A surface that is giving up
Cracks and alligatoring
Alligatoring is a network of shallow cracks that looks like reptile skin. It is the bitumen surface drying out and shrinking. It tells you the roof is aging, even if it is not leaking yet.
- Sign of ageing
- Often fixable with a coating
- Deep cracks need patching
What you are looking at
Alligatoring is a pattern of fine, interlocking cracks in tar, asphalt or the coating on a roof. Deeper cracks, running in lines or around the edge of the roof, are splits. Both mean the top layer has lost its flexibility.
Single-ply roofs crack differently. EPDM shows surface crazing or cuts at corners, and old TPO can craze at seams. Whatever the material, a crack deep enough to show a lower layer is no longer cosmetic.
Why the surface cracks
Sun and age
UV light dries out the oils in asphalt. Without gravel or a cap sheet to shade it, the surface hardens within years.
Temperature swing
Ontario roofs go from deep cold to hot sun. Each cycle expands and contracts the roof, and a stiff surface cracks.
Movement at joints
Splits tend to line up over deck joints, parapet edges and where insulation boards meet.
Poor-quality or thin coating
Cheap roof cement spread thickly on a hot day shrinks and crazes as it cures.
How urgent is it?
Fine alligatoring on a dry roof is a warning, not an emergency. The roof still sheds water, but the protective skin is thin. Splits wide enough to put a fingernail in are another matter: water can enter, and any freeze–thaw cycle widens them.
The right moment to act is before the first leak, when the plies underneath are still dry and the options are widest.
What you can safely do yourself
Watch from a window or ladder at the edge
Do not walk on a roof with a brittle surface; you can crack it more. Look at the edges, drains and wall joints.
Photograph each year
Take pictures in the same spot every spring. A change in crack width is the useful information.
Skip hardware-store tar
A thick coat of roof cement on a cracked surface seals it for a season, then cracks again and traps moisture.
Options for a cracked roof
Confirm the roof is dry
An infrared or moisture scan finds wet insulation. If the roof is dry, repair and restoration are real options.
Patch the splits
Open cracks are cleaned and patched with reinforced material, not just smeared with sealant.
Restore with a coating
A reflective or elastomeric coating can seal a dry, aged roof and slow further wear. Coatings commonly last 7 to 20 years, depending on the coating and are priced from $3 per sq ft. See roof coating.
Replace if saturated
If the cracks are deep, widespread and the insulation is damp, a new system is the cleaner answer.
Cracks and the material underneath
For built-up roofs, alligatoring on the surface of a roof already past 20 to 30 years usually means replacement is near. For mod bit, cracks in the cap that expose the base sheet are a call to repair soon. See built-up roofing and modified bitumen for how each system ages.
Cracks and alligatoring questions
Does alligatoring always mean a leak?
No. Many alligatored roofs stay watertight for a while. It means the protective layer is thin, so risk rises each winter.
Can I seal cracks with a caulk gun?
Only as a short temporary measure. Sealant without reinforcement and cleaning rarely survives a freeze–thaw cycle.
Is a coating cheaper than replacing?
Often, if the roof is dry underneath. Compare the two at repair or replace.
Leak, damage or an old flat roof?
Send a few details and photos. We reply by email with next steps and a written quote.