Why Are There Purple or Iridescent Marks On My Shingles?
A purple or iridescent sheen, a bit like an oil stain on asphalt, shows up on certain sections of the roof, particularly visible from a certain light angle. This unusual appearance generally has an explanation directly tied to the shingles' own composition.
What causes this iridescent look
This sheen generally comes from asphalt oils present in the shingle's composition migrating to its surface, a phenomenon creating a visual effect similar to an oil film on water. This process, sometimes called "blooming," is more visible on certain manufacturing batches or certain sun exposure conditions.
Is it a sign of shingle failure
| Interpretation | What it means |
|---|---|
| Purely cosmetic, temporary phenomenon | The most common case, with no impact on the shingle's performance |
| Early sign of a manufacturing defect in the shingle batch | Less common, but possible if accompanied by other signs of premature wear |
| Confusion with algae growth (black stains) | Different from this phenomenon — algae produces a black tint rather than purple or iridescent |
How to tell this phenomenon apart from a real problem
This particular sheen generally fades over time as the oils naturally dissipate under sun and rain, leaving no permanent shingle damage. A real manufacturing problem would generally come with other signs like a shingle warping, cracking, or losing granules abnormally.
Why this phenomenon sometimes affects only certain sections of the roof
Shingles from the same manufacturing batch can have slightly different compositions, and varying sun exposure depending on the roof's orientation can speed up or slow down this oil-migration process at different rates by section.
What to do if this phenomenon persists or worries you
If the sheen persists for several seasons without fading, or comes with other wear signs like shingles lifting or cracking, a professional inspection can confirm whether it's just a normal cosmetic phenomenon or a more serious sign worth pursuing under a manufacturer's warranty.
Frequently asked questions
Does this phenomenon affect the manufacturer's warranty?+
Generally not for a purely cosmetic, temporary phenomenon, but document the situation with photos if it persists, in case a claim becomes relevant.
Can I clean this sheen myself?+
This generally isn't necessary or recommended — the phenomenon fades naturally, and aggressive cleaning could damage the shingle's granules.
Is this sheen more common on new shingles?+
Yes, oil migration is generally more visible in the first few years after installation, before gradually decreasing with exposure to the elements.
How do I tell this phenomenon apart from black algae stains?+
Color is the best clue — a purple or iridescent sheen differs clearly from the black tint characteristic of algae growth on shingles.
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By Éric Gagné — RBQ-licensed roofer, roofing and roof snow removal specialist
Updated on July 22, 2026