Roof Algae in Florida: What Every Homeowner Should Know: Expert Q&A with Jonathan Thiele, J&M Roofing St. Augustine

Q: Jonathan, let’s start with the basics. What exactly is roof algae, and why do Florida homeowners see it so much?
Jonathan: What you’re seeing on the roof — those dark streaks, usually black or greenish-black — that’s Gloeocapsa magma. It’s a blue-green algae that feeds on the limestone filler in asphalt shingles. Florida is basically a perfect environment for it. We’ve got humidity year-round, warm temperatures even in winter, and regular rainfall that keeps surfaces moist. Up north, algae gets killed off by cold winters. Here in Northeast Florida, it just keeps growing. I’ve worked on roofs in St. Augustine and Jacksonville where the algae had been building for years because homeowners didn’t realize what they were looking at.

Q: Is it just a cosmetic problem, or does it actually damage the roof?
Jonathan: Both — and that’s what people don’t realize until it’s too late. The streaking is ugly and it kills your curb appeal, but the real damage is happening underneath. Algae retains moisture against your shingles. In Florida’s heat, that trapped moisture causes the shingles to crack, curl, and lose their granules faster than they should. A shingle roof in Florida should last 15 to 20 years with proper care. I’ve seen algae take years off that lifespan when it’s been left untreated. And once moisture gets into the shingle layers consistently, you start getting into decking issues — rot, mold — that turn a cleaning job into a full replacement conversation.

Q: You mentioned moisture and mold — is there a health risk for the family inside the home?
Jonathan: It can get there. Algae itself isn’t the health threat — it’s what algae enables. When it retains moisture consistently and that moisture finds its way into the attic space, you create conditions for mold and mildew to grow. Mold in an attic affects air quality throughout the house. It’s not an immediate risk from some dark streaks on the roof, but if you let it go long enough that moisture is actually penetrating, yes — it becomes a health issue on top of a structural one. I always tell homeowners: the streaks you see on the outside are the early warning sign. Don’t wait until you smell something in the attic.

Q: What about energy bills — does algae actually affect how much it costs to cool a Florida home?
Jonathan: Yes, and this one surprises people. A clean roof reflects more heat. An algae-covered roof absorbs it. In Florida where you’re running your AC nine months of the year, that extra heat absorption adds up on your electric bill. It’s not the most dramatic effect compared to the structural damage, but it’s real and it compounds over time. I’ve had homeowners tell me their bills went down after we cleaned or replaced an algae-covered roof, and that tracks with what we know about how heat absorption works.

Q: Why are some Florida roofs worse than others? Is it location, age, material?
Jonathan: All three, honestly. Here in the St. Augustine and Jacksonville area specifically, homes near the coast deal with salt air and ocean breezes that accelerate algae growth on top of the normal humidity. Homes surrounded by trees are also worse — the shade keeps the roof surface moist longer after rain, and organic debris from branches drops onto the roof and feeds the algae. Older roofs with standard shingles that don’t have algae-resistant treatment are more vulnerable than newer roofs with copper-treated granules. And north-facing roof sections are almost always worse because they get less sun and stay damp longer. If I’m inspecting a roof and I see heavy algae on the north side and minimal on the south, that tells me a lot about what’s going on with drainage and shade on that property.

Q: What can homeowners do to prevent it? Where do you start?
Jonathan: A few things. First — get regular inspections. I recommend twice a year in Florida, before hurricane season in May and after it in November. Catching algae early when it’s just starting is a cleaning job. Catching it late when it’s been growing for years is a different conversation. Second, if you’ve got trees with branches hanging over the roofline, get them trimmed back. Ten feet of clearance from the roofline makes a real difference in how quickly algae establishes itself. Third, if you’re at replacement time, choose algae-resistant shingles — the ones with copper granules built in. That’s what we install at J&M for most Florida replacements. The difference in algae resistance over a 10 to 15 year period is significant.

Q: What about zinc or copper strips along the ridge — do those work?
Jonathan: They do work, to a degree. When it rains, the metal releases ions that run down the roof and inhibit algae growth in their path. The limitation is coverage — they’re most effective on the roof sections directly below the strip, and on a complex roofline with multiple valleys and angles, you’re not getting full protection. I think of them as a supplementary measure, not a replacement for algae-resistant shingles or regular maintenance. If you’ve got an older roof that’s not ready for replacement yet and you’re dealing with recurring algae, zinc strips can help slow it down while you get to replacement.

Q: If a homeowner already has algae on their roof, how should it be treated? What’s safe and what’s not?
Jonathan: The right way is low-pressure washing with a biodegradable cleaning solution designed for roofs. What you absolutely do not want is high-pressure washing — it damages shingles directly, blasts granules off the surface, and forces water under the shingles. I’ve seen DIY pressure washing jobs that did more damage than the algae itself. Bleach-based solutions are another thing to be careful with — they can harm your landscaping and the surrounding environment if not handled correctly. If you’re not confident doing it yourself, call a professional. The cleaning cost is small compared to the cost of replacement.

Q: When do you tell a homeowner it’s beyond cleaning and they need a replacement?
Jonathan: When the algae has been long enough that it’s contributed to real shingle damage — significant granule loss, cracking, curling — and the roof is already aging, cleaning it doesn’t make economic sense. You’d be spending money to clean a surface that needs to be replaced anyway. I look at three things together: the extent of the algae, the age of the roof, and the condition of the shingles. If a roof is 15 years old, heavily covered, and the shingles are already showing deterioration, I’m going to be honest and tell the homeowner their money is better spent on a replacement with algae-resistant shingles than on cleaning something that won’t last another five years regardless.

Q: Last question — what’s the one thing you wish every Florida homeowner understood about roof algae?
Jonathan: That it’s not cosmetic. That’s the thing. People see dark streaks and think it’s just ugly and they’ll deal with it later. But later usually means the damage has had another two or three years to compound. The streaks are your roof telling you something. In Northeast Florida’s climate — the humidity, the coastal air, the warm winters — algae doesn’t go away on its own, it gets worse. The homeowners who get the most life out of their roofs are the ones who catch problems early and address them. A cleaning or an inspection is a small investment. A premature replacement because the problem was ignored is a big one.