If your home's siding has developed a green tint — particularly on certain walls or in certain areas — you are looking at an algae or mold colonization that is both a cosmetic problem and a potential maintenance issue. In Georgia's climate, green siding is one of the most common exterior complaints we address, and it is almost entirely preventable with the right maintenance approach.
This guide explains exactly why siding turns green, which walls and home types are most vulnerable, how professional soft washing eliminates the problem, and what you can do to slow recurrence.
What Is Actually Growing on Your Siding?
The green color can come from several different organisms, and they are not all the same:
Green algae (most common): True algae, including common species like Chlorella and various filamentous green algae, are photosynthetic organisms that require light, moisture, and a surface to anchor to. They produce the characteristic bright to olive green color. Green algae is soft and filamentous when actively growing, and can be smeared or wiped — though this does not remove the organism's anchoring structure from the siding surface.
Cyanobacteria (sometimes called blue-green algae): A photosynthetic bacterium that appears green, blue-green, or olive colored. Related to the Gloeocapsa magma that causes black roof streaks, cyanobacteria on siding tends to be more persistent than true algae and requires biocidal treatment to eliminate.
Green-tinted mold or mildew: Some mold species produce greenish pigments or appear green when colonizing with other organisms. True mold growth on siding is typically black, gray, or dark brown — but mixed colonies of mold and algae can appear varied in color.
For practical purposes, the distinction matters less than the treatment: all of these organisms respond to sodium hypochlorite-based soft wash treatment. The organism type affects concentration and dwell time, but not the fundamental approach.
Why North-Facing and Shaded Walls Turn Green First
Walk around any Georgia neighborhood and observe which sides of homes show the most green growth. In the vast majority of cases, it is the north-facing walls — and the pattern is not coincidental.
North-facing walls in the northern hemisphere receive no direct sunlight during most of the year. In Atlanta (latitude approximately 33.7°N), north-facing walls may receive minimal indirect light during summer months when the sun is high, but for most of the year they exist in near-perpetual shade. The consequences for exterior surfaces are significant:
Extended wet periods. Surfaces that do not receive direct sun take much longer to dry after rain or morning dew. A south-facing wall may dry within 1-2 hours of a rain event on a sunny day. A north-facing wall on the same home may stay damp for 6-8 hours or until the next day. Every hour of dampness is additional time for algae and biological organisms to feed and grow.
Lower temperatures. Shaded surfaces are cooler. Cooler surfaces accumulate condensation more readily — and condensation is essentially providing a regular watering cycle for biological growth without any visible rain event occurring.
No UV inhibition. UV radiation from direct sunlight is modestly inhibitory to algae growth. Shaded walls provide none of this natural deterrent.
Heavily shaded south or east walls — shaded by mature trees, neighboring structures, or architectural features like deep eaves — also turn green at rates similar to north walls. In Stone Mountain and Decatur, where many homes sit in mature hardwood canopy, every wall of a home can potentially show green growth regardless of orientation.
How Georgia's Climate Accelerates Siding Algae
Metro Atlanta sits in a humid subtropical climate (Köppen classification Cfa) that is nearly ideal for biological growth on exterior surfaces. Here is what works against Atlanta homeowners specifically:
Year-round growing season for algae. Unlike northern states where winter temperatures kill or dormancy-arrest biological growth for several months, Atlanta's mild winters allow algae to grow slowly even in January and February. Growth slows but does not stop. By the time spring arrives, existing colonies are ready to expand rapidly.
High annual rainfall spread across all seasons. At roughly 52 inches per year with no pronounced dry season, Atlanta surfaces rarely go more than a few weeks without significant wetting. This sustained moisture availability is the primary driver of algae growth frequency.
High pollen season feeding organic food to surfaces. Atlanta's spring pollen season is among the most intense in the country. Pollen that settles on siding provides both a food source and a moisture-retaining coating that accelerates biological colonization. After a heavy pollen event, siding that would normally be dry absorbs moisture through the pollen layer and stays damp longer.
Red clay soil splash-back. Georgia's red clay soils, prevalent throughout DeKalb and Gwinnett counties, create iron-rich splash-back when rain hits bare soil near foundation areas. This creates the right conditions for combined clay, algae, and moisture staining on lower siding courses.
