Mandeville’s towering live oaks, dense pine canopy, and proximity to Lake Pontchartrain make it one of the most beautiful places to live on the North Shore — and one of the toughest environments for a residential roof. The same shade and moisture that keep summer temperatures a few degrees cooler than Baton Rouge also create ideal growing conditions for moss, algae, and lichen on asphalt shingles. Big Easy Roofers works with homeowners across St. Tammany Parish to stop biological growth before it shortens the life of their roof.
Mandeville sits on the north shore of Lake Pontchartrain, and the lake’s influence on local humidity levels is measurable. Morning fog rolls in from the water regularly during spring and fall, keeping roof surfaces damp well into the late morning hours. Average relative humidity in St. Tammany Parish runs between 75 and 90 percent for most of the year, which is exactly the moisture range that moss and algae need to thrive.
The city’s tree canopy compounds the problem. Neighborhoods like Beau Chene, Tchefuncte Country Club, and the older sections along Lakeshore Drive are heavily shaded by mature live oaks, water oaks, and longleaf pines. Trees that overhang the roofline block sunlight from drying the shingle surface, trap leaf debris in valleys and behind dormers, and create pockets of persistent moisture that biological organisms colonize quickly.
Mandeville’s soil is also a factor. The area’s acidic, sandy soil supports the kind of vegetation that sheds spores and organic material onto rooftops. Pine needles accumulate in valleys and along roof edges, holding moisture against the shingles and creating a nutrient-rich bed for moss growth. Homes on larger wooded lots in the Fontainebleau State Park area or along the Tchefuncte River deal with this more than subdivisions with younger, smaller trees.
These three organisms show up on Mandeville roofs regularly, but they’re not the same thing and they don’t cause the same type of damage.
Algae (Gloeocapsa magma) is the most common. It shows up as dark black or greenish-black streaks, usually starting on the north-facing slopes where shade is heaviest. Algae is a single-celled organism that feeds on the limestone filler in asphalt shingles. It doesn’t have roots, so it sits on the surface rather than penetrating the shingle. The dark streaking is actually the algae’s protective outer coating, which darkens the roof and increases heat absorption.
Moss is a plant with a shallow root system that grabs onto the shingle surface. It forms thick, green, spongy mats — usually in shaded valleys, behind chimneys, and along the north side of dormers. Unlike algae, moss holds water against the shingle surface like a sponge, and its root-like structures (rhizoids) can work their way under shingle edges and lift them, creating pathways for water infiltration.
Lichen is a combination of algae and fungus living as a single organism. It forms crusty, circular patches that are light green, gray, or yellow. Lichen bonds tightly to the shingle surface and is the hardest of the three to remove without damaging the granule layer. It’s less common than algae or moss on Mandeville roofs but shows up on older shingles that have been in shade for years.
Moss and algae aren’t just cosmetic problems. Left unchecked, they reduce your roof’s functional lifespan and increase the risk of water damage to your home’s interior.
Shingle deterioration. Moss roots pry up shingle edges and break the seal between overlapping courses. Once that seal is broken, wind-driven rain gets underneath and reaches the underlayment or deck. Algae’s limestone consumption gradually weakens the shingle matrix, making granule loss happen faster than it would from normal weathering.
Moisture retention. A thick moss mat on a roof section holds water long after the rest of the surface has dried. In Mandeville’s climate, where afternoon thunderstorms roll through from May to October with regularity, moss-covered sections may never fully dry out during the summer months. Persistent moisture accelerates the aging of both the shingles and the underlayment beneath them.
Increased cooling costs. Dark algae streaks absorb significantly more solar radiation than clean shingles. On a home where algae covers half the roof surface, the attic temperature can run noticeably hotter than it should, forcing the AC system to work harder. Over a full Louisiana cooling season, that adds up on your Entergy bill.
Reduced curb appeal and property value. A roof covered in dark streaks and green moss patches looks aged and neglected, which affects the home’s perceived value. In Mandeville’s real estate market, where homes in established neighborhoods command premium prices, a dirty roof can make the difference in how quickly a listing sells.
Metal strips installed along the roof ridge are one of the most effective long-term defenses against algae and moss growth. Here’s how they work and what to consider for Mandeville installations.
When rain washes over a strip of zinc or copper mounted just below the ridge cap, it carries microscopic metal ions down the roof surface. These ions create an environment that’s toxic to algae and moss, effectively preventing colonization on the shingle surface below the strip.
Zinc strips are the more affordable option and are widely available at roofing supply houses in the Mandeville and Covington area. They typically last 10 to 15 years before the zinc layer oxidizes enough to reduce effectiveness. Zinc strips work well on roofs with moderate shade and average algae pressure.
Copper strips are more expensive but last longer and release ions more effectively. They’re the better choice for heavily shaded roofs under dense oak canopy, where algae pressure is constant. Copper strips can remain effective for 20 years or more.
Installation involves sliding the upper edge of the metal strip under the ridge cap shingles so that 2 to 3 inches of metal are exposed. The strips should run the full length of the ridge. On hip roofs or complex roof lines with multiple ridges and valleys, additional strips may be needed at mid-slope to ensure coverage reaches the lower sections of the roof.
One limitation: metal strips protect the area directly below them, but steep roofs or roofs with dormers may have sections that don’t receive runoff from the strip. For these areas, supplemental treatment with a cleaning solution is still necessary.
If you’re planning a roof replacement, algae-resistant (AR) shingles are worth the modest upfront premium — especially in Mandeville’s environment. These shingles contain copper granules blended into the surface layer that leach out slowly over time, inhibiting algae colonization.
