Those Black Stripes on Your Gutters Are Not Dirt

Here is a conversation we have most weeks. Someone has just had their gutters cleaned out. The downspouts run freely, the troughs are empty, the job was done properly. And the front of the gutters still has those dark vertical stripes running down it.

They assume the cleaner cut corners. Almost always, the cleaner did exactly what was asked. Cleaning the inside of a gutter and brightening the outside of one are two unrelated jobs.

What tiger striping actually is

The marks are commonly called tiger stripes, and they are a chemical bond rather than a layer of dirt.

Aluminium gutters leave the factory with a baked-on enamel finish. Under years of UV exposure that finish slowly oxidises, and the surface develops a fine chalky texture. Meanwhile, every time it rains, water running off your roof carries asphalt shingle residue, atmospheric pollutants, pollen and road film down over the gutter face.

Rain does not sheet evenly off a gutter lip. It channels, running down in the same narrow paths every storm. Where that contaminated water repeatedly runs over oxidised aluminium, the residue bonds into the degraded finish and stains it.

That is why the stripes are vertical, evenly spaced, and always on the front face. They map the paths the water takes.

Two things follow from this, and they explain nearly every frustration homeowners have with the problem:

• It is below the surface. The staining has bonded with the oxidised layer of the finish. There is nothing sitting on top to be rinsed away.
• It is worst where there is most runoff. Under valleys, below long roof runs, and on older roofs that are shedding more granules.

Why ordinary washing does nothing

Soap and water, even a pressure washer, only remove what is sitting loose on a surface. Tiger striping is not sitting on anything. You can wash a gutter until the water runs clear and the stripes will be exactly where they started.

High pressure is actively counterproductive here. It will not shift the stain, but it will drive water up behind the gutter and into the fascia, and on a gutter with any existing oxidation it can leave the finish patchier than before.

This is genuinely one of the most misunderstood exterior problems we deal with. People conclude their gutters are worn out and start pricing replacement, when what they have is a surface finish problem that costs a fraction of that to correct.

What actually removes it

The treatment is a dedicated gutter brightener: a surfactant-based cleaner, usually mildly acidic, formulated to break the bond between the bonded residue and the oxidised aluminium.

The process is unglamorous and mostly manual:

1. Wash the house first. Brightening gutters below dirty siding gets the order wrong. You will run algae and dirt straight back down over the clean gutter face.
2. Wet the gutter face and the surrounding area, including landscaping below.
3. Apply the brightener to the gutter face and let it dwell. Timing matters; left too long on a heavily oxidised gutter it can lighten unevenly.
4. Agitate by hand. This is the part that cannot be skipped. A soft brush on a pole, worked along the full run. It is the agitation that lifts the bonded layer, not the chemical alone.
5. Rinse thoroughly, top down, and rinse the beds below again.

Done properly the difference is dramatic and immediate. Gutters that looked like they needed replacing come back to near-original white.

The honest limitations

Worth setting expectations, because no one benefits from overselling this:

Severely oxidised gutters will not return to new. If the enamel has broken down badly enough that the surface is visibly chalky across the whole run, brightening will improve it significantly but will not restore a factory finish. At that point the realistic options are accepting a big improvement or repainting.

Results vary along a single run. The section under a valley that takes the most runoff may not come back as far as the rest.

It is not permanent. Oxidation continues. Expect to revisit it every two to three years, though subsequent treatments are easier than the first because less has accumulated.

Some staining is physical damage. If a gutter has been scoured by a previous high-pressure cleaning, that texture is permanent, and brightener will not fix it.

Where it fits in the order of work

If you are having exterior cleaning done, sequence matters:

1. Roof first, if it needs it. Everything washes downhill.
2. House wash next.
3. Gutter brightening after the house, so the clean gutter face stays clean.
4. Driveway, walkways and hardscape last, since runoff from everything above ends up there.

A contractor working in the reverse order is going to undo their own work, and you will see it in the result.

Is it worth doing on its own?

Usually it makes most sense bundled with a house wash. The crew is already on site and set up, so the combined price is lower than booking the two separately, and it avoids the sequencing problem entirely.

As a standalone job it still makes sense in one specific case: you are about to list the house. Gutters frame the entire roofline. Clean white gutter runs against freshly washed siding is one of the highest-impact, lowest-cost things you can do to the front elevation before photography.

If you are anywhere around Kennesaw, Marietta or Acworth and you are not sure whether you are looking at tiger striping or genuine finish failure, send a photo. The difference is usually obvious from one close-up, and it is the difference between a straightforward clean and a painting job.