Alligatoring is a network of intersecting cracks that forms in asphalt or bituminous coatings, creating a pattern visually similar to alligator skin. The cracks typically range from 0.5 to 2 inches across and often penetrate through the bitumen layer to expose the substrate or underlying membrane. This is a failure mode, not a normal weathering pattern, and indicates the coating has lost its flexibility and structural integrity.

On built-up roofs and modified bitumen systems, alligatoring develops when the bitumen becomes brittle due to ultraviolet exposure, thermal cycling, or oxidation over time. Built-up roofs with multiple plies can show alligatoring in the top ply while lower plies remain sound. On spray polyurethane foam (SPF) roofing, alligatoring occurs in the protective topcoat, typically an elastomeric acrylic or polyurethane spray-applied coating, when that coating fails prematurely from incompatibility with the foam or UV degradation.

When and Why It Happens

Alligatoring accelerates in climates with large daily temperature swings and intense sun exposure. The bitumen expands and contracts with thermal stress, and once it loses elasticity, this repeated movement causes it to crack rather than flex. Poor slope or water ponding can trap moisture beneath the membrane, which expands during freeze-thaw cycles and forces the coating outward. On SPF roofs, using incompatible topcoat materials or applying them too thick without proper surface preparation will cause adhesion failure and stress concentration that leads to alligatoring.

The term itself derives from the visual resemblance of the irregular polygonal crack pattern to the bumpy hide of an alligator, a comparison that became standard industry language in the mid-20th century as single-ply and built-up roofing systems became dominant in commercial construction.

Once alligatoring begins, it typically spreads. Water infiltrates through the cracks, breaks down adhesion, and accelerates further degradation. Roofing contractors distinguish alligatoring from other crack patterns like ridging (directional waves) or crazing (very fine surface checks). Alligatoring is grounds for replacement or repair, depending on the extent and the warranty requirements of the original system. Prevention relies on adequate slope, appropriate materials selection, and proper installation, particularly on SPF roofs where topcoat compatibility and cure time are critical.