Major manufacturers — GAF, Owens Corning, CertainTeed, and Atlas — all offer AR versions of their standard architectural shingle lines. The algae-resistance warranty period typically runs 10 to 15 years, which is the period during which the copper granules are most active. After that, the shingles still function normally as a weatherproofing layer but lose their anti-algae properties.
AR shingles cost roughly 10 to 15 percent more than standard shingles of the same grade. On a typical Mandeville home with 2,000 to 2,500 square feet of roof area, that translates to $300 to $600 more in material costs — a small investment compared to the cost of repeated cleaning or premature shingle replacement caused by biological growth.
When selecting AR shingles, match the product to the level of shade exposure on your specific roof. Homes in open subdivisions with minimal tree cover may do fine with a standard AR shingle. Homes on wooded lots in Beau Chene or along the Tchefuncte River, where shade coverage exceeds 60 to 70 percent, should consider a premium AR product with higher copper granule concentration combined with zinc strips at the ridges.
Pressure washing is the most common DIY approach to cleaning a roof, and it’s also the most damaging. A standard pressure washer operating at 2,000 to 3,000 PSI will blast the protective granules right off asphalt shingles, exposing the asphalt layer to direct UV radiation and accelerating deterioration.
Granules are the shingle’s primary defense against sun damage. Once they’re removed by pressure washing, the exposed asphalt dries out, cracks, and curls — the same symptoms you’d see on a shingle that’s 20 years old, except it might only be 8 years into its life. In Mandeville’s intense UV environment, granule loss from pressure washing can cut a shingle’s remaining lifespan in half.
Most shingle manufacturers — including GAF, Owens Corning, and CertainTeed — explicitly state in their warranty documents that damage caused by pressure washing is not covered. If you pressure wash your roof and then file a warranty claim for premature shingle failure, the manufacturer will deny it. The evidence (missing granules in a pattern consistent with pressure washing) is obvious to any trained inspector.
For older roofs in Mandeville — homes with shingles that are 15 to 20 years old and have already lost some granule coverage through normal weathering — pressure washing is especially risky. The remaining granules are less firmly embedded, and the shingles are more brittle. A pressure washer on an aging roof can create more damage than the moss and algae it removes.
The Asphalt Roofing Manufacturers Association (ARMA) recommends a low-pressure chemical wash as the safest and most effective method for removing algae and moss from asphalt shingles. Here’s what that involves:
The cleaning solution. A mixture of 50 percent water and 50 percent household bleach (sodium hypochlorite), applied with a garden sprayer at low pressure, kills algae on contact. For moss, the solution needs to sit on the surface for 15 to 20 minutes before rinsing. Some commercial roof cleaning products add surfactants to help the solution cling to sloped surfaces longer.
The application. Spray the solution onto the affected areas from the ridge down, working with the shingle overlap rather than against it. Never spray uphill under the shingle edges — this forces water and chemicals into the seams. Use a pump sprayer or a low-pressure hose attachment, not a pressure washer nozzle.
The rinse. After the dwell time, rinse the roof with a standard garden hose. The algae and moss will die and wash off gradually over the following weeks with normal rainfall. Heavy moss may need gentle brushing with a soft-bristle broom after the chemical treatment loosens its grip.
Plant protection. Bleach runoff can damage landscaping below the roofline. Wet shrubs and ground cover thoroughly with plain water before applying the cleaning solution, and rinse them again afterward. Covering sensitive plants with plastic sheeting provides additional protection.
For homeowners who prefer not to handle the job themselves, professional soft-wash roof cleaning services in the Mandeville and Covington area use the same low-pressure chemical method with commercial-grade equipment and plant protection protocols.
Keeping moss and algae under control on a Mandeville roof requires year-round attention, not a once-a-year cleaning. Here’s a practical maintenance schedule based on the North Shore’s climate patterns:
Spring (March – May): This is the best time for an annual professional roof inspection and maintenance visit. Winter storms may have loosened shingles or displaced flashing. Spring is also when moss enters its active growth phase as temperatures warm and spring rains increase moisture levels. If moss is present, treat it now before it spreads during the summer wet season. Clear all debris from valleys and behind dormers.
Summer (June – August): Afternoon thunderstorms keep roof surfaces wet and feed algae growth. Monitor for new dark streaking, especially on north-facing slopes. Keep gutters clear so water drains quickly rather than backing up under roof edges. Trim any branches that have grown to within 6 feet of the roofline — overhanging branches drop debris, hold moisture, and provide shade that algae and moss need.
Fall (September – November): Hurricane season runs through November, and post-storm inspections are a must. Fall is also when deciduous trees drop leaves en masse. Clear leaves from valleys, gutters, and flat sections weekly during peak leaf drop. Wet leaf piles on a roof surface create the exact conditions moss needs to establish new growth.
Winter (December – February): Mandeville’s mild winters (average lows in the 40s) don’t kill moss the way northern freezes do. Growth slows but doesn’t stop. This is a good time for zinc or copper strip installation, since cooler temperatures are more comfortable for rooftop work and the strips will be in place before spring rains activate them.
If your Mandeville roof is already showing signs of moss or algae damage, or if you’re considering algae-resistant shingles for your next replacement, contact Big Easy Roofers for a professional assessment. We serve homeowners throughout St. Tammany Parish, including Mandeville, Covington, Madisonville, and Slidell.
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Aug 1, 2025