A Georgia homeowner looking at dark streaking down a north wall wants two things settled: what the growth is, and what removes it without ruining the surface underneath. The category sold back to them is "mold and mildew removal", which names a marketing service rather than an organism. The organism matters, because mold is a fungus, the black staining on most Southeastern roofs is a cyanobacterium, and moss is a plant. Three different kingdoms of life get one label, and the label is the reason so many homeowners buy the wrong treatment twice.
The three groups are separated below, each claim carries the published source behind it, and each section states what the primary documents do not say. Thrare Contracting cleans exteriors across metro Atlanta, and the sourcing below is the same material used to write a scope of work before a crew mobilizes.
What is actually growing on a Georgia exterior wall or roof?
Three unrelated groups of organism produce the discoloration Georgia homeowners call mold. The Asphalt Roofing Manufacturers Association states the roof case directly in its 2012 bulletin on algae and moss: "Often mistaken as mold or mildew, the black discoloration and streaks visible on many roofs across the United States and Canada are actually algae." The same bulletin names the organism: "The most common type, gloeocapsa magma or blue green algae, protects itself from damaging ultraviolet rays by producing a dark pigmented sheath."
Blue-green algae is the older common name, and the modern classification is not algae at all. The NCBI Taxonomy database maintained by the National Library of Medicine records the genus Gloeocapsa under taxonomy ID 102231 with the lineage "cellular organisms; Bacteria; Bacillati; Cyanobacteriota/Melainabacteria group; Cyanobacteriota; Cyanophyceae; Chroococcidiopsidales; Chroococcidiopsidaceae". Gloeocapsa sits in the domain Bacteria, phylum Cyanobacteriota. Mold is a fungus. A cyanobacterium and a fungus are about as distantly related as two living things on a wall get.
Moss is the third group, and the Asphalt Roofing Manufacturers Association describes moss as "a non-vascular plant that obtains water through its leaves, unlike most plants that gather water through its root system." Moss grows in the thick shaded seams where debris collects, holds water in the material it grows on, and does not respond to sunlight the way the pigmented sheath on Gloeocapsa does.
Two limits belong on that paragraph. The Asphalt Roofing Manufacturers Association bulletin is written about asphalt shingle roofs, so ARMA's sentence is not a claim about siding, brick or concrete. NCBI Taxonomy is a classification database rather than a field survey, so it establishes what Gloeocapsa is and not what share of any particular Georgia wall carries it. Nobody should publish a percentage there, and Thrare Contracting does not.
Why does the name of the organism change the correct treatment?
Fungi and cyanobacteria feed differently, and the difference decides what a cleaning has to remove. A fungus consumes organic material, so mold on a wall is growing in pollen, dust and decomposing debris that landed there. A cyanobacterium photosynthesizes, so Gloeocapsa needs light and moisture and mineral surface rather than a food layer. Photosynthesis is why a scrubbed-clean inorganic surface still darkens, and why removing the color without killing the organism buys a short interval.
Chlorine chemistry reaches all three groups. EPA describes the registered use of hypochlorite salts in its 1991 R.E.D. FACTS document 738-F-91-108: "The hypochlorites control bacteria, fungi, and slime-forming algae that can cause diseases in people and animals." Oxygen chemistry is registered against a narrower list. EPA's 2002 fact sheet for sodium carbonate peroxyhydrate lists the target pests as "Algae, moss, liver worts, slime molds and their spores."
Neither EPA document is a cleaning instruction for a building, and neither lists house siding among its registered use sites. Both establish what the chemistry is registered to kill, which is the part a homeowner cannot look up on a bottle in a store aisle.
What keeps a metro Atlanta wall wet long enough for anything to grow?
Rainfall totals are the figure everyone quotes and the wrong one for biological growth. Growth tracks how long a surface stays wet, not how much water fell. NOAA's National Centers for Environmental Information publishes both numbers for station USW00013874, Atlanta Hartsfield International Airport, in the 1991-2020 US Climate Normals.
- Annual precipitation normal: 50.43 inches.
- Average number of days per year with precipitation of at least 0.01 inch: 116.3 days.
- Average number of days per year with precipitation of at least 0.10 inch: 76.5 days.
- Annual mean temperature normal: 63.6 degrees F.
A wall at that station gets wet on roughly one day in three across a normal year. NOAA's hourly normals for the same station describe the other half of the mechanism. For the calendar date July 1, the temperature normal at 6 in the morning is 71.6 degrees F and the dew point normal is 67.2 degrees F, a gap of 4.4 degrees. By 3 in the afternoon on the same date the temperature normal has risen to 86.1 degrees F while the dew point normal sits at 66.6 degrees F. Overnight the air is close to saturation and a shaded surface that has radiated its heat away is close to condensing; by mid afternoon a sunlit surface has dried and a shaded one has not.
