Across Florida · Vetted local pros
Why do black streaks keep coming back on my roof shingles?
A straight answer first, then what you can safely check yourself and when to get a pro. Free estimates from a vetted MB pro near you.
Those black streaks are algae — a living organism that feeds on the limestone filler in asphalt shingles — not dirt, soot or mildew stains. They keep coming back because Florida's heat, humidity and daily wet-season rain give airborne spores a perfect place to land and re-grow, and because a cleaning that only blasts the surface clean leaves the organism behind to spread again within a season or two.
The method used matters as much as the timing. High-pressure washing can knock the dark color off while stripping protective granules from the shingles, which shortens roof life and leaves a rougher surface that holds moisture and algae even better next time. A low-pressure soft wash that treats the growth itself is the approach generally preferred on roofs.
Why it happens
Roof algae spreads by spores carried on wind and rain, so a streaked roof two doors down is a steady supply line. You will usually see the streaks running downhill from the ridge, worst on the shaded north-facing slope and under tree cover, because those areas stay damp longest after a storm.
Florida's climate keeps the cycle running year-round. Algae, mildew and black streaking peak during and after the humid wet season from roughly June through October, which is why a roof cleaned in spring can look stained again by the following winter if the growth was never fully treated.
Anything that holds water on the roof speeds it up: overhanging branches dropping shade and leaf litter, clogged gutters backing water under the eave, debris trapped in valleys, or a roof that simply dries slowly. Where exposed galvanized or copper flashing sheds trace metal in the rain, you will often see a clean stripe below it — a hint of why some roofs streak faster than others.
What you can check yourself
Look at which slopes are streaked
From the ground or a window, compare the shaded side with the sunny side. Streaks concentrated on shaded, slow-drying slopes point to algae rather than staining from something above.
Check what's shading the roof
Note any branches overhanging the roof or leaf litter sitting in valleys and along the eaves. Shade plus trapped debris keeps shingles damp and feeds regrowth.
Look for a clean stripe under metal
A clear band running down from a metal vent, chimney flashing or roof-mounted fixture is a classic algae tell. It also shows the pattern a treatment should be reproducing across the whole slope.
Look in the gutters from the ground
Granule buildup washing out of downspouts can mean the shingles have already lost surface protection, often from too much pressure in a past cleaning. Mention it to the pro before anyone goes up.
Check walls, soffits and walkways too
Green or black growth on stucco, siding and shaded pavers usually means the same damp conditions are affecting the whole exterior. Algae on walkways and pool decks is also slippery.
When to call a pro
Stop and get a pro once the work involves going onto the roof — ladders, slopes and wet algae are a fall risk, and walking a hot asphalt roof can damage it. Call sooner if streaks returned within months of a cleaning, if granules are collecting in gutters, if shingles look curled, soft or patchy, or if you are facing an HOA cleanliness notice with a deadline.
Pressure washing does not require a state contractor license in Florida; a local business tax receipt, liability insurance and workers' compensation compliance are the usual requirements. Roof cleaning with chemicals can overlap with separate regulation, so verify current rules with the Florida Department of Agriculture and Consumer Services and your local jurisdiction. No building permit is needed to wash, but wash water is regulated under local stormwater and NPDES rules in many Florida cities and counties — runoff with detergents generally may not enter storm drains or waterways. If the inspection turns up damaged or lifted shingles, Florida law requires a licensed roofing contractor for roofing repairs; you can verify any license on the Florida DBPR website.
Your MB pro handles the permit when the job needs one. You can look up a contractor's license at Florida DBPR.
What your MB pro will do
- Assess the roof before touching itYour MB pro will look at shingle condition, age and granule loss and tell you whether cleaning is appropriate at all. Ask whether the method matches your shingle manufacturer's guidance, since pressure can void a warranty.
- Soft wash instead of blastingLow-pressure soft washing treats the algae rather than scouring the surface. Ask how long results typically hold on a shaded Florida roof and what affects that.
- Protect landscaping and manage runoffPlants get pre-wet and rinsed and runoff is kept out of storm drains, canals and waterways. Ask how the crew handles discharge on your lot.
- Clean the rest of the exteriorPros in our network also handle driveways, walkways, patios and pool decks with a surface cleaner, soft washing of stucco and siding, decks, fences, screen enclosures and rust or oil stain treatment.
- Give you a free estimate in writingYou will get a free estimate covering scope and method before any work starts.
How it works
Step 1
Tell us what you need
Call or send the short form with your city and the job. Estimates are free.
Step 2
We match you with a vetted pro near you
We pick a vetted pro from our network who works in your part of Florida and book the visit.
Step 3
Your pro gives a free estimate and does the work
Your MB pro looks at the job and gives you a free estimate. You decide whether to go ahead — and we stay your point of contact.
Related questions
- Can I stop the streaks from coming back at all?
- You can slow them down considerably. Trimming branches back so the roof dries faster, keeping gutters and valleys clear of debris, and asking your pro about treatments or metal strips that discourage regrowth all extend the time between cleanings — but in Florida humidity, periodic maintenance is part of owning a roof.
- Is the algae actually damaging my shingles?
- It holds moisture against the roof surface and feeds on the filler in the shingle mat, which does not help shingle life. The faster damage usually comes from high-pressure cleaning that strips granules, which is why soft-wash methods are generally preferred on roofs.
- When is the best time of year to have it done?
- Streaking peaks during and after the wet season from roughly June through October, so late summer through winter is when most homeowners notice it. The drier winter and spring months usually give the best working conditions and line up with HOA compliance season.
- What drives the cost of a roof cleaning?
- Roof size and pitch, how reachable the slopes are, the roof covering, how heavy the growth is, how much landscaping and runoff protection the lot needs, and whether driveways, walls or a screen enclosure are included. There is no permit cost for washing. Your MB pro gives you a free estimate after seeing the property.
- How do I get a pro out to look at it?
- Call (954) 707-6550 or use the form on this page, and we match you with a vetted pro near you and book the visit. MB Home Improvements keeps offices in Tamarac and Plantation and stays your point of contact — one number for every home service, anywhere in Florida.
Request a free pressure washing estimate
Tell us what's going on. We'll match you with a vetted pro near you.
- 1Tell us what you need.
- 2We match you with a vetted pro near you.
- 3Your pro gives a free estimate and does the work.
Prefer to talk?
(954) 707-6550