Quick Answer

Black mould on roof tiles is usually Gloeocapsa magma — a blue-green algae that turns black over time. Remove it naturally using a white vinegar spray or an oxygen bleach (sodium percarbonate) solution. Apply on a dry day, leave for 30-60 minutes, then rinse. A single treatment costs under $20 and lasts 1-2 years.

Why Melbourne Roofs Get Black Mould

SE Melbourne’s humid summers and mild winters create ideal conditions for algae and mould growth on roof tiles. Concrete tiles, terracotta, and even Colorbond valleys are affected. Homes in leafy suburbs like Berwick, Narre Warren, and Pakenham with tree canopy overhead are most susceptible — shaded, damp tiles never fully dry out.

The dark staining is primarily Gloeocapsa magma algae, not structural mould. It’s cosmetic but left untreated it retains moisture, accelerating tile degradation. Chemical treatments (chlorine bleach) work but can damage tile coatings, harm garden beds, and corrode metal flashings. Natural alternatives are safer and nearly as effective.

What You’ll Need

  • White vinegar (5L, supermarket — $4-6) or sodium percarbonate / oxygen bleach ($12-15 at Bunnings)
  • Garden pump sprayer (1-2L, $15-25 at Bunnings)
  • Stable ladder reaching eave height
  • Rubber gloves and safety glasses
  • Old clothes (solutions can bleach fabric)
  • Garden hose with trigger nozzle
Safety warning: Do not walk on roof tiles — they are slippery when wet and terracotta tiles crack under foot pressure. All treatment should be applied from a stable ladder at eave height or by a professional roof cleaner. Never work on a roof in wet or windy conditions.

Method 1: White Vinegar Spray (Mild Staining)

  1. Fill your pump sprayer with undiluted white vinegar.
  2. Choose a dry day with no rain forecast for 24 hours.
  3. From your ladder, spray vinegar liberally onto affected tiles. Work in sections you can reach safely without overreaching.
  4. Leave for 30-60 minutes. Do not rinse — let the vinegar dry into the tile.
  5. After 1-2 weeks, rain will wash away the dead algae. Repeat annually.
Pro tip: Vinegar works best on early-stage algae. For heavy black staining (tiles turning dark grey-black), use oxygen bleach instead.

Method 2: Oxygen Bleach Solution (Heavy Staining)

  1. Mix oxygen bleach (sodium percarbonate) with warm water — 200g per 4 litres. Stir until dissolved.
  2. Wet the tiles with plain water first using your hose (prevents solution soaking in too fast).
  3. Apply the solution via pump sprayer to affected tiles.
  4. Leave for 45-60 minutes. Keep damp if it dries out quickly by respraying.
  5. Rinse thoroughly with a hose. Avoid runoff into garden beds — oxygen bleach can affect soil pH.
  6. Staining fades over 1-3 weeks as dead algae washes away.

Troubleshooting

Problem Likely Cause Fix
Staining returns within 3-6 months Heavy tree canopy keeping tiles damp Trim overhanging branches to improve airflow; increase treatment frequency to twice yearly
White residue left on tiles after treatment Oxygen bleach mineral deposits Rinse more thoroughly after treatment; use less concentrated solution next time
Green moss (not black algae) Persistent damp from poor drainage or shade Vinegar and oxygen bleach work on moss too — but persistent moss indicates a drainage issue worth investigating
Tiles still stained after 3 weeks Deep-set staining penetrating tile surface Professional pressure-washing and tile sealing ($300-600) may be needed for severe cases

When to Call a Professional

Call a licensed roof cleaner if: the affected area is large (half the roof or more), you have broken or displaced tiles that need repair before cleaning, or the roof pitch is too steep to safely reach from a ladder. Professional roof cleaning in SE Melbourne costs $300-600 for an average home and includes pressure-washing and optional tile sealing.

Prevention

  • Trim tree branches overhanging the roof — shade and leaf debris are the primary causes of mould growth.
  • Clean gutters twice yearly (autumn and spring) so water drains freely and doesn’t back up under tiles.
  • Annual vinegar spray treatment prevents algae establishing — much easier than treating heavy staining.
  • Copper or zinc strips installed at the ridge line release ions that prevent algae growth with each rainfall.

Frequently Asked Questions

Is the black on my roof mould or algae?

It’s usually Gloeocapsa magma algae — technically a cyanobacteria. It starts green and turns black as it ages. True roof mould (Aspergillus, Cladosporium) is less common on exterior tiles but can occur under tile lips in poorly-ventilated areas. Both respond to the same treatments.

Is it safe to use bleach on roof tiles?

Chlorine bleach works but is aggressive — it can strip tile coatings, kill garden plants, and corrode metal flashings and gutters. Oxygen bleach (sodium percarbonate) is a safer alternative that breaks down into water and oxygen. Vinegar is the gentlest option for mild cases.

How long does natural roof mould treatment last?

Vinegar and oxygen bleach treatments typically last 1-2 years before regrowth appears. In heavily shaded gardens (common in Berwick and Narre Warren), you may need to treat annually. Copper/zinc ridge strips provide longer-term prevention.

Can I pressure-wash my roof tiles?

With care, yes — but only using a low-pressure setting (under 1500 PSI) and never directly into tile joints or under tile lips. High pressure forces water under tiles and into the roof space. Professional roof cleaners use purpose-built equipment at safe pressures.

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