Understanding the proper seasoning of timber is crucial for its durability and performance, especially when used in construction. Air seasoning is a traditional method that relies on natural air circulation to reduce the moisture content in timber.

Timber Air Seasoning Zones According to IS 287:1993

The Indian Standard code IS 287:1993 provides guidelines related to the maximum permissible moisture content for timber used in building purposes. A key aspect of achieving the desired moisture content through air seasoning depends significantly on the climatic conditions, particularly the relative humidity of the region.

To account for the varying climatic conditions across India, IS 287:1993 divides the country into specific zones based on relative humidity. This zoning helps in determining the expected equilibrium moisture content that timber will reach when air seasoned in that region, and thus influences the choice of timber and seasoning methods for different applications.

According to the specifications laid down in Indian Standard (IS 287:1993), India is categorized into different zones for the purpose of air seasoning of timber based on relative humidity. The standard specifies that India is divided into four such zones.

Significance of Relative Humidity Zones for Timber Seasoning

The relative humidity of the air affects the rate at which timber dries and the final moisture content it will attain during air seasoning. Timber exchanges moisture with the surrounding air until it reaches equilibrium moisture content (EMC). High relative humidity means slower drying and a higher EMC, while low relative humidity leads to faster drying and a lower EMC.

The four zones defined in IS 287:1993 essentially represent regions with different ranges of average annual relative humidity. This classification is important because:

• It helps predict the minimum moisture content attainable by air seasoning in a particular region.
• It guides engineers and builders in selecting timber with appropriate initial moisture content or deciding if further seasoning (like kiln seasoning) is required to achieve the moisture content suitable for the intended use (e.g., furniture, structural elements).
• It ensures that the timber used in buildings is seasoned to a moisture content that is in equilibrium with the local environment, thereby minimizing problems like shrinkage, swelling, warping, and cracking after installation.

Therefore, the division of India into four zones based on relative humidity as per IS 287:1993 is a practical approach to manage timber seasoning effectively across diverse climatic conditions found in the country.

Revision Table: IS 287:1993 Timber Seasoning Zones

Additional Information: Timber Seasoning Concepts

Seasoning is the process of reducing the moisture content in timber to an acceptable level. This process improves the strength, stability, and durability of wood.

• Why Season Timber? Timber loses moisture unevenly as it dries, which can cause defects like warping, splitting, and checking. Seasoning controls this drying process. Properly seasoned timber is lighter, stronger, more resistant to decay and insect attack, and holds fasteners better.
• Methods of Seasoning:
  • Air Seasoning: Natural drying in the open air, protected from rain and direct sun. It is slow but cost-effective. The final moisture content depends on local climate.
  • Kiln Seasoning: Artificial drying in a controlled environment (kiln) using heat and humidity. It is faster than air seasoning and can achieve lower moisture content.
• Equilibrium Moisture Content (EMC): The moisture content at which wood neither gains nor loses moisture when exposed to air at a specific temperature and relative humidity. Different climatic zones have different average EMC values for timber.