Four cautions belong with those figures. NOAA publishes them for one station rather than for a county, a normal is a 30 year average and not a measurement of any year, the hourly figures are for a specific calendar date rather than a month, and NCEI publishes dew point hourly normals for this station without relative humidity. No metro Atlanta relative humidity percentage appears on this page for that reason.
Airborne debris is the other input, and metro Atlanta has a certified measurement of it. Atlanta Allergy & Asthma operates what its pollen page describes as the only pollen counting station in the Atlanta area certified by the National Allergy Bureau, and publishes a tree pollen severity scale running from low at 0 to 14 grains per cubic meter through extremely high at 1,500 and above. The severity scale reports symptom severity rather than the mass of material landing on a roof, so nobody should convert a pollen count into a debris load, and this page does not.
Does growth on the outside of the house mean there is a mold problem inside?
Growth on an exterior wall is not evidence of an indoor mold problem, and the federal guidance on indoor mold is written for a different question. EPA's guide to mold, moisture and the home states the governing principle in one line: "There are many types of mold, and none of them will grow without water or moisture." EPA also states the ceiling homeowners keep asking for: "Keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity."
CDC publishes a tighter number in its own guidance, telling homeowners to keep humidity levels in the home as low as possible and no higher than 50 percent all day long. EPA and CDC differ by 10 points, which is a reason to quote both agencies by name rather than blending them into a single figure. CDC also states plainly that CDC does not recommend mold testing.
CDC describes the health picture without overstating it: "Exposure to damp and moldy environments may cause a variety of health effects, or none at all." For sensitive people CDC lists "a stuffy nose, sore throat, coughing or wheezing, burning eyes, or skin rash", and identifies the higher-risk groups as people with asthma, people allergic to mold, immune-compromised people and people with chronic lung disease. Nothing in either agency document says that washing exterior siding changes anyone's symptoms, and Thrare Contracting makes no such claim.
Why does EPA advise against bleach indoors, and does that ruling reach an exterior wall?
EPA's position on biocides in indoor mold cleanup is explicit: "The use of a chemical or biocide that kills organisms such as mold (chlorine bleach, for example) is not recommended as a routine practice during mold cleanup." EPA wrote that sentence about cleaning up mold inside a building, where the fix is removing the moisture and the affected material rather than killing what is visible. EPA reinforces the point elsewhere in the same guide: "It is impossible to get rid of all mold and mold spores indoors; some mold spores will be found floating through the air and in house dust."
Extending that sentence to a Georgia exterior would misrepresent it. EPA and CDC publish no exterior wall equivalent, and neither agency publishes a house washing method. The governing document outdoors is the registered label on the product being applied, which is the subject of the next two sections. A contractor who quotes EPA as forbidding bleach on siding, or as endorsing it, is citing a document that does not address the surface.
What does sodium hypochlorite do, and what does sodium percarbonate do instead?
Sodium hypochlorite and sodium percarbonate are both oxidizers, and both are registered pesticide active ingredients. The similarity ends at the oxidation. EPA's R.E.D. FACTS document for sodium and calcium hypochlorite salts describes them as "chlorinated inorganic disinfectants used in laundries, swimming pools, ponds, drinking water, and other water and wastewater systems", first registered for use as pesticides in 1957. EPA records the hazard in the same document: "Sodium and calcium hypochlorite are extremely corrosive and can cause severe damage to the eyes and skin. They have been assigned to Toxicity Category I, indicating the highest degree of toxicity, for these acute effects."
EPA also records what hypochlorite becomes: "In the presence of oxygen, these compounds react easily with organic matter and convert readily into sodium chloride (table salt) and calcium chloride (road salt)." The reaction with organic matter is exactly why a hypochlorite mix loses strength as it works, and why the concentration in the tank is not the concentration reaching the last panel on a dirty wall.
Sodium percarbonate is the compound sold as oxygen bleach, indexed by PubChem under CID 159762 as sodium carbonate peroxyhydrate, CAS 15630-89-4. EPA's 2002 fact sheet gives the mode of action in three sentences: "When water is present, the compound breaks down into hydrogen peroxide and sodium carbonate. The hydrogen peroxide oxidizes and thus kills the target pests. After contact, the hydrogen peroxide breaks down harmlessly into water and oxygen." EPA describes the end product as an oxidizing agent that "kills the target algal and fungal pests."
Oxygen bleach is sold as the gentle option and EPA does not describe it as gentle. The same fact sheet records "severe irreversible eye damage" in testing and states that "Workers are required to wear appropriate protective equipment to protect themselves, especially their eyes, from exposure to this corrosive substance." The practical difference for a homeowner is what each chemistry does to the substrate and the plants beside it, not which one is safe to splash.
