How to Clean Mould and Lichen from Terracotta Roof Tiles

Terracotta roof tiles are a classic part of the Glenelg streetscape, from Federation homes near Moseley Square to the character cottages and renovated villas around Brighton Road and Partridge Street. Terracotta is durable and attractive, but it is also porous, which makes it a magnet for mould, algae, moss and lichen. Glenelg's coastal position, with salt-laden sea air, morning dew and cooler, damp winter months, creates ideal conditions for these growths to take hold. This guide explains how to identify what is growing on your roof, how to clean it safely, and how to keep it from coming back in 2026 and beyond.

Why Terracotta Tiles Attract Mould and Lichen

Terracotta is a fired clay product with a naturally porous surface. Over time, tiny pits and crevices absorb moisture and trap airborne dust, pollen, salt and organic matter. That creates a perfect environment for biological growth, especially on south-facing slopes and areas shaded by trees, neighbouring buildings or chimneys.

In Glenelg, several local factors make the problem more common:

Coastal humidity: Moist sea air keeps tile surfaces damp for longer, particularly overnight and in the cooler months.
Shaded roof sections: Slopes that miss direct sun dry slowly, encouraging algae and moss.
Older roofs: Tiles that have weathered over decades develop rougher surfaces that hold more moisture.
Overhanging trees: Leaves and debris trap moisture against the tiles and feed growth.
Low roof pitch: Gentler slopes shed water more slowly, so tiles stay wet longer.

Identifying What Is Growing on Your Roof

Different growths need slightly different treatment, so it helps to know what you are dealing with.

Mould and Algae

Mould and algae usually appear as black, dark green or brown streaks and patches. They are most visible on the shaded side of the roof. They are mainly a cosmetic issue at first, but left alone they hold moisture against the tile and can speed up surface deterioration.

Moss

Moss forms thick, green, cushion-like clumps, often along tile edges, in valleys and where tiles overlap. It holds a lot of water and can lift tiles slightly, which may allow water to get underneath.

Lichen

Lichen is a combination of fungus and algae living together. It appears as flat, crusty circular patches in grey, green, yellow or orange. Lichen grips tightly to the tile surface and is much harder to remove than algae or moss. It is also slow to regrow when properly treated, but it is stubborn and often needs a dedicated treatment product and patience.

Safety First: Before You Start

Roof cleaning is one of the more hazardous home maintenance jobs. Falls from roofs cause serious injuries every year in Australia, and terracotta tiles become extremely slippery when wet or covered in growth. Consider these points before you begin:

Never walk on a wet or mossy roof. Wait for dry conditions and avoid windy days, which are common near the Glenelg foreshore.
Use appropriate fall protection, including a harness and secured anchor points, if you are working at height.
Walk on tile ridges or overlaps with soft-soled shoes, and be aware that old terracotta tiles can crack under body weight.
Keep clear of power lines, solar panel wiring and TV antennas.
Wear eye protection, gloves and long sleeves when handling cleaning chemicals.
Do not work alone, and consider hiring a licensed professional for steep, double-storey or heritage-listed roofs.

If your home is in a heritage area or is a listed property, check with the City of Holdfast Bay about any requirements before carrying out significant work or changing the appearance of the roof.

Step-by-Step: How to Clean Terracotta Roof Tiles

Here is a practical process for https://glenelgpressurecleaning.com.au/sanitation-cleaning/ cleaning mould, algae, moss and lichen from terracotta tiles. Take your time and work in sections.

Clear loose debris. Remove leaves, twigs and loose moss by hand or with a soft-bristled broom. Clear gutters and valleys at the same time.
Inspect the tiles. Look for cracked, chipped or loose tiles and damaged pointing along ridge capping. Mark these for repair before any washing, because high-pressure water can force moisture through damaged areas.
Protect surroundings. Cover or wet down garden beds, lawn and plants below the roof. Close windows and doors and move outdoor furniture and vehicles away.
Apply a suitable roof treatment. Use a purpose-made roof cleaner or fungicidal wash designed for masonry and clay tiles. Apply it evenly with a low-pressure sprayer, working from the ridge downward so product does not run over untreated areas.
Allow time to work. Most products need to sit for the time listed on the label, which can range from 20 minutes to several hours. Do not let the solution dry on the roof unless the product instructs it.
Gently remove growth. Use a soft brush or low-pressure rinse to lift dead moss and algae. Work along the length of the tile in the direction of water flow.
Rinse thoroughly. Wash residue away with clean water, again working from top to bottom.
Reassess and repeat if needed. Lichen often needs a second treatment, and it can take several weeks for treated growth to die off and fade naturally.

