Black streaks on gutters: what 'tiger stripes' are

Tiger stripes are the dark vertical streaks on the outside face of an eavestrough. They form when overflowing water carries roof grime, soot and pollution over the lip, and sun and heat bake it onto the painted aluminum. A hose won't remove them; a gutter-specific cleaner and soft brush usually will, without harming the finish.

What causes tiger stripes?

Roof runoff is dirty. It carries fine asphalt residue from shingles, soot from traffic and chimneys, pollen and airborne grime. When a gutter works properly, that water goes into the trough and down the spout. When it overflows, it runs over the front lip in the same spots every rain, leaving a thin film each time. Sun and heat bake the film onto the paint, and layer after layer builds the stripes.

That is why the stripes are evenly spaced on some houses: water crests the lip at the lowest points of the front edge, often between hangers. Under heavy tree cover, tannins leached from wet leaves can add a brown tint to the grey.

Is it mould, dirt or something else?

Rub a streak with a damp finger. Grey-black grime that smears slightly is the classic tiger stripe. A green or fuzzy growth, usually on shaded faces, is algae or mildew and cleans more easily. A chalky white residue on the whole face is the paint itself breaking down with age, which cleaning cannot fix. Black streaks on the shingles above, rather than on the gutter, are a different thing again: roof algae, covered in moss on roofs and gutters.

Are tiger stripes a problem?

The stripes themselves are cosmetic. What they reveal is not: water has been coming over the front of the gutter instead of going down the spout. That usually means a clog, a partly blocked downspout, lost slope or a gutter undersized for its roof. Why clean gutters still overflow walks through the causes. Clean the stripes and fix the overflow together.

How do you remove tiger stripes?

1. 1Work in the shade or on a cool day so the cleaner does not dry on the surface.
2. 2Rinse the gutter face with a garden hose to remove loose dirt.
3. 3Apply a cleaner made for gutter faces, or a mild degreaser diluted as directed, to a small test area first.
4. 4Wipe with a soft brush or microfibre cloth along the gutter, not up and down, working a section at a time.
5. 5Rinse thoroughly, and protect plants below by wetting them before and rinsing them after.
6. 6Check the result when dry; stubborn stripes may need a second pass.

How do you stop them coming back?

• Keep the gutters clean so water goes into the trough, not over the lip.
• Flush the downspouts; a partly blocked spout is the most common hidden cause of overflow.
• Restore the slope on runs that pool: checking gutter pitch.
• Wash the faces once a year or so, before the film builds into stripes.

Why does curb appeal matter so much in Oakville?

Oakville's stately homes near the lake and high-end rebuilds everywhere are built for curb appeal, and bright white or colour-matched eavestroughs frame the whole facade. Towering oaks and maples in Old Oakville and along the creek corridors drop heavy leaf loads, so gutters there overflow early in the fall if they are not kept clear, and the stripes follow.

The gutter-face wash is an optional add-on to any cleaning: $99, a hand wash of the front face of the eavestrough so it looks new from the curb. Pair it with a full gutter cleaning in Oakville, $299 to $499 before HST for most homes, and the flush that stops the overflow causing the stripes in the first place.