One more limit belongs here. EPA registered sodium carbonate peroxyhydrate for "Ornamental plants, turf grasses, terrestrial landscapes, in commercial greenhouses, garden centers, nurseries and storage areas", and states there are no food uses. House siding, roofs and hardscape do not appear on that list. A percarbonate product sold for washing a deck is a different registered product with its own label, and the label is what governs its use.
Is a mold and mildew remover a regulated product, and who decides the dwell time?
A product that claims to kill mold, mildew or algae is making an antimicrobial pesticide claim, and EPA regulates antimicrobial pesticides. EPA defines them as "substances or mixtures of substances used to destroy or suppress the growth of harmful microorganisms such as bacteria, viruses, or fungi on inanimate objects and surfaces." EPA's non-public-health category covers products that "control growth of microorganisms of economic and aesthetic significance", and the examples EPA lists include algae.
Federal law makes the label enforceable rather than advisory. Under 7 U.S.C. 136j(a)(2)(G), it is unlawful "to use any registered pesticide in a manner inconsistent with its labeling." Under 7 U.S.C. 136j(a)(1)(A), it is unlawful to distribute or sell "any pesticide that is not registered under section 136a of this title or whose registration has been canceled or suspended", subject to the exceptions the statute names. EPA states the registration requirement in its own words in the hypochlorite document: "All pesticides sold or used in the United States must be registered by EPA, based on scientific studies showing that they can be used without posing unreasonable risks to people or the environment."
The label rule settles the question homeowners ask most about soft washing, which is how long the solution sits before rinsing. No regulator publishes a universal dwell time for an exterior wall. Manufacturers publish dwell times for their own materials, and those numbers agree with each other more than the trade web does.
- Asphalt Roofing Manufacturers Association, for asphalt shingle roofs, 2012 bulletin: apply with a sprayer "and allow the solution to dwell on the roof surface for 15 to 20 minutes, and then rinse thoroughly with low pressure water."
- Sto Corp., for acrylic and acrylic elastomeric finishes: apply the cleaning solution "and allow to soak for a period of 15-20 minutes", and bleach that "is not allowed sufficient time (about 15 minutes) to sit on the surface and kill the micro-organisms" leaves algae or mildew to re-occur.
- Sika Corporation, LaHabra bulletin for EIFS and stucco: "Allow the solution to soak the wall for approximately 15 minutes, but do not allow the cleaning solution to dry on the wall."
Three manufacturer documents converge on roughly 15 minutes for three different materials, and none of the three is a regulation. Where a registered product label states a contact time, 7 U.S.C. 136j(a)(2)(G) makes the label the controlling number and the manufacturer bulletin the secondary one. Sika adds a warning specific to this climate: "Trisodium phosphate detergent is a food source for mold. Do not use trisodium phosphate detergent without bleach in hot humid climates."
What cleaning pressure does each Georgia wall material's own document publish?
No single pressure is correct for a house, because the published limits differ by a factor of six across the materials on one elevation. Each figure below comes from the document that governs that material, and each applies only to the material its own document names.
| Material | Published figure | Source document |
|---|---|---|
| Brick and mortar | Do not exceed 400 psi at the nozzle tip unless the brick manufacturer permits it; 25 to 50 degree fan tip held a minimum of 12 inches from the surface | Brick Industry Association, Technical Notes 20, "Cleaning Brickwork", August 2018 |
| Fiber cement siding | Wide fan tips kept a minimum of 6 feet from the wall, pressures under 1500 psi; improper cleaning affects warranty coverage | James Hardie Building Products, Technical Bulletin #20, November 2018 |
| Vinyl siding | 40 degree tip, pressure set between 1,500 and 2,400 psi, washer held straight at eye level and never aimed upward | Georgia-Pacific Vinyl Siding and Ply Gem care guides |
| EIFS and acrylic finish | Approximately 200 to 500 psi and do not exceed 500 psi; nozzle no closer than 2 feet; no steam or high temperature cleaning | Sto Corp. and Sika Corporation manufacturer bulletins |
| Asphalt shingle roof | No pressure washing at any setting | Asphalt Roofing Manufacturers Association, 2012 |
The roof line is the one with no range at all. The Asphalt Roofing Manufacturers Association states it as an absolute: "Never use a pressure washer to clean an asphalt shingle roof as this will cause granule loss and very likely premature failure of the roof system." Sika states the underlying principle for coated walls in a single sentence: "The chemicals provide the cleaning, not the water pressure."
Every figure in that table comes from a trade association or a manufacturer rather than from a building code, and manufacturer literature governs that manufacturer's product only. A vinyl wall of a different brand needs that brand's instruction, which is what the trade association itself says. Thrare Contracting identifies the cladding on each elevation before a pressure is chosen, and details of the method by surface sit on the exterior cleaning page and the building washing methods guide.
