Effects of Moisture on Timber

Almost all building materials deteriorate over time when they are exposed to moisture. Moisture causes fungal decay and mould in timber and chemicals from corroded metal fixings can also damage timber.

Dry Rot


Spore germination 28 – 30% moisture content
Optimum growth 30 – 40% moisture content
Minimum moisture content for continued growth 20%
Requires high humidity and acidity for establishment

Wet Rot


Optimum growth 50 – 60% moisture content
Minimum moisture content for continued growth 30%
High moisture required / sensitive to drying
Tolerant to many preservatives

Soft Rot


Prefers high moisture

Moisture Content Readings

Acceptable levels of moisture are generally below 20%; anything above this level can cause damage to building elements over time and may require further investigation. Where moisture levels in excess of 25 – 30% moisture content are recorded, we will generally recommend that an invasive investigation is undertaken to check the structural integrity.

Moisture readings, unless otherwise stated are taken with a non-invasive moisture meter, so they are not the exact moisture readings of the framework.


Moisture Content in Timber

Types of Rot Conditions
< 18% Decay is highly unlikely
18 – 30% Less than fibre saturation point of 30% decay is unusual except for dry rot
> 30% Close to wood saturation – decay is common =

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  • Pre-purchase and pre-sale inspections
  • Moisture and leak detection
  • Roof inspections
  • Heat loss inspections
  • Leaky home diagnostics
  • Weathertightness reports