Defects due to fungi occur when the moisture content of timber is above ______.
🍄 Conditions for Fungal Growth (Decay)
Fungi are living organisms that cause timber to decay or rot by feeding on the wood's cellular structure, eventually turning it into powder. For fungi to survive and thrive, four essential conditions must be met simultaneously: food (the wood itself), suitable temperature (warmth), oxygen, and, most critically, sufficient moisture.
If any one of these conditions is removed, fungal growth stops. The most practical condition to control in construction is the moisture content of the timber.
📝 Detailed Analysis of the Options
(c) 10% & (d) 15%
These values are incorrect. A moisture content of 10-15% is typical for properly seasoned timber used in interior applications. At this level of dryness, the wood is considered safe from fungal attack as there is not enough free water to support the life cycle of the fungi.
(b) 25%
While fungal growth certainly occurs at 25% moisture content, the critical threshold at which it *begins* is lower. This value is higher than the generally accepted minimum.
(a) 20%
This is the correct answer. A moisture content of above 20% (with respect to its dry weight) is the widely accepted threshold for fungal attack in wood to begin. Below this level, there is insufficient moisture for the fungi to survive and cause decay. This is the primary reason for seasoning timber—to bring its moisture content well below this critical 20% mark to ensure its durability.
📊 Four Requirements for Fungal Decay
| Requirement | Condition |
|---|---|
| Food | Wood (Cellulose/Lignin) |
| Temperature | Warmth (typically 10°C to 40°C) |
| Oxygen | Presence of air |
| Moisture | Content > 20% |
💡 Study Tips
- The 20% Rule: This is a critical number to memorize in timber engineering. Above 20% moisture content means danger of rot; below 20% means safety.
- Seasoning's Goal: Understand that the main goal of seasoning is to get the moisture content below this 20% threshold to protect the wood.
- Remove One, Stop the Rot: Remember that all four conditions are necessary. You can't easily remove the wood (food), oxygen, or control the temperature, but you *can* control the moisture. This is why it's the most important factor in preventing decay.