Where does the rinse water go, and why is that part of the job rather than an afterthought?
Wash water carrying a biocide is not rainwater, and EPA records the reason containment matters in the hypochlorite document itself: "Sodium and calcium hypochlorite are low in toxicity to avian wildlife, but they are highly toxic to freshwater fish and invertebrates." EPA also notes that in fresh water the hypochlorites "break down rapidly into non-toxic compounds when exposed to sunlight", which describes what happens after a discharge and not permission to make one.
The federal rule is structural. Under 40 CFR 122.26, a discharge to a municipal separate storm sewer system that is not composed entirely of storm water requires authorization under an NPDES permit, and wash water carrying detergent or biocide is not composed entirely of storm water. Gwinnett County's surface cleaning guideline WQ-1 removes the most common workaround in one sentence: "There are NO 'environmentally friendly' or 'biodegradable' cleaning products that are approved for discharge to storm drains."
On a house on a wooded lot, controlling the rinse means landscape saturation, blocking a drain inlet and directing flow to a vegetated area. On a townhome, a shared drive or any multifamily or commercial elevation, controlling the rinse means recovery equipment. Gwinnett County's consumer guidance states the buyer-side test bluntly: "The management of polluted wash water often requires the use of specialized equipment. Not all contractors have this equipment and may not be able to comply with the law." No dollar penalty figure appears on this page, because published penalty amounts for this rule circulate through secondary sources rather than from the statute.
What is included when Thrare Contracting washes a Georgia exterior, and what is not?
An estimate from Thrare Contracting names the method and the chemistry for each surface rather than quoting one price for a house. Included in a residential exterior wash:
- Identification of each cladding and roof material on the property, and the published pressure and temperature limit that governs it.
- Pre-wetting and protection of landscaping, light fixtures, meters, and any surface not being treated.
- Application of the treatment at the dwell time the product label states, with rinsing top to bottom so untreated runoff does not streak cleaned work below it.
- Control of the rinse so that treated water does not reach a storm drain inlet.
- A written scope naming which elevations, which surfaces, and which method applies to each.
Not included, and quoted separately when a property needs it:
- Gutter interior cleaning, which is a separate scope with separate access requirements. Guidance on why that matters for the wall below sits on the gutter cleaning and foundation guide.
- Roof treatment, which follows the roof method rather than the wall method and is scoped on the roof cleaning page.
- Repair of the moisture source: failed caulk, a leaking gutter joint, a sprinkler head aimed at the wall, an HVAC condensate line discharging against siding.
- Sealing, staining or painting of any surface.
- Interior mold assessment or remediation, which is a different trade and is not work Thrare Contracting performs.
- Vegetation removal or tree trimming, which is the change that most extends the interval and is a separate trade.
Pricing is quoted per property after the surfaces are identified, because a two-story fiber cement elevation with a shaded rear wall and a single-story vinyl ranch in full sun are different jobs with the same square footage.
What does Thrare Contracting need from a property before the crew arrives?
- The cladding brand where the homeowner knows it, because the manufacturer document governs the pressure and the brands publish different numbers.
- A working exterior water source and an accessible spigot.
- Vehicles moved off the drive and away from the elevations being washed.
- Windows and storm windows closed, and any known failed seal or open gap pointed out in advance.
- Location of the nearest storm drain inlet and of any pond, creek or drainage swale on the lot.
- Any plant the owner considers irreplaceable identified before the crew starts, so protection goes beyond the routine.
- Access arrangements for a gated rear yard, and confirmation that pets are inside.
How often does a Georgia exterior need washing?
No regulator, trade association or manufacturer document in the sources below publishes a washing frequency for a house exterior. Any contractor quoting an interval as a standard is quoting a habit. The honest answer is that the interval is set by how long each elevation stays wet, and NOAA's normals explain why that varies so much across one property: a wall that dries by mid-morning and a wall that never leaves shade are on the same lot under the same 116.3 wet days a year.
What lengthens the interval is documented, even where the interval itself is not. EPA's principle that nothing grows without water or moisture applies outdoors as much as indoors, and the moisture sources on a Georgia house are specific: canopy touching or overhanging the wall, a gutter overflowing down one elevation, irrigation striking the siding, a condensate line discharging at grade, and grade that holds water against the foundation. Removing one of those changes the drying time on that elevation permanently. Cleaning again does not.
Thrare Contracting washes house exteriors, roofs and hardscape across metro Atlanta from an office at 8735 Dunwoody Pl, Ste R, Atlanta, GA 30350. Call (678) 748-3578 for an assessment that names the materials on the property and the method for each one. Related scopes sit on the house washing page and the soft washing page, and the roof-specific version of the Gloeocapsa question is answered on the black streaks guide.