Roofs in Miami-Dade should be cleaned with a low-pressure soft wash, not a high-pressure jet. Black streaks on shingles and gray or green growth on concrete tile are mostly algae, and a gentle cleaning solution removes them without stripping granules or loosening tiles. Pressure applied directly to a roof is the most common way a cleaning job turns into a repair bill.
What the dark streaks actually are
On asphalt shingles, the black or brown streaks that run down the slope are typically caused by airborne algae, most often a species called Gloeocapsa magma. The Asphalt Roofing Manufacturers Association (ARMA) describes it as a natural pigment the algae produce, and notes that it can turn light roofs brown or black. The streaks spread along the path of water runoff, which is why they look like drips.
On concrete and clay tile, the picture is a little different. Tile is porous, so mildew, algae, moss and lichen settle into the surface, especially on the north-facing side and under trees. Heavy rain in summer and constant humidity feed that growth. According to the National Weather Service in Miami, the rainy season runs from roughly mid-May to mid-October and delivers close to 70 percent of the area's yearly rainfall, so roofs stay damp for long stretches.
Why high pressure is a bad idea
ARMA's guidance on algae is blunt: do not use a power washer or a brush or broom on algae, because high-pressure washing is likely to damage asphalt roofing. The reason is physical. Shingles are coated with mineral granules that protect the asphalt from sun. High-pressure water knocks those granules loose, and the roof ages faster. Pressure washing can also put a shingle warranty at risk, so read the warranty before anyone climbs up.
Tile roofs face a different risk. Individual tiles can crack under foot traffic. A strong jet can force water beneath tiles, dislodge loose ones or damage the underlayment that actually keeps water out. Clay and concrete tile also hold sealants or coatings on some roofs, and those can be stripped or streaked by aggressive cleaning.
How a soft wash works
A soft wash applies a cleaning solution at low pressure, often similar to a garden hose, lets it dwell for a set time, and rinses gently. ARMA describes a diluted household bleach solution (sodium hypochlorite) left on for about 15 to 20 minutes and rinsed with a gentle spray, without scrubbing, because friction can loosen granules. It also advises that other chemicals be used only with the shingle manufacturer's approval and that the work be left to professionals, because a wet roof is extremely slippery.
The method is similar for tile, although contractors may adjust the mix and dwell time to the surface. The point is the same: the chemistry does the cleaning, not the force. The comparison of soft washing and pressure washing explains where else that principle applies.
Tile versus shingle at a glance
| Concrete or clay tile | Asphalt shingle | |
|---|---|---|
| Common problem | Algae, mildew, moss, lichen, uneven fading | Dark algae streaks, granule loss |
| Main risk when cleaning | Cracked tiles, water forced underneath | Lost granules, shortened roof life |
| Walking on it | Can break tiles; needs care and experience | Slippery when wet; careful footing needed |
| Method | Low-pressure wash with detergent or algaecide | Soft wash with approved solution, gentle rinse |
| Return of growth | Often within a year or two in shady areas | Can return; treated shingles may resist longer |
Safety and permits
Miami-Dade and Broward are designated as the High-Velocity Hurricane Zone under the Florida Building Code, and roofing work there follows strict product-approval and permitting rules, including Miami-Dade Notices of Acceptance for roofing products. Cleaning itself is maintenance, but repairs, re-coating or sealing a roof surface can cross into roofing work. If someone offers to coat or seal the roof as part of a wash, ask whether that requires a permit and a licensed roofing contractor, and verify the answer with the local building department.
Falls from roofs are a serious hazard. Do not climb a wet or algae-covered roof yourself. If a roof is old, brittle or already leaking, have it inspected before cleaning, because even a gentle rinse can reveal weak spots.
Is it worth cleaning at all?
ARMA's guidance treats the streaks mainly as a discoloration problem rather than structural damage, and it notes that cleaning with bleach is often temporary because the stain can return. On the other hand, a clean roof looks better at resale, may reflect more heat than a dark, streaked one, and loses the organic growth that holds moisture against tile and shingles. Some homeowners also clean ahead of an insurance inspection, although the cleaning does not repair damage or change the roof's condition, and it should not be used to hide real problems.
Spending decisions come down to age, condition and appearance. A cost range for roof washing, usually priced per square foot, is in the cost guide. Long-term prevention, such as zinc or copper strips near the ridge, trimming overhanging branches and keeping gutters clear, helps slow regrowth. The article on mold and mildew outdoors covers the moisture side of the problem.
Timing around storms
Clean before hurricane season if the roof needs it, not during a storm watch, and never pressure wash a roof you suspect was damaged by wind. The hurricane season guide explains how to sequence cleaning, inspection and repairs.
Frequently Asked Questions
Can I pressure wash a tile roof?
It is not recommended. Tile can crack under foot traffic and a strong jet can push water under the tiles or damage the underlayment. A low-pressure wash with the right solution is safer.
Will cleaning a shingle roof void the warranty?
It can if the method is not approved. ARMA warns against high-pressure washing and says other cleaning chemicals should be approved by the shingle manufacturer. Check the warranty first.
How long does a roof wash last?
It varies with shade, humidity and nearby trees. Algae can begin to reappear within a year or two, particularly on shaded slopes.
Do I need a roofing license to clean a roof?
Cleaning is generally treated separately from roofing repair, but any repair, coating or sealing may require a licensed roofing contractor and a permit. Verify with the local building department and the state license lookup.
Is bleach safe for my plants?
Not without protection. Plants should be wet before and after, and runoff should be directed away from beds or collected, which a careful contractor will plan for.