Cleaning Products: What Works and What to Avoid

Commonly Used Treatments

Many Australian roof cleaning products contain quaternary ammonium compounds (often called quats) or other biocides designed to kill algae, moss and lichen with minimal damage to tiles. These are generally gentler on terracotta than strong bleach-based solutions, and many are formulated to continue working after application, gradually clearing the roof over a few weeks.

Sodium hypochlorite (pool chlorine or household bleach diluted to a suitable strength) is sometimes used by professionals, but it requires careful handling, proper dilution and thorough rinsing. It can harm plants, corrode metal flashings and gutters, and affect the colour of some tiles if misused.

What to Avoid

High-pressure washing on old tiles: Aggressive pressure strips away the protective surface layer, makes the tile more porous, and can cause cracking. This often leads to faster regrowth.
Wire brushes and scrapers: These scratch the surface and leave tiny grooves where moisture and spores collect.
Acid-based cleaners: Strong acids can damage terracotta and any glaze or coating.
Mixing chemicals: Never combine bleach with ammonia-based or acidic products, as this can release toxic gases.

Protecting Your Garden and the Environment

Runoff from roof cleaning flows into gutters, downpipes and stormwater drains, which in Glenelg ultimately lead toward the coast and Gulf St Vincent. Chemical runoff can harm plants, pets and marine life. To reduce your impact:

Choose biodegradable or low-toxicity products where possible, pre-wet garden beds, divert downpipes onto lawn or garden areas where suitable, and avoid cleaning before expected rain, which can wash chemicals into drains before they have done their work. Check product labels and local council guidance on disposal of wash water. Professional roof cleaners in South Australia are expected to manage wastewater responsibly, so ask about their runoff controls if you hire someone.

Preventing Mould and Lichen From Returning

Cleaning is only part of the job. Prevention helps keep your roof looking good for longer and reduces how often you need to clean.

Trim Overhanging Branches

More sunlight and airflow help tiles dry faster. Trim back branches that shade the roof or drop leaves onto it.

Keep Gutters Clear

Blocked gutters hold water that backs up under the lower row of tiles. Regular gutter cleaning, ideally in autumn and again in late winter, makes a noticeable difference.

Apply a Preventative Treatment

After cleaning, a long-lasting anti-fungal or algaecide treatment can slow regrowth. Some products are designed to be applied as a mist and left to work over time without rinsing. Reapplication every one to two years is common, depending on shade and exposure.

Consider Sealing, With Caution

Penetrating sealers can reduce moisture absorption and make tiles less welcoming to growth, but they must be breathable and suited to terracotta. A poor-quality or incorrectly applied sealer can trap moisture, cause flaking or change the tile appearance. Get advice from a roofing specialist before sealing, especially on older or heritage tiles.

Consider Zinc or Copper Strips

Metal strips installed near the ridge release trace ions when it rains, which inhibit growth on the tiles below. They are a long-term, low-maintenance option, though results vary and they work best on roofs with adequate rainfall runoff.

When to Call a Professional

While light algae and moss can sometimes be handled safely by a confident homeowner, there are plenty of situations where professional help is the better choice:

The roof is steep, high or has limited safe access.
There is extensive lichen or thick moss growth.
Tiles are old, brittle or already cracked.
You notice sagging, leaks, loose ridge capping or damaged pointing.
Your home has solar panels, skylights or complex roof sections.
The property is heritage-listed or in a character precinct.

A qualified roof cleaner or roof plumber can safely clean the roof, inspect for damage, repair loose tiles and re-bed or re-point ridge capping where needed. Ask for evidence of insurance, a clear description of the cleaning method and chemicals used, and a written quote before work begins.

How Often Should You Clean Your Terracotta Roof?

For most Glenelg homes, a thorough clean every three to five years is a sensible guide, with light maintenance in between. Roofs under heavy tree cover or with a southern aspect may need attention more often, while sunny, well-ventilated roofs may go longer. A quick visual check each year, especially after winter, helps you catch growth early when it is easier and cheaper to treat.

Final Thoughts

Mould, algae and lichen are common on Glenelg's terracotta roofs, but they do not have to be a permanent feature. With the right approach, which means gentle cleaning methods, suitable products, careful attention to safety and regular maintenance, you can restore the look of your tiles and help extend their life. Avoid harsh pressure washing, treat lichen patiently, and tackle underlying causes like shade and blocked gutters. If the job feels risky or the roof is in questionable condition, a local professional can get it done safely and keep your roof in good shape for years to come.

Glenelg Pressure Cleaning

Unit 3/2-4 Byre Ave, Somerton Park SA 5044

(08) 7282 0893