Black staining on stained or clear-finished timber weatherboards

Causes

Mould growth on timber surface; occurs most commonly on south-facing walls

Listed under: Exterior, finishes, clear finishes, timber and wood-based, stain, health and safety, stains, timber, cladding, walls, moisture, mould, mould health and safety, wall cladding, stained or clear-finished timber weatherboards

Cause

Mould growth on timber surface; occurs most commonly on south-facing walls

Repair

  • while lack of direct sun on south-facing walls may be the main cause of the problem, check to see if there are other contributing causes that you can do something about. For example, if the guttering above the wall is broken or blocked and keeping the wall wet, fix that. If there are trees growing close to the wall, trim them back
  • wash with water and a mild detergent and a soft-bristled brush. A solution of 1 part household bleach to 4 parts water is also possible, but rinse off thoroughly and ensure bleach solution doesn’t get onto the lawn or garden or into waterways
  • don’t use a water blaster because this can damage the surface of the timber – stained timbers are often softer timbers such as cedar and redwood. Water blasting can also cause long-term problems if water gets into the wall assembly
  • proprietary timber restorative treatments are also available. Ask the weatherboard manufacturer (or a manufacturer of similar products) for their advice
  • clear finishes on timber weatherboards typically require recoating more frequently than paint finishes – as little as every 1–2 years in some cases. The best solution may be preparing the weatherboards as necessary and then applying a new coating

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