Alligatoring and Cracking on a Flat Roof: What It Means and What to Do
| # | Table of Contents |
|---|---|
| 1 | What Alligatoring Is |
| 2 | Which Roofs Get It |
| 3 | Cosmetic or Serious? |
| 4 | Repair Options |
| 5 | When It Points to Replacement |
| 6 | What Owners Should Do |
| 7 | FAQ |
What Alligatoring Is
NRCA defines alligatoring as cracking of a surfacing bitumen, asphalt, coating or mastic "that occurs during the aging process in which the loss of volatile oils and the oxidation brought about by solar radiation produces a pattern or cracks similar to an alligator's hide." The definition adds an important detail: "the cracks may or may not extend through the surfacing material."
In other words, alligatoring starts as a surface problem. Asphalt that sits in the sun slowly loses the oils that keep it flexible, hardens, shrinks and cracks into a grid. On its own, a shallow pattern is a sign of age. The concern is what comes next: as a FacilitiesNet article on built-up roofs puts it, "left uncorrected, the cracks will grow into splits in the membrane."
Which Roofs Get It
- Built-up (tar-and-gravel) roofs where the gravel has washed into drains, blown off corners or been scraped away, leaving the flood coat exposed. See our guide to tar-and-gravel roofs for the system itself.
- Smooth-surfaced asphalt roofs whose reflective or protective coating has worn off.
- Modified bitumen where granules or coating have eroded and the asphalt is exposed.
- Coatings and mastics at flashings, pitch pans and patches, which can alligator even on otherwise newer roofs.
- Spray polyurethane foam roofs whose protective coating has aged, which NRCA's definition specifically includes.
The common thread is exposed asphalt or coating. A maintenance guide from the Canadian Roofing Contractors Association, reproduced in IKO's commercial roofing manual, explains why surfacing matters: "If the gravel or protective coating has eroded, the underlying bitumen and felts will deteriorate." It lists wind scour at roof corners, water flow, foot traffic and air from rooftop vents among the causes of that erosion. In Central Texas, long hours of intense sun speed up the process on any exposed surface.
Cosmetic or Serious?
| Likely early-stage | Needs prompt attention |
|---|---|
| Fine, shallow crack pattern in the surface coating | Cracks you can see into, or that have opened into splits |
| Felts or reinforcement still covered and intact | Exposed felts, fabric or mat |
| Limited to small areas | Spread across large areas of the roof |
| No leaks below; firm underfoot | Leaks, ceiling stains, or soft, spongy areas suggesting wet insulation |
| Dry insulation confirmed | Alligatoring combined with blisters or ponding |
Alligatoring often shows up on the same aging roofs as other drainage problems. If water sits on the roof for days after rain, read about ponding water on a flat roof as well.
Repair Options
The CRCA maintenance recommendations in IKO's manual give a short, practical rule for built-up roofs: for "alligatoring or eroding surface," apply "a compatible surface coating and surfacing." For worn protective layers generally, the remedy is to "recoat the membrane and reapply the surfacing material." Where cracks have opened into holes or splits, the same guide says to first "determine the extent of moisture ingression into the roof assembly," then "remove and replace wet insulation and other materials" and patch the membrane.
- Coating and resurfacing for early alligatoring on a dry, sound roof. Compatibility matters: the coating must be designed to bond to the existing asphalt surface.
- Restoring gravel on built-up roofs where the aggregate has been lost.
- Cutting out and patching split areas with compatible reinforcement.
- Re-flashing where alligatored mastic at walls, curbs and penetrations has failed.
The building code supports coating as an option: both the 2024 International Building Code (section 1512.3) and the 2024 International Residential Code (section R908.4) allow a new protective coating over an existing built-up roof without tear-off.
When It Points to Replacement
Coating a roof does not dry out wet insulation or rebuild brittle felts. Replacement usually makes more sense when alligatoring is widespread and deep, the felts are exposed and brittle, there are leaks in several places, or a moisture check finds wet insulation across large areas. The code also limits shortcuts: a new roof cannot be installed over an existing one that is water-soaked or deteriorated, or that already has two or more roof coverings.
On older roofs, plan one more step before tear-off. The EPA notes that asbestos was used in roofing felts until about the early 1980s and that it cannot be identified by sight, so older felts, mastics and coatings should be sampled by an accredited professional before removal.
What Owners Should Do
- Photograph the cracked areas so you can see whether they are spreading.
- Check ceilings below for stains, and note any soft spots on the roof.
- Keep gravel in place and drains clear, and limit foot traffic on exposed areas.
- Have the roof evaluated before choosing a coating, so you know the insulation underneath is dry.
Our flat roofing team can inspect your roof and tell you whether alligatoring on your roof is a coating job, a repair or a sign that replacement is closer than it looks.
Sources: NRCA Roofing Guidelines glossary ("Alligatoring"); FacilitiesNet, "Common Problems With Built-Up Roof Systems" (Piper, 2015); IKO Commercial Roofing Manual Part 16 (CRCA repair recommendations); 2024 International Building Code section 1512.3 and 2024 International Residential Code section R908.4; U.S. EPA asbestos guidance.
Frequently Asked Questions
What causes alligatoring on a roof?
Aging. NRCA attributes it to the loss of volatile oils and oxidation from sun exposure, which harden the asphalt or coating until it cracks into an alligator-skin pattern. Worn-away gravel or coating speeds it up.
Is alligatoring a serious problem?
Shallow surface cracking is an early sign of age. It becomes serious when cracks open into splits, felts are exposed, leaks appear or the insulation underneath is wet.
Can you coat over an alligatored roof?
Often, if the roof is dry and sound. Industry repair guidance calls for a compatible coating and new surfacing, and building codes allow a protective coating over an existing built-up roof without tear-off.
Does alligatoring mean I need a new roof?
Not by itself. Widespread deep cracking, brittle exposed felts, multiple leaks or wet insulation are what usually tip the decision toward replacement.
Does alligatoring happen on TPO roofs?
The term describes cracking of asphalt, bitumen, mastic and coatings. TPO membranes have other aging signs, but asphalt-based flashings, mastics or coatings on any roof can alligator.
