Seasonal

Black Streaks on Gutters and Other Hot-Climate Weather Wear

By Jacksonville Gutter Repair · October 28, 2026 · 8 min read

Black streaks on gutters are the most common complaint about how gutters look in Jacksonville, and they have nothing to do with the ice dams and heat cables that fill gutter articles everywhere else in early November. The city goes below freezing only a handful of nights a winter, briefly, and a gutter that drains has nothing in it to freeze. What ages a gutter here is sun, salt air and a climate where mildew grows on anything that stays shaded and damp.

This is what each one does to the finish and the metal, wall by wall, and what slows it down.

Where black streaks on gutters come from

On the face of a white or light-colored gutter, the dark vertical stripes usually build up where water runs over the front lip. Shade and humidity let mildew and algae grow in that film, and airborne grime and pollen bond to an aging painted finish in the same streaks. On the north wall and under oak canopies in Riverside or Ortega, the streaks can turn green as well. They rarely hurt the aluminum, but they mark exactly where water has been running over the front, which can be a sign of overflow worth checking.

The south and west walls: UV and gutter chalking

The baked-on finish on aluminum gutter coil is a thin coat of paint, and Florida sun is hard on paint. Ultraviolet breaks down the resin that binds the pigment, and the surface goes from glossy to flat to chalky. Gutter chalking is the white powder that comes off on your hand when you rub an older brown gutter. Fading follows; a dark bronze drifts toward tan, and a white gutter can yellow slightly. The south wall usually goes first and the west wall second, while the north wall may still look new.

Chalking is cosmetic until it is not. Once the pigment layer is worn, the primer shows, and after that the aluminum itself, which dulls to gray but does not rust. The metal outlasts the paint by years. Whether that matters depends on whether you can see the gutters from the street.

What slows it: a quality coil finish to start with (the manufacturer's finish warranty covers chalk and fade beyond a stated level, and that warranty is worth reading), lighter colors, which reflect more heat than dark ones, and a gentle rinse once a year, since dirt and pollen on the surface hold moisture. Repainting a chalked gutter is possible with the right primer and typically runs $2–$5 a foot; on a younger gutter it can be worth it, and on an old one the money is often better put toward replacement.

The Beaches: salt air corrosion on gutters

East of the Intracoastal, in Jacksonville Beach, Neptune Beach, Atlantic Beach, Ponte Vedra and Fernandina Beach, everything metal on a house lives in salt fog. Aluminum itself holds up well; it forms a stable oxide layer and resists salt far better than steel. Salt air corrosion on gutters mostly attacks what is attached to them. Zinc-plated screws and straps rust within a few years. Steel hangers rust and stain the fascia orange. Even the aluminum can pit where a dissimilar metal touches it, usually at a rusted screw head, and the pit can eventually go through.

What slows it: stainless fasteners at the beach, on straps and hangers as well as screws, which is why beach installs cost $1–$2 a foot more. A fresh-water rinse of the gutters and downspouts a few times a year washes the salt off before it concentrates. And look at the screw heads each fall; the first orange bloom is the signal to re-hang with stainless before the run comes loose.

The north wall and under trees: mildew and algae

Jacksonville humidity keeps a shaded surface damp much of the year, and mildew and algae grow on any finish that stays wet. Beyond the look, the growth holds moisture against the fascia behind the gutter. The same conditions grow algae on splash blocks and on walkways under a leaking corner.

Cleaning gutter mildew: a soft wash with a mild cleaning solution made for painted aluminum or exterior siding, applied with a soft brush and rinsed well, takes most of it off; follow the product label and protect plants below. A rinse every spring keeps it from establishing. Do not pressure wash gutters. The pressure drives water behind the gutter and into the fascia, and on thin .027 metal it dents.

By wall, at a glance

A ten-minute finish check, once a year

1. Walk around the house in daylight and look at each wall on its own: south, west, north and east.
2. Rub a finger along the face of the gutter on the south wall. A lot of powder on your skin means the finish is chalking.
3. Near the ocean, look at screw heads, straps and hanger ends for orange bloom, and at the fascia for rust stripes.
4. On shaded walls, look for black or green streaks and note whether they line up under one section. That can mean water spills over there.
5. Check the downspouts as well. They take the same sun and salt and are easy to forget.
6. Write down what you found and the date. Next year you will know whether it is getting worse or holding steady.

Mistakes that age a finish faster

• Scrubbing with an abrasive pad or a harsh cleaner, which strips paint along with the grime.
• Pressure washing, which dents thin metal and forces water behind the gutter.
• Leaving rusted zinc-plated screws in place at the beach, where the rust starts to pit the aluminum around them.
• Painting over chalk without the right primer, so the new coat does not bond well.

What this means for buying gutters

First, gauge and hardware matter more than color. A .032 coil with stainless fasteners at the coast will still be tight and straight when the finish is chalky, and a chalky gutter that works is not a problem. Second, the finish that ages fastest is often on the wall you see least, so a gutter that looks tired on the south side may have a north side that is fine, and the whole system does not need replacing because of paint. The installation page covers what goes into a system built for this climate, the gutter repair page covers the re-hangs and reseals that keep an aging one working, and the Ponte Vedra page goes into the salt-air hardware question for the beach communities specifically.

Common questions

What are the black streaks on gutters?

Usually a mix of mildew or algae growing in the film of water that runs over the gutter's front lip, plus grime and pollen that bond to an aging painted finish. They show most on white and light-colored gutters and on shaded walls. They are mainly cosmetic, but they can point to water regularly running over the front of the gutter.

How do you remove black streaks off white gutters?

Start gentle: a soft brush, a cleaner made for painted aluminum or exterior siding, and a thorough rinse with a garden hose. Test a small area first so you don't dull the finish, and avoid abrasive pads and pressure washers. Streaks that have been there for years may lighten rather than vanish completely.

Will vinegar remove black streaks on gutters?

A diluted vinegar solution can lift light mildew, but it is often too weak for streaks that have bonded to the paint over several years. Rinse well afterward and keep it off plants. If vinegar does not move them, a cleaner made for painted gutters is the next step.

Can rain gutters warp?

Aluminum gutters do not really warp in the sun, but thin .027 metal can ripple, called oil-canning, on hot west-facing walls, and the ripples can become permanent. Vinyl gutters soften in Florida heat and can sag or distort over time. Heavier .032 aluminum hung on closely spaced hangers holds its shape best.