Why Your Gutters Have Black Streaks Down the Front

Black streaks running down the front of a white gutter are called tiger stripes, and they are not mold and not loose dirt. They are roof grit and shingle runoff bonded into the chalky top layer of the gutter paint, left there by water going over the front lip instead of out the downspout. A rinse will not move them.

What tiger striping actually is

A gutter face is painted aluminum, and sun breaks that paint down a little every year until the surface turns faintly chalky and slightly porous. That is the part that matters, because chalky paint holds onto whatever lands on it instead of letting it wash off.

What lands on it is roof water. An asphalt shingle sheds fine mineral granules, and in summer heat it gives up a trace of the asphalt binder that holds those granules on. Add pollen, dust and ordinary soot, run all of it over the lip of the gutter, and it dries in place on a surface built to grab it. Some of it also creeps up under the drip edge and runs down the face from behind. Either way, each overflow leaves one more thin layer in the same track, which is why it reads as stripes rather than an even gray.

The stripes tell you where the water went

Water leaves a gutter in the same spots every storm, so the pattern is a record of what is wrong.

• Heavy striping in one short section usually means a low spot, a tired hanger or a seam right above it.
• Faint striping along an entire run means the trough is routinely running full, so it is holding debris, the pitch is off, or the gutter is undersized for that much roof.
• The worst striping under a roof valley is normal. A valley delivers a lot of water to one point fast and some of it overshoots the trough no matter how clean it is.
• Striping that gets worse toward an end cap points at pitch running the wrong direction.

Why gutters here pick it up faster

The oaks and maples around the Twin Cities keep dropping into November, so troughs plug up at the same time the fall rain is heaviest. More on the timing in when to clean gutters in the Twin Cities.

Winter does a quieter version of the same thing for months. Once a trough is iced in, every sunny afternoon sends snowmelt straight over the front edge, and nobody is watching in February. Spring then adds pollen and whatever grit the roof shed over the winter.

South and west walls show it first, because they take the most sun and their paint chalks years ahead of the north side of the same house.

What takes it off and what does not

Not a garden hose, and not a pressure washer either. High pressure is the wrong tool on a gutter face. It dents thin aluminum, drives water up behind the gutter into the fascia, and strips paint that is already weak, and after all that it still does not break the bond holding the stain on.

What works is chemistry and time. A cleaner made for oxidized aluminum, brushed on, left to sit for a few minutes, worked gently with a soft brush, then rinsed at low pressure. The dwell time is doing the real work, so most of the mistakes here are people scrubbing harder instead of waiting longer. Keep abrasive pads away from painted aluminum entirely.

It also has a ceiling worth knowing about. If the paint is badly chalked, part of the stain sits inside that oxidized layer rather than on top of it, so the gutter comes back much lighter but not new. Fifteen years of striping does not fully reverse, and anyone promising otherwise is overselling.

Whether it is worth paying anybody for

Often it is not, and that is the honest answer. Tiger stripes are cosmetic. They do not cause leaks, they do not slow drainage, and nothing is quietly degrading while you leave them alone. If you cannot see those gutters from the street or the yard you actually sit in, this belongs near the bottom of the list.

The part that is worth attention is the cause, not the stain. A blocked downspout or a bad pitch is sending water over the front every time it rains, and that is a real maintenance item with real consequences, unlike the stripe it leaves behind. Emptying the trough and brightening the face are two separate jobs, and nobody should sell you one as the other. What a gutter cleaning does and does not cover is laid out in what a gutter cleaning should include.

Where the face is worth doing is when appearance is the point: listing photos, fresh paint, new siding, or a front elevation you look at every day. Gutter cleaning starts at $120 and pressure washing starts at $150, both backed by the Gleam Guarantee. I work in St. Paul, Minneapolis and the suburbs out to about 25 miles, and I will tell you on site if the stripes are not going to come all the way out. You can request a free quote online or book a free 10-minute consultation.