How Biological Runoff Can Affect the Look of Shingles

Atlas Roofing
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Roofers are all too familiar with the unsightly stains caused by algae, but there’s also a lesser known, naturally occurring phenomenon that can affect the look of shingles, known as “tobacco juicing.”

So named for its brown color, “tobacco juice” residue is attributed to the weathering of asphalt roofing. The phenomenon has been attributed to environmental contamination, including biological runoff and/or pollutant deposits, such as excessive air pollution accompanied by nighttime dew conditions and a prolonged lack of rain.

Air pollutants can collect on roof surfaces as dew is forming, then run down onto lower areas, including the fascia and siding. While the residue typically does not affect the performance of the roof, it can continue to accumulate until the surfaces are washed off or a significant rainfall occurs.

“The tobacco-juicing substance is a result of water-soluble [residue] leaching from the oxidized surface of exposed asphalt and may be seen on all asphalt-based products, such as cements, coatings and shingles, as well as (low-slope) base and cap sheets,” explains Paul Casseri, product manager for Atlas Roofing.

The issue tends to occur in drier climates, Casseri continues, and can’t be prevented by homeowners.

However, contractors can help to minimize the aesthetic conditions associated with biological runoff by following these recommendations from Atlas:

  • Add or retrofit the drip edge at the eaves and rakes where not previously installed. (Note: The drip edge should have a well-defined kick edge, ensuring that runoff will be diverted away from the residence.)
  • Install gutters to capture and divert the runoff.
  • Extend the overhang of shingles at the eaves and rakes to the maximum length identified by manufacturer instructions (typically up to ¾”) to allow runoff to fall away from the structure.
  • Require edge metal with a drip lip on parapet walls where the metal slopes outward, is rounded and has no existing lip on the outside edge to assure the residue-laden runoff will fall away from the building.

During long, dry periods — especially the first summer after installation — Atlas also recommends regularly hosing down the roof, but resist the urge to power wash the shingles. (Note: This is not recommended without proper fall protection in place or where steps have not been taken to protect exterior surfaces that may come into contact with the wash-off, such as siding on a house without gutters.)

In addition, if the fascia is already stained, Atlas recommends painting it with an exterior silicon-based paint with mold and/or algae inhibitors.

Although tobacco juicing is unsightly, Casseri reiterates that “the residue will not affect the performance of the roof and should not be considered a performance problem.” That means it doesn’t affect the Atlas Roofing Limited Shingle Warranty (that covers manufacturer defects) — which is great news for contractors and homeowners alike!

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