What You Need to Know About Mould on Timber

Building Advice
Frank DiStefano

Keeping your timber dry is the best form of prevention

There have been some recent jobs I have seen when mould on timber has become a concern for the builder. These have ranged from weathered pine framing exposed for long periods and decking boards and joists in enclosed situations. These were showing mould stains with early signs of decay. Moulds are a type of fungi that can grow on any surface that is moist.

When conditions are favourable with temperatures and available nutrients like wood, mould spores in the air will grow and spread. They can look like dark dusty spots that can stain the surface of timber. If moulds are evident on timber, it is a sure sign of excessive moisture content.

In extreme cases, certain moulds can lead to diseases in animals and humans. Poorly ventilated subfloors can develop moulds on the soil, these moulds can develop into a health hazard causing respiratory problems.

Take steps to reduce and prevent the likelihood of mould growth;

  • Keep timber stored off the ground
  • Use appropriate treated or naturally durable timbers that resist decay
  • Use tarps or moisture barrier coatings to protect timber from the rain
  • Provide good ventilation and drainage to sub floors and decks
  • Avoid pooling of water on Particleboard flooring by drilling holes to drain water away
  • Remove door and window bottom plates early to prevent water from being trapped
  • Pre-coat decking and claddings to reduce moisture uptake

Moulds can be found growing on treated timbers as well as untreated timber, though mould fungi does not affect the mechanical or strength properties of timber, unlike decay fungi.

Decay fungi will degrade the structure of the wood by attacking the cell structure of susceptible timber. Decay and mould can be determined in timber by scrapping at the area with a small knife or screwdriver.

“Moulds can be found growing on treated timbers as well as untreated timber, though mould fungi does not affect the mechanical or strength properties of timber, unlike decay fungi.”

If the surface beneath is solid, similar to new timber, then it is only surface mould. If the timber is spongy and breaks away as a mash, decay has started. In severe situations, further investigation may be required to assess structural damage.

Delays on site can lead to timber frames being left exposed for a longer period than normal. If timber is left unprotected and exposed to rain, the moisture content will increase. Wood in Australia suggests that timber ‘must have a moisture content above 20% before there is a serious risk [of] decay’ (Wood in Australia, Bootle).

It is common to see framing bottom plates and the lower section of studs darken by moulds in winter. These timbers will have a dangerously high-level moisture content and if not allowed to dry, will lead to the start of rot from decay fungi.

Removing mould starts with eliminating the moisture source. Allow the timber to dry and reduce its moisture content.

There are many simple products available like vinegar and bleaches to help treat mould. Ideally, a mouldicide or antifungal solution or spray will treat the mould after the timber has dried. However, keeping your timber dry is the best form of prevention.

If you have any questions about mould on timber, feel free to contact [email protected].

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