Black Streaks on Gutters: What They Are and How to Remove Them

The vertical black streaks running down the face of eavestroughs — sometimes called "tiger striping" — are one of the most common curb appeal complaints we hear from homeowners. They make even recently cleaned homes look neglected. And despite what you might assume, they don't come off with a garden hose or standard cleaning. Here's what causes them and what actually works.

What Causes Black Streaks on Gutters

Tiger striping is caused by a specific phenomenon called electrostatic bonding. As rain flows over your roof shingles, it picks up asphalt particles, oxidized tannins from organic debris, and microscopic pollution particles. This loaded water then flows over the outer lip of the eavestrough as it fills or overflows, depositing a thin film of these compounds on the aluminum face below.

Over repeated rain events, these deposits build into the distinct dark vertical lines that follow each overflow path. The electrostatic charge that forms between the compound and the aluminum surface makes the bond exceptionally strong — it is not surface dirt. This is precisely why scrubbing with soap and water removes surrounding grime but leaves these streaks completely untouched.

What Doesn't Work

Standard cleaning — soap and water, pressure washing from below, household all-purpose cleaners — doesn't break the electrostatic bond. Pressure washing the exterior face removes loose surface dirt but leaves tiger striping frustratingly intact. Many homeowners conclude the streaks are permanent staining when they're not — they just require the right chemistry.

What Actually Removes Tiger Striping

The compound requires a specialized eavestrough brightener or cleaner that specifically breaks the electrostatic bond:

Gutter Zap / Eavestrough Brightener: A professional-grade alkaline cleaner formulated specifically for tiger striping on aluminum gutters. Applied with a brush or sponge, allowed to dwell 1–2 minutes, and wiped off — the streaks release with minimal mechanical effort. This is the product professional gutter cleaning companies use for this specific problem.

Krud Kutter Gutter Cleaner: A retail-available product that works on tiger striping. Application and dwell time technique matters — don't rush the process.

Simple Green Heavy Duty BBQ & Grill Cleaner or similar alkaline degreasers work reasonably well on moderate staining but require more scrubbing than professional-grade products.

Professional vs. DIY Removal

With the right product, tiger stripe removal is achievable as a DIY project on single-storey homes where you can safely access the eavestrough face from the ground or a short step ladder. Apply the cleaner with a soft brush, work in 2–3 foot sections, allow dwell time, and wipe clean with a microfibre cloth.

For two-storey or difficult-to-access eavestroughs, this becomes a safety issue. Professional eavestrough cleaning that includes exterior face brightening is the practical choice for most homes — the time and risk involved in safe DIY work at height rarely justifies the saving versus the cost of professional service.

Can You Prevent Tiger Striping?

Tiger striping can be slowed by keeping gutters clean — overflowing gutters accelerate staining because more loaded water contacts the eavestrough face. Ensuring gutters are properly pitched so they drain fully also reduces the overflow events that cause most striping.

There's no practical topcoat that prevents electrostatic bonding entirely on aluminum, but regular exterior face cleaning as part of annual eavestrough maintenance prevents it from building to severe staining. D&D Home Services includes eavestrough face brightening as part of our full gutter cleaning service. Book a cleaning to have both the inside of your gutters cleaned and the outside faces brightened in one visit.

D&D Home Services Team Exterior Cleaning Experts — Kitchener-Waterloo

With years of hands-on experience cleaning hundreds of homes across Kitchener, Waterloo, Cambridge, and Guelph, our team shares practical, locally-relevant advice you can trust.