Short answer: Black tiger stripes running down the face of a gutter are oxidation and grime, not mold or algae. They form as the gutter's finish reacts to the elements over time, and dirt streaks down with every rainfall, leaving vertical dark lines that only get more visible the longer they are left alone.
What is actually causing the stripes
The dark streaking on a gutter face is oxidation and grime, a different cause entirely from the green mildew that shows up on shaded walls or the black algae that streaks a roof. It is a surface level buildup rather than a living organism spreading its way across the metal.
Why it runs in vertical stripes
Every time it rains, water runs down the face of the gutter and carries dirt and grime along the same path each time, which is exactly why the staining builds up as distinct vertical lines instead of settling as an even coat across the whole gutter run.
Why older gutters show it more
A gutter's finish holds up better when it is newer, so homes with older gutters tend to show tiger stripes more clearly and sooner than a recently installed system does under the same weather conditions.
How it gets removed
Since tiger stripes are oxidation and grime rather than embedded biological growth, gutter brightening is built specifically to address this staining on the gutter face by hand, without scratching the paint. It is priced as an add-on at $95 or $145 depending on home size. Brightening is a separate job from a gutter clean-out, which clears leaves and debris out of the trough instead.
What to expect from the result
A brightening visit brings the gutter face back to a much cleaner look, though very heavily oxidized or long neglected gutters may lighten more than they fully return to like-new, depending on how deep the staining has set in over the years.
Easy to mistake for something else
From a distance, dark vertical streaking on a gutter can look like mildew or algae staining elsewhere on the house. Since it's oxidation and grime rather than a living growth, it responds to gutter brightening rather than a wall soft wash. While you're at it, check the roofline too: the black streaking up there comes from a different cause and needs its own service.
In Volusia County
Volusia County property records show that 56% of homes here were built before 1990. Decades-old gutters on that many properties are exactly the kind most likely to show heavy tiger stripes by the time a homeowner finally books a first cleaning for them.
Questions people ask
Is gutter brightening the same as a gutter clean-out?
No, a clean-out clears debris from the trough and runs $175 to $249 by home size, while brightening specifically targets the oxidation and grime staining on the gutter face and runs $95 or $145.
Will the stripes fully disappear after one visit?
Most staining comes out well, though gutters with years of buildup may lighten significantly rather than look completely new after just a single visit.
Do new gutters get tiger stripes too?
They can, though a newer finish generally holds up longer before showing the same visible streaking that an older gutter does under the same rain and sun exposure.
Does rain make tiger stripes worse over time?
Yes, since every rainfall running down the same path on the gutter face is part of what builds and deepens the striping pattern in the first place, year after year.
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