Roof Soft Wash and Algae Treatment in Fort Worth
The black streaks running down the north side of your roof aren't dirt, and the shingles aren't staining. That's Gloeocapsa magma, a cyanobacterium that feeds on the limestone filler in asphalt shingles, and Fort Worth's Gulf humidity keeps it fed all year. We kill it with a low-pressure chemical wash, closer to a garden hose than a pressure washer, so the granules stay where they belong.
How we do it
A pressure washer at 3,000 PSI will make a roof look clean for one season. It'll also cost you years of shingle life. High pressure blasts off the ceramic granules that shield the asphalt mat from UV, and spraying up-slope drives water behind the courses and into the decking. So we do the opposite. We use a sodium hypochlorite solution with a surfactant that holds it on a pitched surface long enough to work, applied at under 100 PSI. Hypochlorite kills Gloeocapsa magma outright, gelatinous sheath included. That's the part that matters. The percarbonate and soap blends sold as gentler alternatives mostly knock the visible layer down and leave a living colony behind to grow back. On concrete tile, common on the newer Southlake and Colleyville builds, we keep our weight on the bottom third of each tile, where it laps the course below and sits over a batten. Step on the raised center and it cracks, and that crack shows up as a leak two storms later. On metal, the mix comes down and the rinsing goes up. Hypochlorite left to dry will dull a factory finish, and Galvalume in particular reacts badly to a strong mix. On exposed-fastener panels there's a second reason to keep it wet and moving, since the rubber washers under the screw heads are already the first thing on that roof to fail.
The visit, step by step
- 1
Read the roof before anything gets wet
We identify what's actually growing. Black streaking is algae, and it rinses away once it's dead. Lichen is gripped into the shingle surface and needs a longer dwell, often a second application. Moss holds water and gets handled differently again. We check for brittle or lifted shingles and note the roof type, because asphalt, tile, and metal each get walked and rinsed differently.
- 2
Find where the runoff comes out
Everything sprayed on a roof leaves through the gutters, so the whole job concentrates at a handful of downspout outlets instead of spreading out. We locate those first and keep clean water running at them during the wash and after it. Cars come off the driveway, because sodium hypochlorite spots clear coat and there's no fixing that afterward.
- 3
Apply the solution down-slope at low pressure
Sodium hypochlorite with a surfactant, roughly 2 to 4 percent at the shingle, which is about where ARMA's 50:50 mix of laundry bleach and water lands. It goes on at under 100 PSI, from the ridge down, never up under a course, so nothing gets pushed behind the shingles into the decking. We start early. A deck sitting at 150 degrees flash-dries the solution before it can do anything, and that leaves stripes.
- 4
Let it dwell, then rinse
Dwell runs fifteen to twenty minutes, and it can't be allowed to dry on the shingle. The black turns brown and fades while you're standing there, because the organism is dying, not being scrubbed off. Then a low-pressure rinse from the ridge down, plus the siding, windows, and walkways the runoff crossed. The dead biofilm comes out of the gutters too, so it doesn't pack a downspout.
- 5
Set the roof up to stay clean longer
A zinc or copper strip near the ridge sheds ions down the slope every time it rains, which holds the top courses back. The band it protects narrows the farther down you go, so it slows regrowth rather than stopping it. Overhanging limbs get flagged for trimming, since shade and trapped moisture are what let a colony hold on in the first place. If the gutters are backed up we say so, because standing water at the eave keeps the bottom courses damp year round.
Why it matters here
The sheath grips the granules, so when a colony finally lets go, in a hailstorm or under someone's pressure washer, it takes granules with it. Granules are the only UV shield the asphalt mat has. DFW averages around eighteen 100-degree days a year, nearly all of them in July and August, with a long run of 90s on either side. A bare mat dries, curls, and cracks years ahead of schedule. The colony makes it worse while it's alive, too. It's dark, so it holds heat. It's absorbent, so it holds moisture against the shingle through every humid night. And it moves downhill. Every storm carries live cells from ridge to eave, which is why the thin streak you notice from the curb this spring is a whole slope a few years out. A roof replacement in Tarrant County is a five-figure decision. Keeping algae off one isn't.
Signs it's time
- ✦Dark streaks running downward on the north or northeast slope, while the south slope facing the street still looks fine.
- ✦A clean stripe running straight down from a galvanized chimney flashing or a plumbing vent boot, with black on both sides of it. That's metal ions killing the algae, and it's proof the rest of the slope has it.
- ✦Crusty gray-green patches with a raised, scabby texture, usually higher up the slope. That's lichen, and it's gripped into the shingle surface rather than sitting on top of it.
- ✦Shingle granules collecting in the gutters or piling up at the bottom of a downspout splash block.
- ✦Spongy green growth in a valley that sits under a live oak and never gets full sun.
What it costs
Roof soft wash is one of the twelve visits in The Homestead Plan, so members don't pay separately for it. As a one-time job it starts at $500 and runs to about $700, depending on roof size, pitch, and how much lichen has taken hold. Most Fort Worth roofs price at $550.
Questions
Will washing my roof void the shingle warranty?
Not when it's done this way, but pull up your own warranty and check, because terms vary by manufacturer. ARMA, the asphalt roofing manufacturers' trade group, publishes a bulletin on algae discoloration that calls for a 50:50 mix of laundry-strength bleach and water, applied gently and rinsed at low pressure. That's a soft wash. What manufacturers warn against in writing is the pressure washer, because high pressure strips granules off the mat and forces water under the courses. We haven't read your warranty document, so we're not going to tell you what's in it.
Will the bleach kill my landscaping?
Not if the job is done right, and this is the part that separates a good roof wash from a bad one. Beds get soaked with clean water before any solution goes up, so whatever lands is already diluted. Plantings tight to the wall get tarped. Downspout outlets get flooded during and after, because that's where the concentrated runoff comes out. Diluted sodium hypochlorite breaks down into salt and water fairly quickly. What burns turf and foliage is concentrate landing on dry leaves and sitting there in the sun, and that's avoidable.
How long before the black streaks come back?
Two to four years on a north slope is the usual range in this climate. Faster on a roof that sits under live oaks and never fully dries out. A zinc or copper ridge strip stretches that out, and so does cutting back the limbs shading the slope. The Homestead Plan includes a roof treatment once a year, which keeps it from building back up to where you'd notice it from the curb.
My shingles are algae resistant. Why do I have streaks?
Algae-resistant shingles blend copper-bearing granules into the surface, and rain carries copper ions down the slope so the algae can't get established. Two things wear that down. The copper leaches out over the years, which is why a lot of those algae warranties run a fixed term, ten or twenty years, rather than the life of the roof. And the copper granules are a blend mixed into the surface, not full coverage, so protection is thinner in some spots than others. Worth knowing: those warranties cover blue-green algae and nothing else. Lichen and moss are specifically excluded, and on an older roof that's often what you're actually looking at.