Short answer: the dark streaks running down many Alpharetta roofs are a type of algae, usually Gloeocapsa magma. They aren't dirt, and they won't wash off in the rain. They should be removed with a low-pressure soft wash — not a pressure washer, which can strip the protective granules off asphalt shingles.

Here's what causes the streaks, why they show up where they do, what not to do, and how to keep them from coming back.

What causes black streaks on a roof?

The streaks are colonies of algae. Its spores travel on the wind and land on roofs across the Southeast, and they grow best where shingles stay damp and shaded. As the colony grows, it darkens, and rain carries it down the slope — which is why it shows up as streaks rather than spots.

You'll usually see it first on:

The north-facing slopes, which get the least direct sun.
Sections shaded by tall trees.
Areas below where water collects or drains slowly.

Algae, moss or lichen?

Algae lies flat and looks like dark staining. Moss is green, fuzzy and raised, and tends to grow in thick shade. Lichen forms crusty, round patches that cling tightly to the shingle. Algae is by far the most common on Georgia roofs. Moss and lichen hold more moisture and grip harder, so they may take more than one treatment to fade fully.

Why you shouldn't pressure wash shingles

Asphalt shingles are covered in granules that protect them from the sun. High-pressure water can knock those granules loose, and once they're gone, they don't come back. That's why roofing manufacturers advise against pressure washing asphalt roofs. Standing on a wet, slick roof with a pressure wand is also genuinely dangerous.

How a soft wash removes the streaks

A soft wash applies a cleaning solution — industry-standard sodium hypochlorite with biodegradable surfactants — at low pressure. The solution kills the algae, and a gentle rinse removes it. The streaks fade without blasting the shingle surface. Before the solution goes on, plants below the roofline are pre-rinsed and protected, and they're rinsed again afterward.

Is roof algae actually harmful?

Mostly it's a curb-appeal problem — a streaked roof makes a well-kept house look older than it is, and buyers and neighbors notice. But algae also holds moisture against the shingles, and left alone it keeps spreading across the roof. Treating it while it's limited to a slope or two is easier than waiting until the whole roof is dark. If you see raised green moss, lifted shingles or missing granules, have a roofer take a look as well.

What to expect on cleaning day

A roof soft wash uses low-pressure equipment, so it's gentler on the shingles than most people expect. Close windows, move cars away from the eaves and keep pets inside. Much of the dark staining lifts during the treatment and rinse; any faint shadow that remains can keep fading over the next few rains as the dead algae weathers off.

If you're thinking about doing it yourself

Ladder and roof safety come first. Wet shingles are slippery, and many Alpharetta homes have steep or two-story roof lines.
Never use high pressure on shingles.
Protect your landscaping. Cleaning solution running off the roof can harm plants if they aren't pre-wetted and rinsed.
Check your shingle warranty for any cleaning guidance from the manufacturer.

Keeping the streaks away

Trim overhanging branches so the roof gets more sun and dries faster.
Keep gutters clear so water drains off the roof edge instead of backing up.
Consider zinc or copper strips near the ridge. Rain washes small amounts of metal down the roof, which discourages algae growth. Some shingles are also made with algae-resistant granules; ask your roofer if you're replacing.

When to clean

There's no fixed schedule — clean when the streaks become visible. Spring and fall are comfortable times of year for roof work, and pairing a roof cleaning with a house wash means the siding below gets cleaned of anything that rinses down. Most Georgia homes do well with a full house wash every 12–18 months.

Pricing

Roof cleaning is $0.30 per sq ft with a $450 minimum. House washing is $0.18 per sq ft with a $200 minimum, and driveway cleaning is $0.15 per sq ft with a $200 minimum. You can get a price range in about two minutes with our instant price tool, and every house wash comes with our 6-Month Spot-Free Warranty.

Roof cleaning in Alpharetta

Kodesh Pro-Wash is family-owned and owner-operated, locally owned and serving Gwinnett & North Fulton. Visit our Alpharetta pressure washing page for services, real reviews and common questions, or call or text (678) 916-8616 — we answer 7 AM – 10 PM, 7 days a week.

See the matching cleaning service and local service-area details, or request your property quote. Call or text (678) 916-8616. Serving Suwanee and Gwinnett/North Fulton.

Black Streaks on Your Alpharetta Roof: A Homeowner's Guide