TECHNICAL NOTES
MOULD AND ALGAE ON EXTERIOR
SURFACES
MAINTAINING YOUR HOME – MOULD AND ALGAE ON EXTERIOR SURFACES
Organic growth on external surfaces is typically naturally occurring micro-organisms of fungi or algae
which left unchecked can cause blemishes or staining of painted or unpainted surfaces. The resultant
blemishes are the result of site and environmental conditions that support or “feed” their growth.
The key to reducing the risk of contamination is managing the conditions that otherwise fuels growth.
MOULD OR ALGAE?
The distinction becomes important when we consider control measures.
Algae are plants that derive their energy from chloroplast requiring light energy from the sun.
Mould is actually a form of Fungi which thrive in conditions of low or no light providing there is
adequate moisture and a nutrient source (eg any organic matter such as leaves, wood, dirt etc). Fungi
can be spread by airborne spores that are present in surrounding environments.
Treatment of either Fungi (mould) or Algae is similar typically involving cleaning and sterilising.
Prevention of growth / re-growth requires maintenance of the surface and environment to eliminate
one or more of the conditions required for growth.
PAINTED SURFACES
Fungi and Algae can exist on virtually any surface (even glass) provided the right conditions for
growth are met.
Visible growth on painted surfaces is typically caused by contaminants present together with the
presence of high enough levels of moisture to support growth.
Whilst paints can be “fortified” with agents restricting growth, protection relies on reaching the
growth and the active agent remaining “active”.
Agents in paints become ineffective where they cannot “touch” the growth source (eg where growth
emanates from deposits on the film). Additionally the active agents are “consumed” in the process
such that protection is time limited where conditions support ongoing growth.
KEEPING SURFACES FREE OF MOULD/ALGAE
Mould and Algae are essential parts of the earths’ biomass & natural growth will occur where
favourable conditions exist. Typically algae is more prevalent on roofing and horizontal surfaces
receiving maximum light and moisture.
Unlike Fungi, Algae requires light to grow.
Fungi (mould) do not require light but thrive in damp environments.
In all cases maintenance of surfaces and surrounding environments is required to restrict growth /
regrowth involving:
Regular cleaning of surfaces to remove dirt and airborne deposits
Restriction of nutrient sources such as leaves, plant matter, waste
Specific attention to areas of high moisture and/or poor ventilation creating ideal growth
conditions
BEST PRACTICE TO MAINTAIN YOUR EXTERIOR FACADE.
Organic growth on external surfaces is a sign that the prevailing conditions that exist (eg. high surface
or substrate moisture levels, organic nutrient availability, airborne mould spores) are conducive to
organic growth. Once initiated, natural growth will develop and continue until treatment is effected
and one or all of the conditions are controlled.
Where you see mould on a surface you need to look for potential causes - generally dirt accumulating
on the surface (containing contaminants or the “food source”) and excess moisture (potentially poor
ventilation, drainage, leaks allowing the moisture level to stay permanently high)
Overwhelmingly, the easiest condition to control is the reduction of surface contamination through
regular and simple cleaning Dirt, organic matter, airborne pollution left unchecked provides both the
nutrient source and increased moisture retention.
PREVENTION IS BETTER THAN CURE!
Whether your building is cement render, brick, masonry or timber, all exterior painted surfaces will
benefit greatly from an annual “spring clean”. Dependant on the site conditions and location more
frequent cleaning may be required, examples include:
Coastal environments continuously subjected to wind spray may collect more dirt and airborne matter
and benefit from more regular cleaning
Rural locations subject to increased airborne plant / organic matter or industrial areas are at higher
risk of surface contamination supporting growth.
Locations with high airborne yeasts (such as near breweries or bakeries) are at increased risk.
A simple but routine maintenance program of the external surface involving inspection & cleaning
also provides for identification of potential issues before they develop into costly repairs. Pay
particular attention to likely water ingress points such as around windows & flashings, gutters &
downpipes and drainage away from walls.
Prepare a bucket of warm water mixed with a moderate amount of household detergent. Caustic-based
preparations should not be used, as they may damage the coating. Hose down walls to remove loose
dust and dirt.
Apply the cleaning solution to wet walls with a soft bristle brush, or broom. Localised grime or
ingrained dirt should be removed by cleaning with gentle scrubbing. Before the cleaned walls areas
dry, hose it down thoroughly with clean water.
Note : Cleaning by High Pressure Wash is generally not recommended as damage my occur where
pressures are excessive. If used in conjunction with soft brushing or brooming or for rinsing a
maximum pressure of 450-psi is recommended using a wide fan jet angled at 45 degree and kept at
least 20cm away from the surface.
Dependant on location and environment professional cleaning and recoating every 5-7 years is
recommended to maintain premium aesthetics. Warranty periods of materials (including paint)
typically do not cover normal exposure weathering and aesthetics.
TREATMENT OF EXTERIOR WALLS WITH MOULD OR ALGAE
Where organic growth has initiated and spread, remedial action is required to treat the infection -
Cleaning alone typically will not prevent further growth
In minor instances where growth is minimal, isolated to a specific area, and discoloration of the
surface is not ingrained, spot treatment with a mild household bleach solution (5% Sodium
Hypochlorite) may be practical.
It should be noted that in such cases :
The spot treatment may “bleach” the colour - a test area in an inconspicuous area is recommended
The treatment provides an “initial kill” but offers no residual protection – attention to the conditions
supporting growth as defined is required for longer term protection.
Where ingrained growth has developed and staining of the surface has occurred,
Pre-Treatment and Recoating System approach is required consisting of:
Pre-Cleaning, Sodium Hypochlorite Treatment and Residual Effect Sanitising of the surface
Encapsulation using a Sealer Coat to restrict moisture penetration and stain migration
Recoating with 2 coats of “Tropical Strength” (high Fungal / Algal protection) Topcoat.
