Specifications on Roof Cleaning SoftWashing from the Manufacturers

Published: July 30, 2023
Written by: blueadmin2

Technical Bulletin by BlueScope Steel – Issue Date May 2022

This Technical Bulletin’s goal is to shed light on the fungus’ occurrence, characterization, identification, and cleansing. Fungal growth can occur on a number of building materials, however the information in this Technical Bulletin mainly applies to COLORBOND® prepainted steel.
As many as 10,000 fungal spores may be present in one cubic metre of the air surrounding us, according to estimates1,2. These spores are continually adhering to surfaces of all kinds. Although most spores are washed away by rain, in specific environments or micro-environments, some species may adhere and multiply to create colonies that can result in the surface in question becoming noticeably darker.

Figure 1. An example of lichenous
growth across the surface of a tiled roof

Figure 2. A close up of a) fungal and b) lichenous growth

No other building material is more prone to fungus growth and discoloration than COLORBOND® steel. For instance, skylights made of glass or polycarbonate, painted wood, PVC pipes, powder-coated handrails, and roof tiles are all susceptible to fungus growth and discoloration. So, where COLORBOND® steel installations exhibit fungal staining, nearby buildings and fences will often exhibit comparable signs of infestation. Lichens are occasionally visible in areas with extremely good local environmental conditions, typically covering the entire roofing surface. An illustration of lichenous growth on a tile roof is shown in Figure 1.

A range of building materials, including COLORBOND® steel, have shown signs of fungal development in several tropical and temperate regions of Australia and beyond. Fungal growth happens in places where the “local environment” or a solitary “micro-environment” is favourable, that is, when moisture, heat, humidity, and nutrients are present in the proper proportions. In the context of roofing, examples of favourable microenvironments include the ridge-capping, the vicinity of bathroom vents, or other sources of moisture and nutrients.

Based on the organism and dispersion, there are numerous fundamental types of fungus-affected areas. Fungus and lichen (a symbiosis of fungus and algae), two types of visible organisms on roofs, can be broadly categorised. While lichen appears green and frequently has leaves, fungus is typically black. Look at Figure 2.

Table 1. Guidelines for dilution of different strength bleaches to achieve a 2% sodium hypochlorite solution

NOTE: 1% = 10 grams of sodium hypochlorite per 1 litre of water.

Figure 3. Roof made from COLORBOND® steel showing isolated fungal growth near the ridge capping

Figure 4. Roof made from COLORBOND® steel showing spotty lichen and fungus growth spread across the structure

Figure 5. Roof made from COLORBOND® steel showing uniform coverage of fungus

Distribution can come in 3 different forms:

  1. Individual growth
    Around the ridge-capping of a building, it’s common to notice dense fungal growth. Look at Figure 3.
    The ‘ridge cap’ effect is thought to be caused by the conducive micro-environment for fungal growth that exists in this area as a result of moisture and/or nutrients leaving the roof cavity at the intersection of the roofing sheets and the ridge capping.
  2. erratic growth
    On the roof, there may be globular or scattered organic stuff. Patches of lichen are a frequent sign of this. Look at Figure 4.
  3. Consistent coverage
    It is also possible for growth to spread throughout the entire roof area rather than being restricted to a single location. Any outdoor construction that is in a favourable environment will typically see this type of expansion. On lighter-colored roofs, this form of growth can develop very heavily, changing the roof’s overall colour and look. Look at Figure 5.

A helpful starting point for fungal identification is the data shown in the Characteristics section above.
It is therefore advised to conduct a spot test using a sodium hypochlorite solution. Bleach is the most practical way to obtain sodium hypochlorite. The hypochlorite/available chlorine concentration of the bleach should ideally be over 3%, and it should be as fresh as feasible. The solution should always be handled carefully because it is alkaline. Always wear rubber gloves and adhere to the manufacturer’s safety recommendations. Any places that have been tested should then be properly washed with clean, potable water.

Apply a drop of the bleach solution to the suspected region, wait a few minutes, and then subtly indicate the area with chalk or a coloured pencil the extent of the drop. If fungus is present, the bleach will eliminate the black stuff, leaving a spotless drop area.
Please take note that since bleach can also impact other species like algae and bleachable organic waste, this test is NOT 100% accurate for identifying fungi. However, it is a useful test since it can tell fungi apart from regular inorganic soil, which cannot be bleached.

  • Marking with a black “lead” or graphite pencil is not recommended; more details can be found in Corrosion Technical Bulletin CTB-12 Dissimilar metals.

Application and Cleaning

It is advised to wash down a COLORBOND® steel roof with a 2% sodium hypochlorite solution to clean it. Bleaches sold in stores can be used to create a sodium hypochlorite solution. Table 1 illustrates the dilution needed for various bleach strengths to produce a 2% solution to help in solution preparation.
Apply the bleach with a soft-bristled broom, let it sit for three to five minutes, and then thoroughly rinse it off with clean, drinkable water. If additional non-ionic detergent is required to promote wetting, such as dishwashing detergent or high surfactant with high grip, a tiny amount can be added to the bleach.

It is IMPORTANT to remember that the long-term performance of COLORBOND® steel products may be harmed by the use of greater than advised concentrations of sodium hypochlorite solution, extended contact durations, or the application of commercially available fungicides.
When employing hypochlorite solution, caution MUST be exercised to prevent any solution from washing into water tanks.
Since the fungus is a natural occurrence on COLORBOND® steel, cleaning is still the owner’s obligation.


  1. J. Fröhlich-Nowoisky, D. A. Pickersgill, V. R. Després and U. Pöschl ‘High diversity of fungi in air particulate matter’ Proceedings of
    the National Academy of Sciences, 2009, 106, pp. 12814 – 12819.
  2. W. Elbert, P. E. Taylor, M. O. Andreae, and U. Pöschl ‘Contribution of fungi to primary biogenic aerosols in the atmosphere: wet
    and dry discharged spores, carbohydrates, and inorganic ions’ Atmospheric Chemistry and Physics, 2007, 7, pp. 4569 – 4588.

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The information and advice contained in this Technical Bulletin (‘Bulletin’) is of a general nature only and has not been prepared with your specific needs in mind. You should always obtain specialist advice to ensure that the materials, approach and techniques referred to in this Bulletin meet your specific requirements. BlueScope Steel Limited makes no warranty as to the accuracy, completeness or reliability of any estimates, opinions or other information contained in this Bulletin and to the maximum extent permitted by law, BlueScope Steel Limited disclaims all liability and responsibility for any loss or damage, direct or indirect, which may be suffered by any person acting in reliance on anything contained in or omitted from this Bulletin. COLORBOND®, BlueScope and the BlueScope brand mark are registered trademarks of BlueScope Steel Limited. © 2022 BlueScope Steel Limited ABN 16 000 011 058. All rights reserved.

Last updated: July 30 2023

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