Algae Moss - Expert Removal & Treatment Guide
Algae Moss - Expert Removal & Treatment Guide
What is Algae Moss?
Algae and moss growth on stone is a common outdoor problem, particularly in damp, shaded areas. Algae appears as green, red, or black discolouration on the surface, while moss forms thicker cushion-like growths in grout lines and textured areas.
Biological growth occurs when moisture, shade, and organic nutrients create favourable conditions. North-facing walls, shaded terraces, areas under trees, and surfaces near water features are most susceptible. The growth is not just cosmetic - it makes surfaces dangerously slippery and can cause long-term damage by producing organic acids and retaining moisture against the stone.
Effective treatment requires both removal of existing growth and preventive measures. Simply pressure-washing removes the visible growth but leaves behind spores and roots that quickly regrow. A chemical treatment that kills the organisms and continues to work preventively gives much longer-lasting results.
How to Identify It
Algae appears as green, reddish, or black patches that feel slippery when wet. Moss is more clearly a plant growth, often in grout lines and crevices. Black algae (cyanobacteria) forms dark patches that can be mistaken for dirt. To test: wet the surface - if dark patches become slippery, it is biological growth. If the patches do not become slippery, it may be soot, industrial fallout, or rubber marks.
Common Causes
- Persistent moisture from rain, irrigation, or condensation
- Shade from buildings, trees, or fences
- Poor air circulation around the surface
- Organic debris (leaves, soil) providing nutrients
- Porous or rough-textured surfaces that trap moisture
- Inadequate slope or drainage on horizontal surfaces
Prevention
- Improve drainage and ensure adequate surface slope (min 1-2%)
- Trim overhanging trees and vegetation to increase sunlight
- Keep surfaces free of organic debris (regular sweeping)
- Seal porous stone to reduce moisture retention
- Apply a preventive algae treatment annually before growing season
Treatment by Surface Type
Natural Stone
Risk level: Medium
Apply Lithofin ALGEX undiluted on dry surface. Leave 24-48 hours. Rinse with water (garden hose or gentle pressure washer). For stubborn growth: scrub with stiff brush before rinsing. ALGEX also works preventively.
- Do not pressure-wash delicate stone at close range
- Marble and limestone may etch from acidic biological cleaners
- Sandstone is soft - use low pressure only
Ceramics
Risk level: Low
Apply Lithofin ALGEX to affected areas. Leave overnight. Scrub grout lines with brush. Rinse thoroughly. Apply Lithofin KF Grout Protector to grout lines to reduce regrowth.
- Algae mainly grows in grout lines on outdoor tiles
- Glazed tiles resist growth but grout does not
Artificial Stone
Risk level: Medium
Apply Lithofin ALGEX or Lithofin MN Outdoor Cleaner. Leave 24-48 hours. Rinse with pressure washer at safe distance (30cm+). For pavers, re-sand joints after cleaning. Seal with MN Stain-Stop for longer protection.
- Concrete pavers are very susceptible due to porosity and texture
- Do not use high-pressure close to joints - can dislodge sand
Cotto Terracotta
Risk level: Medium
Apply Lithofin ALGEX. Leave 24-48 hours. Brush off and rinse gently. Seal with Lithofin TC Impregnator to reduce moisture absorption and future growth.
- Outdoor cotto is susceptible to biological growth
- Do not pressure-wash unsealed cotto - too porous
When to Call a Professional
For large outdoor areas (>100m²) with heavy growth, professional cleaning companies with appropriate equipment can be more efficient. If biological growth is causing structural damage to the stone or mortar, a mason should assess and repair before cleaning.
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