What Soft Washing Does That Pressure Washing Cannot
This is a critical distinction for siding cleaning. Pressure washing physically removes visible green contamination but leaves the organism's anchoring structures embedded in the siding's surface texture. Soft washing kills the organism entirely.
The soft wash process for green siding:
- Pre-rinse and protection setup. Adjacent plantings, hardscape, and surfaces that should not receive chemical treatment are protected or pre-rinsed.
- Low-pressure chemical application. A sodium hypochlorite solution (typically 1-3% concentration for siding, lower than roof application) mixed with a surfactant is applied to the siding surface using a soft wash pump that delivers water at under 100 PSI. This is critical — vinyl siding at high pressure can warp, crack, and allow water infiltration behind the panels.
- Dwell time. The chemical solution is allowed to remain on the surface for 5-15 minutes. During this time, the sodium hypochlorite is breaking down the cell walls of the algae and oxidizing the pigment. You will often see the green color fading or yellowing during this stage.
- Rinse. The surface is rinsed with clean water at moderate pressure, removing the dead organism material and chemical residue.
Results are immediate on surface algae and improve over 24-48 hours as remaining dead pigment washes away. In severe cases, a second treatment may be required 2-3 weeks after the first to address areas where the initial treatment needed longer dwell time to penetrate.
Siding Material Considerations
Vinyl siding is the most common siding type in Georgia and is excellent at resisting algae once properly cleaned. Its smooth, non-porous surface provides little texture for algae to anchor into. However, older vinyl that has oxidized and developed a chalky, rough surface texture becomes significantly more susceptible. High-concentration bleach solutions can discolor vinyl if not properly diluted — always confirm your contractor is using siding-appropriate concentrations.
Fiber cement siding (HardiePlank and similar products) is common in higher-end Atlanta neighborhoods and is more susceptible to biological growth than vinyl because its surface is slightly more porous and textured. Pre-primed fiber cement also tends to absorb more moisture. Soft wash treatment works well on fiber cement, but higher concentrations may be needed for established growth.
Wood siding is highly susceptible to biological growth due to its organic, porous nature. Algae actually feeds on wood directly. Wood siding in Georgia needs more frequent attention — typically annually — and may require a stronger treatment solution. After cleaning, a wood-appropriate biocidal primer or sealant dramatically extends the clean period.
Stucco and EIFS are highly textured and porous, providing excellent surfaces for algae to establish. The irregular texture holds moisture and provides anchoring sites. Soft wash is appropriate but requires careful control to prevent chemical saturation of the stucco, which can cause efflorescence (salt leaching) if over-applied.
Prevention: Airflow, Sunlight, and Post-Clean Treatment
After a professional cleaning, several steps can slow algae recurrence:
Improve airflow. Dense foundation plantings that trap humid air against siding are a major contributor to algae growth. Trimming shrubs to maintain 6-12 inches of clearance from the siding surface improves airflow and allows surfaces to dry faster after rain. This is free maintenance that pays dividends in cleaning frequency.
Manage tree canopy. Trimming branches that shade north and east walls improves UV exposure and reduces leaf litter that composts against the siding. Tree limbs that touch the house also provide a highway for insects and biological material to transfer directly to the surface.
Address drainage issues. Gutters that overflow deposit water down the siding face rather than channeling it away from the structure. Keeping gutters clean (see our gutter cleaning service) prevents this type of chronic wetting. Downspout extensions that discharge water away from the foundation reduce splash-back.
Post-clean preventive spray. A light diluted SH treatment applied to affected walls 6-12 months after the initial cleaning resets the biocide effect without requiring a full wash. This is a cost-effective way to extend the clean period, particularly on north walls that experience faster recolonization.
How Frequently Should Georgia Homes Be Washed?
The honest answer varies by the specific site conditions:
- Good sun exposure, no overhanging trees, well-maintained gutters: Every 2-3 years
- Partially shaded with some north-facing walls: Every 2 years
- Heavily wooded lot, significant north-facing walls, or history of rapid regrowth: Annually or every 18 months
We recommend a brief annual inspection even if cleaning is not due — catching early-stage growth before it establishes deeply makes the next cleaning faster and less expensive. Contact Thrare Contracting at (678) 748-3578 for a free inspection and estimate anywhere in the metro Atlanta area.