Black streaks on exterior walls are almost always biological growth running in the direction of water flow. Soft washing kills it at the root so it doesn't come back within months.
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Black streaks on exterior walls — running vertically down from eaves, window sills, overflow pipes and wall junctions — are one of the most common cosmetic issues reported by Queensland homeowners. These dark streaks are almost always caused by biological growth (algae or mould running in the direction of water flow), or dirty runoff from overhead surfaces (roof staining or dust washed off eaves by rain). In both cases the solution is professional house washing — soft washing that kills biological growth at the root and removes all surface staining in a single treatment.
Algae and mould that establish on rendered or weatherboard walls produce dark pigmentation as they grow — creating the characteristic dark streaking in the direction of water flow. Soft washing kills the organism at root level — results last 12 to 18 months.
Dirty runoff from overhead roof surfaces, eaves and window sills creates a different pattern — typically more concentrated below specific points rather than distributed across the whole wall. This type of streaking is surface contamination carried by water rather than biological growth, and responds to standard house washing.
Streaking that follows water flow from specific points is typically runoff staining. Streaking that appears more uniformly across the wall face in the direction of rain flow is typically biological growth. Both are addressed in a single house wash.
Keeping gutters clear so water flows correctly away from walls reduces runoff streaking. Soft washing every 12 to 18 months manages biological streaking before it establishes deeply.
Black streaks are almost always biological growth running in the direction of water flow, or dirty runoff from overhead surfaces. Both are removed by professional house washing.
With soft washing, biological growth takes 12 to 18 months to re-establish. With pressure washing it returns in 3 to 6 months.
Yes — soft washing is specifically the correct method for render. High pressure risks surface damage.
Included in house washing from $499 including free gutter clean. Free quote provided.
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