Florida's humidity provides a hospitable environment for several organisms that grow on roof surfaces. Most are cosmetic; some are corrosive. All three commonly seen — moss, algae, and lichen — respond to different treatments. The wrong treatment for the wrong organism causes more damage than the organism itself.
Algae (Gloeocapsa magma)
The dark streaks running down a roof's north slope. Algae thrives in shaded, humid conditions. It does not penetrate the roof material; it grows on the surface.
On asphalt: largely cosmetic, but the discoloration is significant. Algae-resistant shingle products (most premium designer asphalt) include copper or zinc additives in the granules that suppress algae growth.
On tile (clay or concrete): cosmetic. The algae does not damage the tile.
On metal: rare. Algae cannot adhere to smooth metal surfaces in any significant volume.
Treatment: A 50/50 mixture of household bleach and water, applied via low-pressure pump sprayer, kills the algae. Allow to sit for 15 minutes, then rinse with low-pressure water. Higher-pressure water (above 1,500 PSI) can damage shingle granules and concrete tile surfaces; we use 800-1,000 PSI on roof cleaning.
Prevention: Zinc or copper strips installed near the ridge. Rain washes traces of the metal down the slope, suppressing algae growth on the entire field. Largely a cosmetic concern; do not pay a premium for treatment if your roof is concrete tile or metal.
Moss
The green spongy growth that holds moisture against the roof surface. Moss is less common in South Florida than in cooler climates, but appears on heavily shaded slopes.
On asphalt: damaging. Moss roots penetrate the granule layer and the asphalt mat. Moss is a real lifespan issue.
On tile: cosmetic. The roots cannot penetrate fired clay or concrete.
On metal: very rare. Moss cannot adhere to smooth metal.
Treatment: Same bleach mixture as algae. Allow to dry; the dead moss can be brushed off with a soft brush. Do not scrub with stiff bristles on asphalt — you will remove granules.
Prevention: Address the shade issue. Trim back canopy. If trimming is not possible, expect periodic moss removal.
Lichen
The pale gray-green crusty growth that looks like map continents. Lichen is a composite organism (algae and fungus in symbiosis). It penetrates more aggressively than algae and is harder to remove.
On asphalt: damaging. Lichen roots penetrate deeper than moss.
On tile (especially clay): cosmetic but stubborn. The growth does not damage the material but is difficult to remove without damaging the surface.
On metal: very rare.
Treatment: Stronger than algae treatment. Commercial roof-cleaning chemicals (sodium hydroxide-based) can remove lichen but must be carefully rinsed. The bleach solution above is sometimes inadequate.
Prevention: Same as moss — address the shade if possible.
What we do not do
Pressure-wash at high pressure: removes granules from asphalt, damages mortar joints on concrete, etches the surface of clay tile. Standard pressure-washing equipment runs 2,500-4,000 PSI; this is too much for most roof surfaces.
Use undiluted bleach: damages most roof materials over repeated applications. The 50/50 dilution is the practical maximum.
Scrub aggressively: removes granules from asphalt, damages clay tile glaze.
Annual maintenance
We include a roof-surface cleaning in our maintenance program for clients in heavily shaded areas. Roughly $400-$900 per visit on a typical residential roof. The cleaning is annual; the prevention is permanent.
For clients without our maintenance program, we recommend the cleaning every 18-36 months for shaded asphalt or tile roofs. Metal and slate roofs typically do not require cleaning.
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