Tiger Stripes on Gutters in Whatcom County

What tiger striping actually is

It is not dirt in the ordinary sense.

Rain landing on the roof picks up dissolved material on its way down — including from the asphalt shingle surface itself — and in heavy rain some of that runoff overshoots the trough and sheets over the gutter’s front edge. As it runs down the outside face it leaves the dissolved fraction behind.

The reason it stays there is the surface it lands on. Painted aluminium oxidises with UV exposure; the paint film goes microscopically porous and slightly chalky. Residue arriving on that surface does not sit on top of it, it keys into it. That is the bond general washing cannot break, and it is why people conclude their gutters are simply old.

The striping pattern comes from the overshoot being uneven. Water leaves the roof edge in concentrated runs rather than a uniform sheet, so the residue arrives in vertical bands with clean metal between them — which is exactly what makes it read as stripes rather than as general dinginess.

Why the pattern tells you something

Two things are worth reading off the stripes before anyone touches them.

Spacing that matches the roof above. Regular, evenly spaced striping is normal overshoot and nothing more. A single heavy band, darker than the rest, usually sits under a valley or a roof junction that concentrates runoff at one point — that section will re-stripe faster than the rest of the run, and it is worth knowing rather than being surprised by.

Striping that stops abruptly. Where a run is clean on one elevation and striped on another, the difference is almost always sun. A south or west face oxidises faster, so it takes the bond more readily. The north face often looks better and is dirtier — growth rather than residue, and it comes off differently.

Brightening versus cleaning

These are two jobs on two surfaces, and conflating them is the single most common source of disappointment on a gutter quote.

Gutter cleaning is the inside of the trough: debris out, silt flushed, outlets and downspouts confirmed running. It restores function. It does nothing at all to the outside face, and a freshly cleaned gutter can look exactly as striped as it did before — which is a reasonable thing to be annoyed about if nobody explained it first.

Gutter brightening is the outside face: a treatment made for bonded residue, applied to the front and the visible returns, dwelled, agitated where the surface allows it, and rinsed. It restores appearance and changes nothing about how the gutter performs.

Most houses want both, booked together because the ladder work overlaps. They are still separate lines on the quote, because they are separate work.

What the treatment does and does not do

It removes residue. It does not resurface metal.

Where the paint film underneath is intact, the result is close to the original finish and it is a startling difference from the ground — brightening is one of the highest-visual-return jobs on a house, out of proportion to what it costs, because a roofline reads as maintained or not maintained from a long way away.

Where the paint film has already chalked through to bare oxidised aluminium, the residue still comes off but the surface underneath is dull and will stay dull. That is a finish that has failed, and cleaning is not the remedy for it. We would rather identify that at the quote than have you discover it at the end of the job.

Material matters too. Painted aluminium is the common case and the one that responds best. Galvanised steel takes the treatment but wants more care around any point where the coating is already compromised. Copper gets treated conservatively or left alone — the wrong chemical lifts the patina unevenly and produces something worse than what you started with, and we will say plainly when that is the recommendation.

What to do now

1. Wet a finger and run it down the stripe. If nothing transfers, it is bonded residue rather than dirt, and washing will not touch it.
2. Check whether the striping is even or concentrated. A single darker band usually sits under a valley or roof junction and will return faster than the rest of the run.
3. Compare elevations. A sunny face striped and a shaded face merely dirty is the normal pattern, and it tells you which surface needs which treatment.
4. Rub a dry hand on the gutter face. If it comes back powdery, the paint film has chalked and brightening will only go so far.
5. Book brightening alongside a gutter clean, not instead of one — two surfaces, two jobs.

Where it fits in the schedule

Brightening is not seasonal in the way cleaning is. There is no window that has to be hit. In practice it is booked alongside a gutter clean, most often in spring, because that is when the house is being looked at properly and when the difference is going to be visible for the longest.

Every few years is the usual interval, and each subsequent treatment is easier than the first, because the residue has had less time to key into the surface.

Full method and scheduling for the trough itself is on the gutter cleaning page. If the gutter is also overflowing, deal with that first — see gutters overflowing in heavy rain.